SANRAL Geometric Design Guide
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Chapter
Contents Glossary
1
Introduction
2
Design Philosophy
3
Design Controls
4
Design Elements
5
Alignment Design
6
Intersections
7
Interchanges
8
Roadside Safety
9
RRR
10
Grade Separations
11
Toll Plazas Bibliography Covers
Glossary A
summation for all vehicles. Also referred to as
Acceleration lane. An auxiliary lane used by an
space mean speed whereas time mean speed is
entering vehicle to accelerate before entering
simply the average of all recorded speeds.
the travelled way.
Axis of rotation. The line about which the pavement is rotated to superelevate the roadway.
Access control. The condition whereby the road
This line normally maintains the highway profile
agency either partially or fully controls the right of abutting landowners to direct access to and
B
from a public highway or road.
Barrier sight distance. The limiting sight distance below which overtaking is legally prohibit-
Access interchange. An interchange providing
ed.
access to a freeway from the adjacent non-freeway road network.
Boulevard. The area separating sidewalks from the through lanes.
Arterial. Highway designed to move relatively large volumes of traffic at high speeds over long
Bridge. A structure erected with a deck for car-
distances. Typically, arterials offer little or no
rying traffic over or under an obstruction and
access to abutting properties.
with a clear span of six metres or more. Where the clear span is less than six metres, reference
Auxiliary lane. Short lane located immediately
is to a culvert.
adjacent to the basic or through lane to accommodate some or other special circumstance
Broken-back curve. Two curves in the same
such as a turning movement to right or to left,
direction with a tangent shorter than 500 metres
acceleration to or deceleration from the speeds
long connecting them.
cles reduced to crawl speeds on a steep
C
upgrade.
Camber. The slope from a high point (typically
Average Daily Traffic (ADT).
at the centre line of the highway) across the
The number of
lanes of a highway. Negative camber refers to
vehicles per day passing a point on the highway
a central low point, usually with a view to
during a defined period. If this period extends
drainage of a small urban street or alley.
from 1 January to 31 December, reference is to Annual Average Daily Traffic (AADT).
Capacity.
The maximum number of vehicles
Average running speed. The distance summa-
that can pass a point on a highway or in a des-
tion for all vehicles divided by the running time
ignated lane in one hour without the density
i Glossary
Geometric Design Guide
prevailing on the travelled way or heavy vehi-
being so great as to cause unreasonable delay
Collector-Distributor road. A road used at an
or restrict the driver’s freedom to manoeuvre
interchange to remove weaving from the
under prevailing roadway and traffic conditions.
through lanes and to reduce the number of entrances to and exits from the through lanes.
Carriageway. Roadway forming part of a divided highway and intended for movement in one
Compound curve. A combination of two or more
direction only – hence dual carriageway as an
curves in the same direction without intervening
alternative name for divided highway.
tangents between them.
Catchwater drain. Located above a cut face to
Criterion. A yardstick according to which some
ensure that storm water does not flow down the
or other quality of the road can be measured.
cut face causing erosion and deposition of silt
Guideline values are specific numerical values
on the roadway.
of the criterion. For example, delay is a criterion of congestion.
Channel grading.
Where side channels are
designed to gradients that differ from those of
Critical length of grade. The maximum length of
the road centreline, typically on either side of the
a specific upgrade on which a loaded truck can
highest points on crest curves and the lowest
operate without an unreasonable reduction in
points on sag curves where the centreline gradi-
speed. Very often, a speed reduction of 15 km/h
ent is less than 0,5 per cent.
or more is considered “unreasonable”. Cross fall. See camber. In the case of cross
Channelisation. The use of pavement markings
fall, the high point is at the roadway edge.
or islands to direct traffic through an intersection
Cross-over crownline. The line across which an
Clearance profile. Describes the space that is
instantaneous change of camber takes place.
exclusively reserved for provision of the road or
In the case of a normally cambered road, the
highway. It defines the minimum height of the
centreline is a special case of the cross-over
Geometric Design Guide
soffit of any structure passing over the road and
crownline.
the closest approach of any lateral obstacle to
located anywhere on the road surface and need
the cross-section.
Cloverleaf interchange.
The cross-over crownline can be
not even be parallel to the road centreline. An interchange with
Crosswalk. A demarcated area or lane desig-
loop ramps in all quadrants to accommodate
nated for the use of pedestrians across a road
right turns and outer connectors for the left
or street.
turns. Crown runoff.
(Also referred to as tangent
Collector. A road characterised by a roughly
runout) The rotation of the outer lane of a two-
even distribution of its access and mobility func-
lane road from zero cross fall to normal camber
tions.
(NC). ii Glossary
Culvert. A structure, usually for conveying water
Design hour. The hour in which the condition
under a roadway but can also be used as a
being designed for, typically the anticipated flow,
pedestrian or stock crossing, with a clear span
is expected to occur. This is often the thirtieth
of less than six metres.
highest hour of flow in the design year.
Cut. Section of highway or road below natural
Design speed. The speed selected as the basis
ground level. Sometimes referred to in other
for establishing appropriate geometric elements
documents as a cutting or excavation.
for a section of road.
Cycle lane. A portion of the roadway which has
Design vehicle.
been designated by road markings, striping and
A compilation of the 85th percentile values of the
signing as being exclusively for the use of
various parameters of the vehicle type being
cyclists.
designed for, e.g. length, width, wheelbase, overhang, height, ground clearance, etc.
Cycle path. Also known as a bike way. A path physically separated from motorised traffic by
Design year. The last year of the design life of
an open space or barrier and located either
the road or any other facility, often taken as
within the road reserve or an independent
twenty years although, for costly structures such
reserve.
as major bridges, a longer period is usually adopted.
D Decision sight distance. Sometimes referred to
Directional distribution (split). The percentages
as anticipatory sight distance, allows for circum-
of the total flow moving in opposing directions,
stances where complex decisions are required
e.g. 50:50, 70:30, with the direction of interest
or unusual manoeuvres have to be carried out.
being quoted first.
As such, it is significantly longer than Stopping Divided highway. A highway with separate car-
Sight Distance.
riageways for traffic moving in opposite direc-
given length of road.
Usually averaged over Driveway. A road providing access from a pub-
time and expressed as vehicles per kilometre.
lic road to a street or road usually located on an abutting property.
Depressed median. A median lower in elevation than the travelled way and so designed to carry
E
portion of the storm water falling on the road.
Eighty-fifth percentile speed. The speed below Design domain. The range of values of a design
which 85 per cent of the vehicles travel on a
criterion that are applicable to a given design,
given road or highway.
e.g. lane widths of more than 3,3 metres. iii Glossary
Geometric Design Guide
tions.
Density. The number of vehicles occupying a
F
Gradient. The slope of the grade between two
Footway. The rural equivalent of the urban side-
adjacent Vertical Points of Intersection (VPI),
walk.
typically expressed in percentage form as the vertical rise or fall in metres/100 metres. In the
Freeway. Highest level of arterial characterised
direction of increasing stake value, upgrades
by full control of access and high design
are taken as positive and downgrades as nega-
speeds.
tive.
Frontage road. A road adjacent and parallel to
Guideline.
but separated from the highway for service to
approximate threshold, which should be met if
abutting properties and for control of access.
considered practical.
Sometimes also referred to as a service road.
value whereas a standard is a prescriptive value
A design value establishing an It is a recommended
allowing for no exceptions.
G
H
Gap. The elapsed time between the back of
High occupancy vehicle ( HOV) lane. A lane
one vehicle passing a point on the road or high-
designated for the exclusive use of buses and
way and the nose of the following vehicle pass-
other vehicles carrying more than two passen-
ing the same point. A lag is the unexpired por-
gers.
tion of a gap, i.e. the elapsed time between the arrival of a vehicle on the minor leg of an inter-
High-speed. Typically where speeds of 80 km/h
section and the nose of the next vehicle on the
or faster are being considered.
major road crossing the path of the entering vehicle.
Horizontal sight distance. The sight distance determined by lateral obstructions alongside the
Gore area. The paved triangular area between
road and measured at the centre of the inside
the through lanes and the exit or entrance
lane.
ramps at interchanges plus the graded areas
Geometric Design Guide
immediately beyond the nose (off-ramp) or
I
merging end (on-ramp). Grade line.
Interchange. A system of interconnecting roads (referred to as ramps) in conjunction with one or
The line describing the vertical
more grade separations providing for the move-
alignment of the road or highway.
ment of traffic between two or more roadways Grade. The straight portion of the grade line
which are at different levels at their crossing
between two successive vertical curves.
point.
Grade separation. A crossing of two highways
Intersection sight distance. The sight distance
or roads, or a road and a railway, at different
required within the quadrants of an intersection
levels.
to safely allow turning and crossing movements. iv Glossary
J, K
verse natural slopes are severe and changes in
Kerb. Concrete, often precast, element adja-
elevation abrupt. Many trucks operate at crawl
cent to the travelled way and used for drainage
speeds over substantial distances.
control, delineation of the pavement edge or protection of the edge of surfacing.
N
Usually
Normal crown (NC). The typical cross-section
applied only in urban areas.
on a tangent section of a two-lane road or fourlane undivided road.
Kerb ramp. The treatment at intersections for gradually lowering the elevation of sidewalks to
O
the elevation of the street surface.
Overpass. A grade separation where a minor highway passes over the major highway.
K-value. The distance over which a one per cent change in gradient takes place.
Outer separator.
Similar to the median but
L
located between the travelled way of the major
Level of Service (LOS). A qualitative concept,
road and the travelled way of parallel lanes
from LOS A to LOS F, which characterises
serving a local function if these lanes are con-
acceptable degrees of congestion as perceived
tained within the reserve of the major road. If
by drivers. Capacity is defined as being at LOS E.
they fall outside this reserve, reference is to a frontage road.
Low speed. Typically where speeds of 70 km/h
P Partial Cloverleaf (Par-Clo) Interchange.
An
M
interchange with loop ramps in one, two or three
Median. The portion of a divided highway sep-
(but usually only two) quadrants. A Par-Clo A
arating the two travelled ways for traffic in oppo-
Interchange has the loops in advance of the
site directions. The median thus includes the
structure and Par-Clo B Interchange has the
inner shoulders.
loops beyond the structure.
A Par-Clo AB
Interchange has its loops on the same side of the crossing road.
Median opening. An at-grade opening in the median to allow vehicles to cross from a road-
Passenger car equivalents (units) (PCE or
way to the adjacent roadway on a divided road.
PCU). A measure of the impedance offered by The public facility at
a vehicle to the passenger cars in the traffic
which passengers change from one mode of
stream. Usually quoted as the number of pas-
transport to another, e.g. rail to bus, passenger
senger cars required to offer a similar level of
car to rail.
impedance to the other cars in the stream.
Modal transfer station.
Passing sight distance. The total length visibiliMountainous terrain.
ty, measured from an eye height of 1,05 metres
Longitudinal and transv
Glossary
Geometric Design Guide
or slower are being considered.
to an object height of 1,3 metres, necessary for
R
a passenger car to overtake a slower moving
Ramp. A one-way, often single-lane, road pro-
vehicle. It is measured from the point at which
viding a link between two roads that cross each
the initial acceleration commences to the point
other at different levels.
where the overtaking vehicle is once again back in its own lane.
Relative gradient. The slope of the edge of the travelled way relative to the gradeline.
PC (Point of curvature). Beginning of horizontal curve, often referred to as the BC.
Reverse Camber (RC). A superelevated section of roadway sloped across the entire travelled
PI (Point of intersection). Point of intersection of
way at a rate equal to the normal camber.
two tangents. Reverse curve. A combination of two curves in PRC (Point of reverse curvature). Point where
opposite directions with a short intervening tan-
a curve in one direction is immediately followed
gent
by a curve in the opposite direction. Typically applied only to kerb lines.
Road safety audit. A structured and multidisciplinary process leading to a report on the crash
PT (Point of tangency). End of horizontal curve,
potential and safety performance of a length of
often referred to as EC.
road or highway, which report may or may not include suggested remedial measures.
PVC (Point of vertical curvature) The point at which a grade ends and the vertical curve
Roadside. A general term denoting the area
begins, often also referred to as BVC.
beyond the shoulder breakpoints.
PVI (Point of vertical intersection). The point
Road bed.
where the extension of two grades intersect.
shoulder breakpoints.
The extent of the road between
Geometric Design Guide
The initials are sometimes reversed to VPI. Road prism. The lateral extent of the earthworks.
PVT (Point of vertical tangency). The point at which the vertical curve ends and the grade
Road reserve. Also referred to as Right-of-way.
begins. Also referred to as EVC.
The strip of land acquired by the road authority
Q
for provision of a road or highway.
Quarter link. An interchange with at-grade intersections on both highways or roads and two
Roadway. The lanes and shoulders excluding
ramps (which could be a two-lane two-way road)
the allowance (typically 0,5 metres) for rounding
located in one quadrant. Because of its appear-
of the shoulders.
ance, also known as a Jug Handle Interchange. vi Glossary
Rolling terrain. The natural slopes consistently
Single point urban interchange.
rise above and fall below the highway grade
interchange where all the legs of the inter-
with, occasionally, steep slopes presenting
change meet at a common point on the crossing
some restrictions on highway alignment.
road.
In
A diamond
general, rolling terrain generates steeper gradients, causing truck speeds to be lower than
Speed profile. The graphical representation of
those of passenger cars.
the 85th percentile speed achieved along the length of the highway segment by the design
Rural road or highway. Characterised by low-
vehicle.
volume high-speed flows over extended distances. Usually without significant daily peaking
Standard. A design value that may not be trans-
but could display heavy seasonal peak flows.
gressed, e.g. an irreducible minimum or an absolute maximum. In the sense of geometric
S
design, not to be construed as an indicator of
Shoulder. Usable area immediately adjacent to
quality, i.e. an ideal to be strived for.
the travelled way provided for emergency stopping, recovery of errant vehicles and lateral sup-
Stopping sight distance. The sum of the dis-
port of the roadway structure.
tance
travelled
during
a
driver’s
perception/reaction time and the distance travShoulder breakpoint. The hypothetical point at
elled thereafter while braking to a stop.
which the slope of the shoulder intersects the line of the fill slope. Sometimes referred to as
Superelevation. The amount of cross-slope pro-
the hinge point.
vided on a curve to help counterbalance, in combination with side friction, the centrifugal
Side friction (f). The resistance to centrifugal
force acting on a vehicle traversing the curve.
force keeping a vehicle in a circular path. The Superelevation runoff.
sents a threshold of driver discomfort and not
superelevation development)
the point of an impending skid. Sidewalk.
(Also referred to as The process of
rotating the outside lane from zero crossfall to reverse camber (RC), thereafter rotating both
The portion of the cross-section
lanes to the full superelevation selected for the
reserved for the use of pedestrians.
curve.
Sight triangle. The area in the quadrants of an
Systems interchange. Interchange connecting
intersection that must be kept clear to ensure
two freeways, i.e. a node in the freeway system.
adequate sight distance between the opposing legs of the intersection.
T
Simple curve. A curve of constant radius with-
Tangent.
out entering or exiting transitions.
The straight portion of a highway
between two horizontal curves. vii Glossary
Geometric Design Guide
designated maximum side friction (fmax) repre-
V
Tangent runoff. See crown runoff
Value engineering. A management technique in Traffic composition. The percentage of vehicles
which intensive study of a project seeks to
other than passenger cars in the traffic stream,
achieve the best functional balance between
e.g. 10 per cent trucks, 5 per cent articulated
cost, reliability and performance.
vehicles (semi-trailers) etc. Verge. The area between the edge of the road Transition curve. A spiral located between a
prism and the reserve boundary
tangent and a circular curve.
W Travelled way. The lanes of the cross-section.
Warrant. A guideline value indicating whether or
The travelled way excludes the shoulders.
not a facility should be provided. For example, a warrant for signalisation of an intersection
Trumpet interchange.
A three-legged inter-
would include the traffic volumes that should be
change containing a loop ramp and a direction-
exceeded before signalisation is considered as
al ramp, creating between them the appearance
a traffic control option. Note that, once the war-
of the bell of a trumpet.
ranting threshold has been met, this is an indication that the design treatment should be con-
Turning roadway. Channelised turn lane at an
sidered and evaluated and not that the design
at-grade intersection.
treatment is automatically required.
Turning template. A graphic representation of a
X, Y, Z
design vehicle’s turning path for various angles
Yellow line break point. A point where a sharp
of turn. If the template includes the paths of the
change of direction of the yellow edge line
outer front and inner rear points of the vehicle,
demarcating the travelled way edge takes place.
reference is to the swept path of the vehicle.
Usually employed to highlight the presence of the start of a taper from the through lane at an
Geometric Design Guide
U
interchange.
Underpass. A grade separation where the subject highway passes under an intersecting highway. Urban road or highway. Characterised by high traffic volumes moving at relatively low speeds and pronounced peak or tidal flows. Usually within an urban area but may also be a link traversing an unbuilt up area between two adjacent urban areas, hence displaying urban operational characteristics. viii Glossary
TABLE OF CONTENTS 1.
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1
Chapter 1 INTRODUCTION The geometric design of a highway or of one of
demands from different sections of the commu-
its many elements is only one step in a multifac-
nity as they endeavour to design safe and oper-
eted process from concept to construction.
ationally efficient roads.
However, the constraints, which the physical
A major objective of any road design guide is to
elements ultimately place on the function and
ensure that designs achieve value for money
form of a highway, pervade every step in the
without any significant deleterious effect on
process. Knowledge of the parameters, which
safety.
govern planning and design together with their
The design philosophy, systems and
techniques developed elsewhere in this docu-
practical application, is thus essential. These
ment have been based on the Design Speed
guidelines seek to meet that need.
approach and related geometric parameters which will result in a much greater flexibility to
The emphasis previously of Geometric Design
achieve economic design in varied and some-
Manuals was on design standards for new conwork is, however, substantially complete and
In line with this, the standards in this guideline
new road works are largely limited to urban
will address a spectrum of road types, varying
developments. This Manual thus deals not only
from multi-lane freeways carrying traffic vol-
with new works but also pays attention to reha-
umes of over 100 000 vehicles per day, to single
bilitation, reconstruction and upgrading projects.
carriageway roads carrying volumes of the order
A feature of these projects is that the designer’s
of 500 vehicles per day. In respect of this latter
freedom of choice is often restricted by develop-
class of road design, recommendations have
ments surrounding the road to be rehabilitated.
been considerably extended to allow greater
In consequence, adherence to rigidly applied
flexibility in design, with particular emphasis on
standards is not possible, in addition to the fact
the co-ordination of design elements to improve
that blind adherence has never been construed
safety and overtaking conditions.
as a thinking designer’s approach to the problem at hand.
The guidelines distinguish between roads in rural areas and those in urban areas and also
These geometric design guidelines are intended
caters for situations where National Roads tra-
for use on National Roads – or on any other
verse the CBDs of smaller municipalities.
roads falling within the domain of the S A National Roads Agency Limited. For this rea-
Overall, the greater flexibility in design intro-
son, the guidelines address a wide range of
duced in these guidelines will enable more eco-
functional uses and requirements. They will also
nomic designs, reducing both the construction
need to cater for a multiplicity of users, and
costs and the impact of new roads and road
designers will be faced with competing
improvements on the environment. 1-1
Chapter 1: Design philosophy and techniques
Geometric Design Guide
times difficult circumstances.
struction. The South African primary road net-
TABLE OF CONTENTS 2.
DESIGN PHILOSOPHY AND TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1 2.1
BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.2
FUNDAMENTAL PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
2.3
DESIGN PHILOSOPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3
2.4
DESIGN TECHNIQUES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 2.4.1 Flexibility In highway design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4 2.4.2 Interactive Highway System Design Model (IHSDM) . . . . . . . . . . . . . . . . . . . . . . . 2-6 2.4.3 The "design domain" concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7 2.4.4 Road safety audits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10 2.4.5 Economic analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-11 2.4.6 Value engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-12
TABLE OF FIGURES Figure 2.1 :The design domain concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8 Figure 2.2 :Example of design domain application - shoulder width. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9
Chapter 2 DESIGN PHILOSOPHY AND TECHNIQUES 2.1
BACKGROUND
the focus to move towards the whole-life economy of the network.
The Department of Transport completed the "Moving South Africa" project in 1999. One of
The network with the shortest overall length
the findings was that, in order to reduce con-
compatible with linking all origins and destina-
gestion, a shift from private to public transport
tions would theoretically have the lowest cost. It
would be required. As many people are captive
could also represent a saving in maintenance
to public transport, it is also necessary to create
cost provided that the attempt to reduce the net-
an environment supportive of public transport.
work length did not adversely impact on the vertical alignment, resulting in very steep gradients
This will require encouraging:
carry a maintenance and construction penalty.
Settlement densities supportive of public
Assuming that maintenance is practical, it is
transport;
•
Network layouts with geometric design
possible that the network, short though it may
standards suitable for bus and taxi
be, forces circuitous travel paths, which would
routes including:
nullify any savings on construction and maintenance. It follows that the shape of the network
-
Safe stopping sight distance;
-
Reduced gradients;
-
Minimum horizontal curvature;
often incorrectly construed as the selection, siz-
-
Intersection layouts that are simple
ing and grouping of a set of components to cre-
to negotiate;
ate a road network, must therefore contain a
-
User friendly bus stops;
-
Terminals and modal transfer sta-
is as important as its overall length in optimising the life-cycle cost. Geometric Design, which is
strong element of Geometric Planning.
Geometric Planning includes careful selection of
tions; and -
the cross-section. The road width and shape has a significant impact on the cost of construc-
High occupancy vehicle lanes.
tion but economizing on the cross-section by Optimising resources requires the building of
reducing the number and width of lanes could
networks with the lowest possible whole-life
have a crippling effect on traffic flow and a con-
costs. This has always been a goal but, histori-
sequential increases in road user costs.
cally, the emphasis tended to be on minimum
such, classification of the various links in the
construction costs and, more recently, on mini-
road network and estimating their traffic vol-
mum combined construction and maintenance
umes is essential for planning a truly economi-
costs. A subsequent shift in emphasis caused
cal road network.
2-1 Chapter 2: Design philosophy and techniques
As
Geometric Design Guide
•
or a poorly drained road, both of which could
Another historic emphasis was on design for
mulation of these laws states that "The change
mobility and accessibility. Design was specifi-
of motion is proportional to the motive force
cally for passenger cars with some attention
impressed; and is made in the direction of the
being paid to the requirements of other vehicles,
right line in which that force is impressed" and
particularly at intersections. However, geomet-
also that
ric designers must now recognize that the road
opposed an equal reaction: or, the mutual
network, particularly in dense settlements,
actions of two bodies upon each other are
serves other functions in addition to mobility and
always equal, and directed to contrary parts".
accessibility.
Community needs, including
Professor Newton clearly understood the impli-
social interaction, relaxation and commerce, are
cations of these laws for he goes on to say "The
becoming ever more important. In urban areas
power and use of machines consists only in this,
there is a trend towards mixed land usage. A
that by diminishing the velocity we may aug-
consequence of this change is that trip lengths
ment the force, and the contrary."
"To every action there is always
are shorter and modes of transport other than By applying the laws of motion, together with
passenger cars and buses become a practical
judicious experimentation, we are able to gain a
option. Walking and cycling can be expected to
reasonable understanding of the interaction
become more pervasive in the urban environ-
between the vehicle and the roadway, as they
ment. The design process will have to make
are essentially deterministic. In essence, this
provision for these mobility options as part of the
understanding describes what a vehicle moving
total package available to the traveller.
along a road can do and not necessarily what the driver wishes to do. Therefore, to properly
As there is a need to consider:
describe a highway operating system these
• •
Geometric Design Guide
•
network reconstruction and rehabilita-
laws must be integrated with the human factor,
tion;
which includes the perceptions, reactions, toler-
the findings of the Moving South Africa
ances and failures of a wide spectrum of indi-
project;
viduals under continuously changing circum-
the whole-life economy of the road net-
stances.
work;
•
Design manuals tend to focus on vehicle
the broader functionality of the road net-
dynamics, with all the frailties of the human
work; and
•
component of the system being summed up in a
the possibility of an increase in non-
single reaction time. The randomness of human
motorised transport;
behaviour is disregarded. Crash investigations
it follows that the focus of geometric planning
often reveal, however, that it is not always the
and design has to change.
road or the vehicle but rather the human com-
2.2
ponent of the system that fails under stress.
FUNDAMENTAL PRINCIPLES
The laws of motion govern the interaction of the
A vehicle moving along a roadway is a highly
vehicle and the roadway. Isaac Newton's for-
complex system with an infinite range of possi2-2
Chapter 2: Design philosophy and techniques
bilities and outcomes. There are numerous crit-
that many drivers manage to make the same
ical elements, each with its own probability of
mistake at the same point along the road. While
failure. When these are factored together, the
it is necessary to reconsider the role of the
sheer number of elements ensures that the
Newtonian models on which geometric stan-
probability of failure of the system as a whole is
dards are based, human factors require careful
very high indeed. We measure these failures as
evaluation.
crashes.
2.3
DESIGN PHILOSOPHY
According to Hauer, roads designed to pub-
Commonly advocated design philosophies tend
lished standards are neither safe nor unsafe and
towards the simplistic and are inclined to ignore
the linkage between standards and safety is
the issues discussed in Section 2.1. In search of
largely unpremeditated. He illustrates his con-
safety they place inordinate reliance on models
tention by reference to the vector diagram that
derived exclusively from Newtonian dynamics.
describes the forces operating on a vehicle tra-
Current philosophy is, in short, based on the
versing a superelevated curve.
This is
assumption that any design that accords with
Newtonian dynamics and, if it offered a proper
established geometric design policies is safe
explanation of the situation, curves should theo-
and that those that do not are unsafe. This is
retically have no accidents at all or, at worst,
taken for granted by designers and often is
should have exactly the same accident rate as
accepted by the courts when making decisions
the tangents that precede and follow them.
on questions of liability.
Furthermore, vehicles leaving the road should be equally distributed between the inside and
Despite many decades of research the complex
the outside of the curve. The reality of the situ-
relationship between vehicle, roadway, driver;
ation is that the accident rate on curves is high-
and operational safety is not always well under-
er than on tangents and most vehicles leaving
stood.
the road do so on the outside of the curve.
investigated the relationships between accident
Although numerous researchers have
rates and specific geometric design elements, Clearly, the vector diagram is not a complete or
the results were often not sufficiently definitive
sufficient exposition of the problem. For exam-
of this research, which, in examining the rela-
after they have passed its starting point and are
tionship between accidents and individual
thus obliged to follow a path with a smaller
design elements, fails to consider the interactive
radius than that provided by the designer. If the
effects of other parameters, which could lead to
designed curve is at minimum radius, the sub-
bias and mask important relationships.
minimum path actually being followed could A panic
From this rather unhappy state of affairs we can
reaction under these circumstances could
only conclude that a new design philosophy is
cause the vehicle to swerve out of control.
warranted.
have unanticipated consequences.
While reference is made to human error as the
A design philosophy should encompass two lev-
prime cause for most crashes, it is noteworthy
els. In the first instance, the focus should be on 2-3
Chapter 2: Design philosophy and techniques
Geometric Design Guide
for practical use. This is due to the narrow focus
ple, drivers sometimes steer into a curve only
Geometric Planning, which has seldom, if ever,
malised expressions of particular objectives and
been discussed in Geometric Design Manuals.
include:
Geometric Planning explicitly addresses the matters discussed in Section 2.1. In a sense, it is these issues that dictate how user-friendly the ultimate design will be to both the road user and the community. Detailed Design is about operational safety,
• • • • • •
Flexibility in highway design;
2.4.1
Flexibility In highway design
Interactive highway design; Design domain concept; Safety audits; Economic analysis; and Value engineering.
which is the second level of geometric design. This is the level on which Manuals typically focus and the effectiveness and the safety of road elements enjoy equal attention. It is pro-
A review of the standards and warrants in this
posed that, in the new philosophy, safety should
manual will quickly reveal that it allows some
be the prime consideration. Sacrificing safety in
degree of design flexibility. The degree to which
the interests of efficiency and economy is not an
this flexibility is employed in the design process
acceptable practice.
is in fact, nothing more than the application of the art and science of engineering.
Geometric Design Guide
A more holistic philosophy should thus be founded on the concept of reducing the proba-
In an attempt to formalise the process and to
bility of failure to the lowest possible level and,
guide the designer towards appropriate choices,
furthermore, should seek to minimise the con-
the United States Department of Transportation
sequences of those failures that do occur. To
published a report in 1997 entitled "Flexibility in
achieve this goal, designs must begin with a
Highway Design". It consists of three main sec-
clear understanding of purpose and functionali-
tions: an introduction to the highway design
ty. From this foundation comes the selection of
process, general guidelines referring to the
appropriate design elements followed by their
major elements of highway design, and exam-
integration into the landform and its current and
ples of six design projects presented as case
future use. The hallmark of professionalism in
studies. The concepts described are now more
road design is the ability to foresee and optimize
commonly referred to as "context sensitive
the conflicting objectives that are inherent in any
design".
project. The most important concept to keep in mind
2.4
throughout the highway design process is that
DESIGN TECHNIQUES
every project is unique. The setting and charTo arrive at an acceptable design there is no
acter of an area, the values of the surrounding
substitute for experience and study. There is,
community, the needs of the highway users and
however, a range of useful tools and techniques
the associated physical challenges and opportu-
at the designer's disposal.
nities are unique factors that highway designers
These are for2-4
Chapter 2: Design philosophy and techniques
For each
later in the process. Public input can also help
potential project, designers are faced with the
to assess the characteristics of the area and to
task of balancing the need for improvement of
determine what physical features are most val-
the highway with the need to safely integrate the
ued by the community and, thus, have the great-
design into the surrounding natural and human
est potential for impact. Awareness of these val-
environments.
ued characteristics at an early state will help
must consider with each project.
designers to avoid changing them during the To accomplish this, highway designers must
project, reducing the need for mitigation and the
exercise flexibility.
likelihood of controversy.
There are a number of
options available to aid in achieving a balanced road design and to resolve design issues.
After working with the community to define the
Among these are the following:
basic project need and to assess the physical character of the area, public involvement is nec-
Use the flexibility available within the
essary to obtain input on design alternatives.
design standards;
• •
Working with the affected community to solve
Recognise that design exceptions may be required where environmental impact
design challenges as they arise is far more
consequences are great;
effective than bringing the public into the
Be prepared to re-evaluate decisions
process only after major design decisions have
made earlier in the project planning and
been made. The public needs to be involved at
environmental impact assessment
all points in the project where there are the
phase;
•
greatest opportunities for changes to be made
Lower the design speed where appropri-
in the design.
ate;
• •
Maintain the road's existing horizontal and vertical geometry and cross section
One of the major and continuing sources of con-
where possible;
flict between highway agencies and the commu-
Consider developing alternative design
nities they serve relates to the topic of function-
standards, especially for scenic or his-
al classification. In particular, the need to iden-
toric roads; and
•
tify the "correct" functional classification for a
Recognise the safety and operational
particular section of highway, and a regular re-
impacts of various design features and
examination of functional classification as
modifications.
changes in adjacent land use take place, would In addition to exercising flexibility, a successful
resolve many potential design conflicts before
highway design process should include the pub-
they take place.
lic. To be effective, the public view should be canvassed at the outset, even before the need
There are a number of other fundamental
for the project has been defined. If the primary
design controls that must be balanced against
purpose and need for the improvement has not
one another. These include:
been agreed on, it would be extremely difficult to
• •
reach consensus on alternative design solutions
The design speed of the facility; The design-year peak-hour level of
2-5 Chapter 2: Design philosophy and techniques
Geometric Design Guide
•
The Design Consistency Module
service on the facility;
• • •
The physical characteristics of the design vehicle;
This module evaluates the operating-speed
The performance characteristics of the
consistency of two-lane highways. The evalua-
design vehicle;
tion is performed using a speed-profile model
The capabilities of the typical driver on
that estimates 85th percentile speeds on each
the facility (i.e., local residents using
element along an alignment. The module gen-
low-speed neighbourhood streets
erates two consistency-rating measures:
versus long distance travellers on inter-
•
urban freeways); and
•
The existing and future traffic demands
percentile speeds and the design speed
likely to be placed on the facility.
of the roadway, and
• 2.4.2
The difference between estimated 85th
The reduction in 85th percentile speed
Interactive Highway System
between each approach tangent-curve
Design Model (IHSDM)
pair.
A suite of computer modules within the CAD
The module will consist of a speed-profile model
environment is currently under development by
and consistency rating measures that have
the U.S. Federal Highway Administration. When
been validated and are applicable to most two-
completed, designers will have a powerful tool
lane, free flowing
with which to assess the safety effects of their
States.
highways in the United
geometric design decisions.
The Driver/Vehicle Module As currently planned, IHSDM will be applicable
This will consist of a Driver Performance Model
to two lane highways. It is composed of six
linked to a Vehicle Dynamics Model. Driver per-
modules.
formance is influenced by cues from the road-
Geometric Design Guide
way/vehicle system (i.e., drivers modify their
The Crash Prediction Module
behaviour based on feedback from the vehicle
This module will estimate crash potential for a
and the roadway). Vehicle performance is, in
design alternative, including all roadway seg-
turn, affected by driver behaviour/performance.
ments and intersections.
Estimates will be
The Driver Performance Model will estimate a
quantitative and will include the number of
driver's speed and path along a two-lane high-
crashes for a given roadway segment or inter-
way in the absence of other traffic. These esti-
section as well as the percentages of fatal and
mates will be input to the Vehicle Dynamics
severe crashes.
Model, which will estimate measurements including lateral acceleration, friction demand,
The module will allow the user to compare the
and rolling moment.
number of crashes over a given time period for different design alternatives or to perform sensi-
The Driver/Vehicle Module will produce the fol-
tivity analyses on a single alternative.
lowing measures of effectiveness and, where 2-6
Chapter 2: Design philosophy and techniques
appropriate, threshold or reference values for
ance with policy will be identified, and an expla-
comparison purposes:
nation of the policy violated will be provided. In
•
response to this information, the user may cor-
Lateral acceleration in comparison with
rect any deficiencies, analyse the design further
discomfort, skid, and rollover threshold
using other IHSDM modules, and/or prepare a
values;
• • •
Friction demand in comparison with the
request for design exception. A summary of the
skid threshold;
policy review will be provided, including a listing
Rolling moment in comparison with the
of all design elements that do not comply with
rollover threshold;
policy. The categories of design elements to be
Estimated vehicle speed in comparison
verified include: horizontal alignment, vertical
with threshold speeds for discomfort,
alignment, cross section, intersections, sight
skidding, and rollover; and
•
distance, and access control/management.
Vehicle path (lateral placement) relative to the lane lines.
The Policy Review Module will notify designers
The Intersection Diagnostic Review
of any design elements that deviate from mini-
Module
ma/maxima set by the AASHTO Green Book,
This module will be used to evaluate the geo-
the "Roadside Design Guide," and the "Guide
metric design of at-grade intersections on two-
for the Development of Bicycle Facilities." The
lane highways and to identify possible safety
Module will also have the capability of reviewing
treatments. The Intersection Diagnostic Review
designs relative to alternative, user-specified
Module will incorporate qualitative guidance
design policies, such as State Department of
from the American Association of State Highway
Transportation design guidelines.
on the geometric design of highways and
The Traffic Analysis Module
streets" (generally referred to as the Green
This module will link highway geometry data
Book) and other design policies, design guide-
with a traffic simulation model to provide infor-
lines based on past research and design guide-
mation on speed, travel time, delay, passing
lines based on expert opinion.
The primary
rates, percentage following in platoons, traffic
focus is to identify combinations of geometric
conflicts and other surrogate safety measure-
design elements that suggest potential design
ments. TWOPAS, a traffic simulation model for
deficiencies, even when each element consid-
two-lane highways, will form the basis for this
ered individually could be regarded as being
module.
within good design practice.
2.4.3
The "design domain" concept
The Policy Review Module
The design domain concept recognizes that
This module is intended for use in all stages of
there is a range of values, which could be adopt-
highway planning and design, including design
ed for a particular design parameter within
review, for both new and reconstruction proj-
absolute upper and lower limits. Values adopted
ects. Design elements that are not in compli-
for a particular design parameter within the 2-7
Chapter 2: Design philosophy and techniques
Geometric Design Guide
and Transportation Officials document "A Policy
design domain would achieve an acceptable
for assessment of safety and operational.
though varying, level of performance in average
These improvements, as well as initiatives in the
conditions in terms of safety, operation, and
assessing and auditing of scheme layouts, have
economic and environmental consequences.
considerably improved the design process.
Figure 2.1 illustrates the concept.
It is now practical to estimate the changes in the level of service, cost and safety when the design
While values within the lower region of the
is changed within the design domain. Where
design domain for a particular parameter are
data are not available, guidance is available to Practical upper limit
Cost or benefit
Absolute upper limit
Absolute lower limit
Practical lower limit
Design domain
Range of values
Guideline
Geometric Design Guide
Figure 2.1 : The design domain concept generally less safe and less operationally effi-
the designer in the literature on the sensitivity of
cient, they are normally less costly than those in
safety to changes in the parameter under con-
the upper region. In the upper region of the
sideration within the design domain.
domain, resulting designs are generally "safer"
evaluations are however limited in comparison
and more efficient in operation, but may cost
to the evaluation of operational adequacy or
more to construct. In fact, the design domain
construction costs.
These
sets the limit within which parameters should be selected for consideration within the value engi-
The benefits of the design domain concept are:
neering concept.
•
It is directly related to the true nature of the road design function and process,
During recent years there have been many
since it places emphasis on developing
advances in road design and in the procedures
appropriate and cost-effective designs,
2-8 Chapter 2: Design philosophy and techniques
•
rather than on those which simply meet
Application of the concept of a design domain in
"standards";
practice presents practical challenges. In some
It directly reflects the continuous nature
cases, the concept of a design domain with
of the relationship between service, cost
upper and lower bounds, and a continuous
and safety and changes in the values of design dimensions.
range of values in between, may not be practi-
It thus reinforces
cal or desirable. Lane widths provide a good
the need to consider the impacts of
•
trade-offs throughout the domain and
example of such a case. In these instances, it
not just when a "standards" threshold
may only be necessary to consider a series of
has been crossed, and;
discrete values for the dimension in question. In
It provides an implicit link to the concept
other instances, there may be no upper limit to
of "Factor of Safety" - a concept that
a design domain other than what is practical or
isused in other civil engineering design
economic. In these cases, the upper boundary
processes where risk and safety are
of the design domain generally reflects typical
important.
upper level values found in practice, or the gen-
The illustration in Figure 2.2 is an example of
eral threshold of cost-effective design.
how different costs and benefits may vary within the design domain for a specific parameter - in
The designer must respect controls and con-
this case shoulder width. The application of this
straints to a greater or lesser degree, depending
concept to all design parameters will lead to an
on their nature and significance.
optimal project design.
designer is faced with the dilemma of being
2-9 Chapter 2: Design philosophy and techniques
Geometric Design Guide
Figure 2.2: Example of design domain application - Shoulder width.
Often, the
unable to choose design dimensions or criteria
considered in the design process. If a design
that will satisfy all controls and constraints, and
involves compromise, it may be more appropri-
a compromise must be reached.
ate to vary several elements by a small amount
These are
engineering decisions that call for experience,
than to alter one element excessively.
insight and a good appreciation of community
important that a design be balanced.
It is
values.
2.4.4
Some design criteria such as vertical clearance at structures are inviolate. Others are less rigid
As the term implies, road safety auditing is a
and some are little more than suggestions.
structured process that brings specialised and
Some of those chosen are for safety reasons,
explicit safety knowledge to bear on a highway
some for service or capacity, while others are based on comfort or aesthetic values.
Road safety audits
project so that it can be quantitatively consid-
The
ered. It is a formal examination of a future or
choice of design criteria is very important in the
existing project in which an independent, quali-
design process and it is essential for the design-
fied examination team reports on the accident
er to have a good understanding of their origin
potential and safety performance of the project.
and background. A design carefully prepared by a designer who has a good understanding, not
The benefits of road safety audits include:
only of the criteria, but also of their background
•
and foundation, and who has judiciously applied
A reduction in the likelihood of accidents on the road network;
•
the community values, will probably create the
A reduction in the severity of accidents on the road network;
desired level of service, safety and economy.
•
An increased awareness of safe design practices among traffic engineers and
For many elements, a range of dimensions is
road designers;
given and the designer has the responsibility of
•
choosing the appropriate value for a particular
A reduction in expenditure on remedial measures; and
application. A designer with economy upper-
•
most in mind may be tempted to apply the mini-
A reduction in the life-cycle cost of a road.
Geometric Design Guide
mum value, reasoning that so long as the value is within an accepted range, the design is "sat-
Australian and New Zealand experience has
isfactory". This may or may not be the case.
shown that road safety audits do not add more than four per cent to the cost of a road project.
The designer might find it appropriate to reduce
It is, however, necessary to equate this cost to
values of design criteria, which is not necessar-
the potential benefits of the road safety audit,
ily a poor decision. However, the consequences
e.g.:
need to be thoroughly understood, particularly
•
as they impacts on safety and also on the costs
A saving in time and cost by changing project details at the planning and
and benefits. Ameliorating measures, such as
design stage rather than by changing or
the use of traffic control devices, may need to be
removing a road element once installed; 2-10
Chapter 2: Design philosophy and techniques
• •
A reduction in the likelihood of accidents
By spending more money on construction, other
and therefore in accident costs; and
costs may be reduced (e.g. travel time or crash-
A reduction in the cost of litigation.
es). However, additional expenditure must create increases in benefits or reductions in other costs.
The objectives of a road safety audit are;
Economic analyses can evaluate the
trade-offs between costs and benefits.
•
To identify and report on the accident potential and safety of a road project;
•
The analysis when applied to a road can be
To ensure that road elements with an
highly complex, depending on the scope of the
accident potential are removed; or
•
project.
That the risk of crashes is reduced.
Many formal or informal evaluations
may have been carried out and decisions made, before the geometric designer gets involved. In
Road safety can be audited at any of the follow-
extreme cases, the designer may be so con-
ing six stages, however, the sooner the better: Stage 1 Road safety audit:
strained by decisions already made, that there
Preliminary
is little or no opportunity to judge many of the
design stage Stage 2 Road safety audit:
potential costs and benefits. It is, however, the
Draft design
designer's task to incorporate those judgements
stage Stage 3 Road safety audit:
into planning and design wherever that freedom
Detailed
exists. The designer should also identify situa-
design stage Stage 4 Road safety audit:
tions where policy decisions may unreasonably
Preconstruct-
constrain a satisfactory design. When present-
ion stage Stage 5 Road safety audit:
ed effectively, arguments made by designers
Pre-opening
may affect the timing and scope of projects and
stage
2.4.5
also influence changes to existing policy.
Existing facility The geometric designer determines the horizon-
Economic analysis
tal and vertical alignment and cross section at
Economic analyses form an intrinsic part of any
every point on the road.
In addition, special
civil engineering project where the "value for
planning is required at every location where
money" concept is important.
roadways intersect, to accommodate diverging, converging and conflicting traffic movements. In
Roads are essential for mobility of people and
selecting design dimensions and layouts, the
goods. The benefits of mobility are attained at a
designer can directly affect some of the benefits,
cost. Roads cost money to build and maintain;
costs and impacts of the road, as well as allow
they consume space and affect the environ-
for future expansion.
ment; road travel consumes time, creates noise and pollution, and brings about crashes, etc. All
The hallmark of professionalism in road design
these are the costs of mobility.
is the ability to optimise and foresee the reper-
2-11 Chapter 2: Design philosophy and techniques
Geometric Design Guide
Stage 6 Road safety audit:
cussions of design decisions on the benefits,
ment technique based on an intensive, system-
costs and impacts of the road.
atic and, especially, creative study of the project to seek the best functional balance between its cost, reliability and performance.
For most, if not all, road projects, the designer will have some scope for value judgements, although this will vary from place to place and
In a road design context, this means that a value
from project to project, governed by policy deci-
engineering exercise should be more than
sions already made. Factors that the designer
merely a way of minimizing construction costs,
may be able to influence include:
but that equal and explicit attention should also be given to the important aspects of safety,
• • • • • • •
Mobility;
operational performance and quality.
Environmental impacts;
value engineering can, and sometimes does,
Safety;
In fact,
result in increased construction costs to reduce
Capital costs;
the life-cycle costs.
Aesthetics; Maintenance costs and
More and more authorities are using the con-
Vehicle operating costs.
cept of value engineering to a more cost-effecIn influencing these factors, the designer will be
tive design. If properly applied, this approach is
guided by jurisdictional policy decisions, such as
a valuable input to the design process where
the relative importance of maintenance cost ver-
functional balances are evaluated explicitly and
sus capital cost or of fuel consumption and air
quantitatively for the full range of life cycle costs
pollution against capital cost.
and benefits and re-evaluated in response to proposed changes in design, construction
2.4.6
sequences and practices. Only in this way can
Value engineering
the true benefits of the value engineering process be realised.
Road design is generally carried out in an envi-
Geometric Design Guide
ronment where a limited budget needs to be stretched as far as possible. For this reason
Engineers acting independently of the design
designers are placed under considerable pres-
team often do value engineering. However, the
sure to minimize costs.
concept is applicable at all times to all projects and, to do a complete job, this design team
While economy and fiscal efficiency is a key
should embody value engineering in its design
goal of all designs and should continue to be so,
process. If this is done, the independent value
it is essential that changes in design should be
engineering process will become less neces-
analysed explicitly, evaluating safety in the
sary.
same manner as other criteria, such as construction and maintenance costs, and environmental and operational impacts. One method is "value engineering" which is a proven manage2-12 Chapter 2: Design philosophy and techniques
TABLE OF CONTENTS 3
DESIGN CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1 3.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.2
HUMAN FACTORS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.3
3.4
3.5
3.6
3.7
3.8
3.2.1
Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
3.2.2
Other road users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4
SPEED. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5 3.3.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5
3.3.2
Speed classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6
3.3.3
Design speed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6
3.3.4
Operating speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
3.3.5
Application of design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9
DESIGN VEHICLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10 3.4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10
3.4.2
Vehicle classifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11
3.4.3
Vehicle characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11
3.4.4
Selecting a design vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13
SIGHT DISTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14 3.5.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.5.2
Deceleration rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.5.3
Object height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-14
3.5.4
Stopping sight distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15
3.5.5
Effect of gradient on stopping sight distance . . . . . . . . . . . . . . . . . . . . . . . . . 3-16
3.5.6
Variation of stopping sight distance for trucks . . . . . . . . . . . . . . . . . . . . . . . . 3-16
3.5.7
Passing sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18
3.5.8
Decision sight distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-19
3.5.9
Headlight sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20
3.5.10
Barrier sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-21
3.5.11
Obstructions to sight distance on horizontal curves . . . . . . . . . . . . . . . . . . . . 3-21
ENVIRONMENTAL FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22 3.6.1
Land use and landscape integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23
3.6.2
Aesthetics of design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-23
3.6.3
Noise abatement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24
3.6.4
Air pollution by vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-24
3.6.5
Weather and geomorphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-25
TRAFFIC CHARACTERISTICS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26 3.7.1
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26
3.7.2
Traffic volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-26
3.7.3
Directional distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27
3.7.4
Traffic composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-27
3.7.5
Traffic growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28
3.7.6
Capacity and design volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28
ROAD CLASSIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-30 3.8.1
Classification criteria for South African roads . . . . . . . . . . . . . . . . . . . . . . . . 3-30
3.8.2
Functional classification concept. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31
3.8.3
Administrative classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32
3.8.4
Design type classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-32
LIST OF TABLES Table 3.1: Typical design speeds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8 Table 3.2: Dimensions of design vehicles (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 Table 3.3: Minimum turning radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-13 Table 3.4: Object height design domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-15 Table 3.5: Recommended stopping sight distances for design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-16 Table 3.6: Passing sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-18 Table 3.7: Decision sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-20 Table 3.8: Equivalent passenger car units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-28 Table 3.9: Road functional classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-31
LIST OF FIGURES Figure 3.1: Five Axle Vehicles and Multi Vehicle Combinations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-12 Figure 3.2: Stopping distance corrected for gradient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-17 Figure 3.3: Horizontal restrictions to stopping sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-22 Figure 3.4: Relationship of functional road classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-33
Chapter 3 DESIGN CONTROLS 3.1
INTRODUCTION
and behavioural characteristics is thus a vital input into the design task.
The design of a road is that of a three-dimensional structure which should ideally be safe, efficient, functional and economical for traffic
Road users do not all behave in the same way
operations, and which should also be aestheti-
and designs should cater for substantial differ-
cally pleasing in its finished form. However, the
ences in the range of human characteristics and
designer uses dimensions and related criteria
a wide range of responses. However, if the per-
within a design context that recognizes a series
ceptual clues are clear and consistent, the task
of design controls constraining what can be
of adaptation is made easier and the response
achieved. These limitations are imposed by the
of drivers will be more appropriate and uniform.
characteristics of vehicle and driver perform-
For roadway design this translates into some
ance as well as by environmental factors. The
useful principles, viz:
•
designer should, therefore, relate the physical
A roadway should confirm what drivers expect based on previous experience;
elements of the road to the requirements of the
and
driver and vehicle so that consistency in the dri-
•
ver's expectations is achieved and, at the same
Drivers should be presented with clear clues about what is expected of them
time, ensures that environmental and other constraints are accommodated.
Driver Workload and Expectations The driver workload comprises
•
Good road design is the art of combining and
Navigation:
trip planning and route following;
balancing the various controls in a perceptive
•
fashion and is not merely an exercise in three-
Guidance:
following the road and maintaining a safe path in
dimensional geometry. In this chapter, the con-
response to traffic condi-
straints and controls on the design process are
tions; and Control:
steering and speed control
3.2
HUMAN FACTORS These tasks require the driver to receive and
3.2.1
Drivers
process inputs, consider the outcome of alter-
An appreciation of driver performance as part of
native actions, decide on the most appropriate,
the road traffic system is essential for effective
execute the action and observe its effects
road design and operation. When a design is
through the reception and processing of new
incompatible with human capabilities (both of
information. There are numerous problems
the driver and any other road user) the opportu-
inherent in this sequence of tasks, arising from
nities for errors and accidents increase.
both the capabilities of the human driver, and
Knowledge of human performance, capabilities
the interfaces between the human and other 3-1
Chapter 3: Design Controls
Geometric Design Guide
•
discussed.
components of the road traffic system (the road
Continuation expectancy. This is the expecta-
and the vehicle). These include inadequate or
tion that the events of the immediate past will
insufficient input available for the task at hand
continue. It results, for example, in small head-
(e.g. during night time driving, as a result of poor
ways, as drivers expect that the leading vehicle
sight distance, or because of complex intersec-
will not suddenly change speed. One particu-
tion layouts). When they become overloaded,
larly
drivers shed part of the input to deal with that
expectance is that of subliminal delineation, e.g.
judged to be more important. Most importantly,
a line of poles or trees or lights at night which
drivers are imperfect decision-makers and may
suggests to the driver that the road continues
make errors, including in the selection of what
straight ahead when, in fact, it veers left or right.
input to shed.
These indications are subtle, but should always
perverse
aspect
of
continuation
be looked out for during design.
The designer must provide all the information the driver needs to make a correct decision
Event expectancy. This is the expectation that
timeously, simultaneously ensuring that the
events that have not happened will not happen.
information is provided at a tempo that does not
It results, for example, in disregard for "at grade"
exceed the driver's ability to absorb it. In the
railway crossings and perhaps for minor inter-
words of the American Association of State
sections as well, because drivers expect that no
Highway and Transportation Officials: (AASH-
hazard will present itself where none has been
TO)
seen before.
A response to this situation is
‘A common characteristic of many high-acci-
more positive control, such as an active warning
dent locations is that they place large or
device at railway crossings that requires that the
unusual demands on the information-pro-
driver respond to the device and not to the pres-
cessing capabilities of drivers.
ence of a hazard.
Inefficient
operation and accidents usually occur where the chance for information-handling errors is
Temporal expectancy. This is the expectation
high. At locations where the design is defi-
that, where events are cyclic (e.g. traffic sig-
Geometric Design Guide
cient, the possibility of error and inappropriate
nals), the longer a given state prevails, the
driver performance increases.'
greater is the likelihood that change will occur.
Prior experience develops into a set of
This, of course, is a perfectly reasonable expec-
expectancies that allows for anticipation and for-
tation, but it can result in inconsistent respons-
ward planning, and these enable the driver to
es. For example, some drivers may accelerate
respond to common situations in predictable
towards a green signal, because it is increas-
and successful ways. If these expectancies are
ingly likely that it will change, whereas others
violated, problems are likely to occur, either as a
may decelerate. A response to this is to ensure,
result of a wrong decision or of an inordinately
to the extent possible, that there is consistency
long reaction time. There are three types of
throughout the road traffic system to encourage
driver expectancy:
predictable and consistent driver behaviour.
3-2 Chapter 3: Design Controls
The combined effect of these expectancies is
brakes. Recognition that complex decisions are
that:
time-consuming leads to the axiom in highway
•
drivers tend to anticipate upcoming situ-
design that drivers should be confronted with
ations and events that are common to
only one decision at a time, with that decision
the road they are travelling;
•
being binary, e.g. "Yes" or "No" rather than com-
the more predictable the roadway fea-
plex, e.g. multiple choice. Anything up to 10
ture, the less likely will be the chance for
seconds of reaction time may be appropriate in
errors;
•
complex situations.
drivers experience problems when they are surprised;
•
Design Response
in the absence of evidence to the contrary, drivers assume that they will only have to react to standard situations;
•
Designers should strive to satisfy the following
the roadway and its environment upstream
criteria:
create an expectation of
•
downstream conditions; drivers experi-
and unexpected, unusual or inconsistent
ence problems in transition areas and
design or operational situations avoided
locations with inconsistent design or
or minimized.
operation, and
•
Driver's expectations are recognized,
•
expectancies are associated with all lev-
Predictable behaviour is encouraged through familiarity and habit (e.g. there
els of driving performance and all
should be a limited range of intersection
aspects of the driving situation and
and interchange design formats, each
include expectancies relative to speed,
appropriate to a given situation, and
path, direction, the roadway, the envi-
similar designs should be used in similar
ronment, geometric design, traffic oper-
situations).
ations and traffic control devices.
•
Consistency of design and driver behaviour is maintained from element to ele-
Driver Reaction
ment (e.g. avoid significant changes in
It takes time to process information. After a per-
design and operating speeds along a
son's eyes detect and recognize a given situa-
roadway). The information that is provided should decrease the driver's uncertainty, not
reaction occurs. Reaction time is appreciable
increase it (e.g. avoid presenting sever-
and differs between persons. It also varies for
al alternatives to the driver at the same
the same individual, being increased by fatigue,
time).
drinking, or other causes. The AASHTO brake
•
reaction time for stopping has been set at 2,5 s
Clear sight lines and adequate sight dis tances are provided to allow time for
to recognize all these factors. This value has
decision-making and, wherever possi-
been adopted in South Africa.
ble, margins are allowed for error and recovery.
Often drivers face situations much more complex than those requiring a simple response
With the major response to drivers' require-
such as steering adjustments or applying the
ments being related to consistency of design, it 3-3
Chapter 3: Design Controls
Geometric Design Guide
•
tion, a period of time elapses before muscular
is worthwhile considering what constitutes con-
between -0,04 and 0,01 results in a fair design.
sistency. Consistency has three elements that
A value of less than -0,04 is not acceptable. A
are the criteria offered for the evaluation of a
negative value for the difference between side
road design:
friction assumed for design and the side friction Design consistency - which cor-
demanded means that drivers are demanding
responds to relating the design speed to actual
more side friction than is assumed to be avail-
driving behaviour which is expressed by the
able - a potentially dangerous situation.
Criterion I
85th percentile speed of passenger cars under
3.2.2
free-flow conditions; Criterion II
Other road users
Operating speed consistency
which seeks uniformity of 85th percentile
Pedestrians
speeds through successive elements of the
The interaction of pedestrians and vehicles
road and
should be carefully considered in road design,
Criterion III
Consistency in driving dynam-
principally because 50 per cent of all road fatal-
ics - which relates side friction assumed with
ities are pedestrians.
respect to the design speed to that demanded at Pedestrian actions are less predictable than
the 85th percentile speed.
those of motorists. Pedestrians tend to select In the case of Criterion 1, if the difference
paths that are the shortest distance between
between design speed and 85th percentile
two points. They also have a basic resistance to
speed on an element such as a horizontal curve
changes in gradient or elevation when crossing
is less than 10 km/h, the design can be consid-
roadways and tend to avoid using underpasses
ered good. A difference of between 10 km/h and
or overpasses that are not convenient.
20 km/h results in a tolerable design and differences greater than 20 km/h are not acceptable.
Walking speeds vary from a 15th percentile
Geometric Design Guide
speed of 1,2 m/s to an 85th percentile of 1,8 In the case of Criterion 2, the focus is on differ-
m/s, with an average of 1,4 m/s. The 15th per-
ences in operating speed in moving from one
centile speed is recommended for design pur-
element, e.g. a tangent, to another, e.g. the fol-
poses.
lowing curve. A difference in operating speed between them of less than 10 km/h is consid-
Pedestrians' age is an important factor that may
ered to be good design and a difference of
explain behaviour that leads to collisions. It is
between
tolerable.
recommended that older pedestrians be accom-
Differences greater than 20 km/h result in what
modated by using simple designs that minimize
is considered to be poor design.
crossing widths and assume lower walking
10
and
20
km/h
is
speeds.
Where complex elements such as
For the third Criterion the side friction assumed
channelisation and separate turning lanes are
for the design should exceed the side friction
featured, the designer should assess alterna-
demanded by 0,01 or more.
tives that will assist older pedestrians.
A difference 3-4
Chapter 3: Design Controls
•
Pedestrian safety is enhanced by the provision of:
•
er behaviour;
• •
median refuge islands of sufficient width at wide intersections, and
•
Driver capability, driver culture and driv-
lighting at locations that demand multi
Vehicle operating capabilities; The physical characteristics of the road and its surroundings;
• • •
ple information gathering and process ing.
Weather; Presence of other vehicles, and Speed limitations (posted speed limits).
Cyclists Bicycle use is increasing and should be consid-
Speeds vary according to the impression of con-
ered in the road design process. Improvements
straint imparted to the driver as a result of these
such as:
• •
factors.
paved shoulders; wider outside traffic lanes (4,2 m mini-
The objective of the designer is to satisfy the
mum) if no shoulders exist;
• • •
bicycle-safe drainage grates;
road users' demands for service in a safe and
adjusting manhole covers to the grade,
economical way. This means that the facility
and
should accommodate nearly all reasonable
maintaining a smooth, clean riding sur-
demands (speed) with appropriate adequacy
face
(safety and capacity) but should not fail com-
can considerably enhance the safety of a street
pletely under severe load, i.e. the extremely
or highway and provide for bicycle traffic:
high speeds maintained by a small percentage
At certain locations it may be appropriate to sup-
of drivers.
plement the existing road system by providing
designed to operate at a speed that satisfies
specifically designated cycle paths. The design
most, but not necessarily all, drivers.
Roads should, therefore, be
elements of cycle paths are discussed in Various studies have shown that the 85th per-
Chapter 4.
centile speed generally exceeds the posted
SPEED
3.3.1
General
speed limit by a margin of at least 10 km/hr when weather and traffic conditions are favourable. For this reason, design speed is typically equated to the 85th percentile speed.
Drivers, on the whole, are concerned with minimising their travel times, and speed is one of the most important factors governing the selec-
The relationship between road design and
tion of alternate routes to gain time savings.
speed is interactive. While the designer shapes
The attractiveness of a specific road or route is
the elements of the road by the anticipated
generally judged by its convenience in travel
speed at which they will be used, taking into
time, which is directly related to travel speed.
account the inherent economic trade-offs between construction and environmental costs
Various factors influence the speed of vehicles
of alternative alignments (vertical and horizon-
on a particular road. These include:
tal) to match desired travel speed, the speed at 3-5 Chapter 3: Design Controls
Geometric Design Guide
3.3
which they will be used depends to a large
rather than for geometric design consid-
extent on the chosen design features.
erations and is aimed at encouraging drivers to travel at appropriate speeds
3.3.2
for all prevailing conditions.
Speed classification
The term "speed" is often used very loosely
3.3.3
Design speed
when describing the rate of movement of road traffic. Road design recognizes various defini-
The most important factor in geometric design is
tions or classifications of speed, all of which are
the design speed. This was previously defined
interrelated. The sub-divisions are:
•
as the highest continuous speed at which indi-
Desired Speed - the speed at which a
vidual vehicles can travel with safety on the road
driver wishes to travel, determined by a
when weather conditions are favourable, traffic
combination of motivation and comfort.
•
Design Speed - the speed selected as a
volumes are low and the design features of the
safe basis to establish appropriate geo-
road are the governing condition for safety. The
metric design elements for a particular
current definition is simply states that the design
section of road and which should be a
speed is the speed selected as the basis for
logical one with respect to topography,
establishing appropriate geometric elements for
anticipated operating speed, the adja-
a section of road. These elements include hori-
cent land use and the functional classifi-
zontal and vertical alignment, superelevation
cation of the road.
•
and sight distance.
Operating Speed - observed speeds
Other elements such as
during free flow conditions. For an indi-
lane width, shoulder width and clearance from
vidual driver, operating speed is gener-
obstacles are indirectly related to design speed.
ally lower than desired speed since
•
operating conditions are not usually
The chosen design speed should be a logical
ideal.
one consistent with the road function as per-
Running Speed - the average speed
ceived by the driver and also one that takes into
maintained over a given route while a
account the type of road, the anticipated operat-
vehicle is in motion. The running time is
ing speed, and the terrain that the road travers-
Geometric Design Guide
the length of the road section divided by the time required for the vehicle to trav-
es. Where a difficult condition is obvious, driv-
el through the section. Thus, in deter-
ers are more apt to accept a lower speed than
mining the running speed, the times en
where there is no apparent reason for it.
route when the vehicle is at rest are not taken into account in the calculations.
Other relevant factors include traffic characteris-
Running speeds are generally used in
tics, land costs, speed capabilities of vehicles,
road planning and capacity and service
aesthetics, economics and social or political
level analyses. The difference between
impacts. A highway of higher functional classifi-
running speed and design speed is
•
strongly affected by traffic volumes.
cation may justify a higher design speed than a
Posted Speed - is a speed limitation set
less important facility in similar topography, par-
for reasons of safe traffic operations
ticularly where the savings in vehicle operation 3-6
Chapter 3: Design Controls
and other operating costs are sufficient to offset
always possible that the signpost may be
the increased costs of right-of-way and con-
obscured, illegible, removed or even simply not
struction. A low design speed, should not be
perceived by the driver. Isolated design speed
assumed where the topography is such that
changes are, therefore, to be avoided.
drivers are likely to travel at high speeds. The need for a multilane cross-section suggests When carefully selected, these factors should
that traffic volumes are high. A design speed of
result in a design speed which is acceptable to
at least 120 km/h should be used if the topogra-
all but a very few drivers.
Above minimum
phy permits. Major roads, even if two-lane two-
design values should be used where feasible
way roads, should also be designed to this
though consistency is essential.
speed if possible. Rolling terrain may, however, necessitate a reduction to 100 km/h in the
When a substantial length of road is being
design speed and, in the case of mountainous
designed, it is desirable to adopt a constant
terrain, it may even be necessary to reduce the
design speed to maintain consistency. Changes
design speed to 80 km/h.
in terrain and other physical controls may, however, dictate a change in design speed on cer-
Secondary and tertiary roads may have lower
tain sections. Each section, however, should be
design speeds than those advocated for the pri-
relatively long, compatible with the general ter-
mary road network. However, where traffic is
rain or development through which the road
likely to move at relatively high speeds on these
passes.
roads, higher design speeds should be select-
The justification for introducing a
ed.
reduced design speed should be obvious to the driver. A case in point is where a road leaves relatively level terrain and starts traversing hilly
There is still debate as to whether speeds
or mountainous terrain. Moreover, the introduc-
greater than 120 km/h should be used for
tion of a lower or higher design speed should
design purposes on freeways.
not be effected abruptly but over sufficient dis-
speeds not only safeguard against early obso-
tance to encourage drivers to change speed
lescence of the highway, but also provide an
gradually.
increased margin of safety for those driving at high speeds. That there is some validity in this
Where design speeds exceed 90 km/h the vari-
statement is reflected by the fact that the design
ation between successive speeds should be lim-
speed of high-type roads is now at least 120
ited to 10 km/h and, below 80 km/h, this varia-
km/h as compared with 56 km/h in 1927, a
tion should be limited to 20 km/h. Where it is
change brought about by the continuing
necessary to change the design speed, the new
increase in vehicle performance.
design speed should apply to an extended section of road. Even if properly signposted, isolat-
The choice of a design speed for a dual car-
ed design speed variations are hazardous as
riageway is much less influenced by construc-
they do not match driver expectations and it is
tion cost than that for other rural roads. In prac3-7
Chapter 3: Design Controls
Geometric Design Guide
Higher design
tice, lower design speeds are often accepted on
120 km/h for unhindered vehicles on a four-lane
single carriageway roads in order to keep con-
divided roadway. Use of a design speed of 130
struction costs within certain limits.
km/h should therefore satisfy driver demands in
There is
danger in this philosophy since, although drivers
most areas.
will obviously accept lower speeds in what are clearly difficult locations, repeated studies have
The selected design speed should be logical
shown that they do not adjust their speeds to the
and in harmony with the topography and the
importance of the facility. Instead they endeav-
functional classification of a road. Careful con-
our to operate at speeds consistent with the traf-
sideration should also be given to its relation-
fic on the facility and its physical limitations.
ship to other defined speeds. While no hard relationships have been established, choice of
Ideally, then, design speed should be chosen to
design speed can simultaneously accommodate
reflect the 85th percentile desired speed that is
and influence desired, operating, running and
likely to materialize. This is often achievable for
posted speeds.
roads for which the primary function is mobility and where severe physical constraints do not exist.
Limited studies in South Africa have
Table 3.1 provides an indication of typical
Geometric Design Guide
shown that the 85th percentile speed exceeds
design speeds for different classes of roads.
3-8 Chapter 3: Design Controls
3.3.4
Operating speed
factors, the driver's initial response is to react to the anticipated situation rather than to the actu-
Operating speed is measured under free flow
al situation. In most instances, the two are sim-
conditions. The term "spot speed" is sometimes
ilar enough not to create conflicts. If the initial
used to denote operating speed. For an individ-
response is incorrect, operation and safety may
ual driver, operating speed is generally lower
be severely affected.
than desired speed since operating conditions are not usually ideal. When reference is made
Some agencies conduct speed surveys to deter-
to the operating speed of all vehicles in the traf-
mine operating speeds at various points along a
fic stream, this is taken as being the 85th per-
section of roadway. The results can be com-
centile of all observed speeds.
pared with the design speed, and may lead to a policy change in the selection of design speeds.
Operating speed has a variety of uses. It is gen-
3.3.5
erally used as a measure of level of service at
Application of design speed
uninterrupted flows. It can also be used to monitor the effect of flow constrictions, such as inter-
Consistency of design is fundamental to good
sections or bridges. Since operating speeds at
driver performance, based on satisfying the dri-
ideal sections of road are indicative of speeds
ver's expectations. Design consistency exists
desired by motorists, they can be used to guide
when the geometric features of a continuous
the selection of design speed on improved or
section of road are consistent with the opera-
new facilities.
tional characteristics as perceived by the driver. The traditional approach to achieving design
When the design speed is less than the desired
consistency has been through the application of
speed, drivers should be warned to modify their
the design speed process. Once selected, the
speed, as studies have shown that crash rates
design speed is used to determine values for
increase as the operating speed of a particular
the geometric design elements from appropriate
vehicle deviates from the mean operating speed
design domains. However, application of this procedure alone
The typical driver can recognize or sense a log-
does not guarantee design consistency. There
ical operating speed for a given roadway based
are several limitations of the design speed con-
on knowledge of the system, posted speed lim-
cept that should be considered during design:
its, appraisal of the ruggedness of the terrain,
1.
traffic volumes and the extent, density and size
date specified design speed does not necessar-
of development. Studies have shown that char-
ily ensure a consistent alignment design.
acteristics, such as the number of access
Design speed is significant only when physical
points, nearby commercial development, road
road characteristics limit the speed of travel.
width and number of lanes, have a significant
Thus, a road can be designed with a constant
influence on vehicle speeds. Based on these
design speed, yet have considerable variation in
Selection of dimensions to accommo-
3-9 Chapter 3: Design Controls
Geometric Design Guide
of the other vehicles on the roadway.
speeds achievable and therefore to a driver
where the design speeds are less than 100
appear to have a wide variation in character. For
km/h at horizontal curves on rural two-lane high-
example, the radii of curves within a section
ways.
should be consistent, not merely greater than
5.
the minimum value.
may have quite different levels of perceived haz-
2.
For horizontal alignments, design speed
ard. Entering a horizontal curve too fast will
applies only to curves, not to the connecting tan-
almost certainly result in loss of control, so driv-
gents. Design speed has no practical meaning
ers adjust their speed accordingly.
on tangents. As a result, the operating speed on
the possibility of a curtailed sight distance con-
a tangent, especially a long one, can often sig-
cealing a hazard is considered as a remote
nificantly exceed the design speed of the road
occurrence. Unfortunately drivers do not gener-
as a whole.
ally adjust their speed to compensate for sight
3.
distance restrictions.
The design speed concept does not
In addition, different alignment elements
However,
ensure sufficient coordination among individual It
To help overcome these weaknesses in the use
controls only the minimum value of the maxi-
of design speed to design individual geometric
mum speeds for the individual features along an
elements, speed profiles are used. A speed pro-
alignment. For example, a road with an 80 km/h
file is a graphical depiction (which can be mod-
design speed may have only one curve with a
elled) showing how the 85th percentile operat-
design speed of 80 km/h and all other features
ing speed varies along a length of road. This
with design speeds of 120 km/h or greater. As
profile helps to identify undesirably large differ-
a result, operating speeds approaching the crit-
entials in the 85th percentile operating speed
ical curve are likely to exceed the 80 km/h
between successive geometric elements, e.g. a
design speed. Such an alignment would comply
curve following a tangent.
geometric features to ensure consistency.
with an 80 km/h design speed, but it would vio-
When an existing road is being improved, actu-
late a driver's expectancy and result in undesir-
al operating speeds can be measured to create
able alignment.
Geometric Design Guide
4.
a speed profile, but interpretation of the profile
Vehicle operating speed is not neces-
can be difficult, depending on the complexity of
sarily synonymous with design speed. Drivers
geometric and other features that may cause
normally adjust speed according to their desired
drivers to change speed. For a new road, pre-
speed, posted speed, traffic volumes and per-
diction of operating speeds is needed to create
ceived alignment hazards. The perception of
a speed profile model.
hazard presented by the alignment may vary along a road designed with a constant design
3.4
DESIGN VEHICLES
3.4.1
Introduction
speed. The speed adopted by a driver tends to vary accordingly and may exceed the design speeds. A report on studies in Australia and the US concluded that 85th percentile operating
The physical characteristics of vehicles and the
speeds consistently exceed design speeds
proportions of the various sizes of vehicles 3-10
Chapter 3: Design Controls
using a road are positive controls in design and
with full trailers.
Buses include single unit
define several geometric design elements,
buses, articulated buses and intercity buses. In
including intersections, on- and off-street park-
establishing the design dimensions for the vari-
ing, site access configurations and specialized
ous vehicle classes, this guide focuses on vehi-
applications such as trucking facilities. It is nec-
cles in regular operation only.
essary to identify all vehicle types using the facility, establish general class groupings and
Vehicles defined in the Road Traffic Act include:
select hypothetical representative design vehi-
•
Passenger cars and minibuses (kombis);
cles, within each design class. The dimensions
• • •
used to define design vehicles are not averages or maxima, nor are they legal limiting dimensions. They are, in fact, typically the 85th per-
Standard single unit buses; Articulated buses ("Bus Train"); Two axle trucks, with and without trailers;
centile or 15th percentile value of any given
• •
dimension. The design vehicles are therefore hypothetical vehicles, selected to represent a
Three and four axle vehicles; Three, four and five axle articulated trucks;
particular vehicle class.
• •
Five and six axle articulated trucks, and
3.4.3
Vehicle characteristics
Multi vehicle combinations.
The dimensions in the previous Geometric Design Manual were based on typical design vehicles in South Africa pertinent to 1965. The range of vehicle types and their operating char-
The dimensions adopted for the various design
acteristics have changed significantly since
vehicles are given in Table 3.2.
then. The vehicle size regulations have also undergone substantial revisions which have
The WB15 vehicle has an overall length of 17 m,
generally resulted in larger trucks on the roads
whereas the regulations allow for a semi-trailer
as well as in an increased use of recreational
to have an overall length of 18,5 m. The multi-
utility vehicles.
ple vehicle combination, being a semi-trailer
3.4.2
ration, can have a maximum overall length of 22
Vehicle classifications
m. Examples of these vehicles are illustrated in Figure 3.1.
Three general classes of vehicles have been selected for this design guide: passenger cars, The passenger car class
f these vehicles are expected to use a route with
includes compacts and subcompacts, recre-
any frequency, the designer will have to careful-
ational utility vehicles and all light vehicles and
ly plan the layout of the intersections to ensure
light delivery trucks (vans and pickups). The
that they can be accommodated. As described
truck class includes single-unit trucks, truck
below, accommodation does not necessarily
tractor-semi trailer combinations, and trucks or
require lane widths sufficient to complete a turn-
truck tractors with semi trailers in combination
ing movement within the lane.
trucks and buses.
3-11 Chapter 3: Design Controls
A degree of
Geometric Design Guide
plus trailer and typically in an Interlink configu-
* Distance between SU rear wheels and trailer front wheels
Geometric Design Guide
Figure 3.1: Five Axle Vehicles and Multi Vehicle Combinations
encroachment on adjacent lanes is permissible
In terms of regulation 355 (a) of the Road Traffic
depending on the frequency of occurrence.
Act, all vehicles must be able to describe a minimum turning radius not exceeding 13,1m.
Turning Radii
Vehicle height
In constricted situations where the templates
Regulation 354 in the Road Traffic Act limits the
are not appropriate, the capabilities of the
overall height of a double decker bus to 4,6
design vehicle become critical. Minimum turn-
metres, and that of any other vehicle to 4,3
ing radii for the outer side of the vehicle are
metres. The 15th percentile height of a passen-
given in Table 3.3. It is stressed that these radii
ger car has been established to be 1,3 m. This
are appropriate only to crawl speeds.
has been selected for design purposes as the
3-12 Chapter 3: Design Controls
passenger car is also an object that has to be
for the purpose of measuring critical turning
seen by the driver in the cases of passing and
dimensions.
intersection sight distance.
Commercially available templates and computer software define the turning envelope of vehicles
Driver Eye Height
in forward motion and also support plotting of
The passenger car is taken as the critical vehi-
the turning envelope of reversing non-articulat-
cle for driver eye height and a figure of 1,05
ed vehicles. Prediction of the reversing behav-
metres is recommended. For buses and single
iour of articulated vehicles is, however, very
unit vehicles a typical value is 1,8 metres and for
complex, mainly because this behaviour is
semi-trailer combinations the height of the eye
inherently unstable, and additional turning con-
can vary between 1,9 metres and 2,4 metres.
trols come into play.
Vehicle Turning Paths
3.4.4
The designer should allow for the swept path of The
swept path is established by the outer trace of
In general, buses and heavy vehicles should be
the front overhang and the path of the inner rear
used as the design vehicle for cross section ele-
wheel. This turn assumes that the outer front
ments, with the car as the design vehicle for the
wheel follows the circular arc defining the mini-
horizontal and vertical alignment.
mum turning radius as determined by the vehi-
major intersections along arterial roads or with-
cle steering mechanism.
in commercial areas, it is common practice to
For most
accommodate the semi trailer. The occasional It is assumed that the turning movements critical
larger vehicle may encroach on adjacent lanes
to the design of roadway facilities are done at
while turning but not on the sidewalk.
low speeds.
At these speeds, the turning
behaviour of vehicles is mainly determined by
Many authorities designate and signpost specif-
their physical characteristics. The effects of fric-
ic truck routes. The intersections of two truck
tion and dynamics can safely be ignored. It is
routes or intersections where trucks must turn to
also assumed that groups of evenly spaced
remain on a truck route should be designed to
axles mounted on a rigid bogie act in the turn as
accommodate the largest semi-trailer combina-
a single axle placed at the centre of the group
tion expected to be prevalent in the turning traf-
3-13 Chapter 3: Design Controls
Geometric Design Guide
the selected design vehicle as it turns.
Selecting a design vehicle
fic stream. Where local residential roads inter-
It is also necessary to consider the terrain or
sect truck routes or arterials, the intersections
obstructions on the inside of horizontal curves
should be specifically designed not to accom-
when evaluating adequate sight distance.
modate trucks easily, in order to discourage
3.5.2
them from travelling through the residential
Deceleration rates
area. Although research in North America has shown On major haulage routes, large tractor-trailer
that drivers can choose (or apply) a deceleration
combination trucks are prevalent and these
of greater than 5 m/sec2, there is a large degree
routes should be designed to accommodate
of variability in driver and vehicle capabilities
them. Raised channelising islands are typically
and the 90th percentile deceleration is of the
omitted in recognition of low pedestrian volumes
order of 3,4 m/sec2.
and other constraints such as right of way and
Transportation Engineers' Traffic Engineering
construction costs.
The absence of raised
Handbook states that decelerations of up to 3,0
islands also allows more manoeuvring area for
m/sec2 are reasonably comfortable for passen-
large trucks.
ger car occupants. This deceleration rate has
The Institute of
been adopted for these guidelines.
3.5
SIGHT DISTANCE 3.5.3
3.5.1
General The object height to be used in calculation of
A critical feature of safe road geometry is ade-
stopping sight distance is often a compromise
quate sight distance. As an irreducible mini-
between the length of the resultant sight dis-
mum, drivers must be able to see objects in the
tance and the cost of construction. Stopping is
road with sufficient time to allow them to
generally in response to another vehicle or large
manoeuvre around them or to stop. Other forms
hazard in the roadway. To recognize a vehicle
of sight distance are pertinent. They are:
•
as a hazard at night, a line of sight to its head-
Passing sight distance, which is
lights or taillights would be necessary. Larger
Geometric Design Guide
required for substantial portions of the
•
•
length of two-lane roads;
objects would be visible sooner and provide
Intersection sight distance, to allow a
longer stopping distances. To perceive a very
driver on the minor road to evaluate
small hazard, for example, a surface obstruc-
whether it is safe to cross or enter the
tion, a zero object height may be necessary.
opposing stream of traffic;
However, at the required stopping sight dis-
Decision sight distance where, for
tances for high speeds, small pavement varia-
example, a driver must be able to see
tions and small objects (especially at night)
and respond to road markings;
• •
Object height
would not be easily visible. Thus, most drivers
Headlight sight distance, typically applied to sag vertical curves, and
travelling at high speeds would have difficulty in
Centre line barrier sight distance.
stopping before reaching a small obstruction.
3-14 Chapter 3: Design Controls
3.5.4
A driver will usually attempt to take evasive
Stopping sight distance
action rather than to stop for small objects on The minimum sight distance on a roadway
the roadway. Although not recommended as a
should be sufficient to enable a vehicle travelling
design parameter, the time available to manoeu-
at the design speed on a wet pavement to stop
vre is a useful measure when examining varia-
before reaching a stationary object in its path.
tions of geometry in restricted situations or reconstruction projects. In this case, the appro-
Stopping sight distance is the sum of two dis-
priate object is the pavement surface.
tances:
•
The designer should adopt an object height
the distance traversed by the vehicle from the instant the driver sights an
based on the probability of a particular object
obstruction to the instant the brakes are
occurring on the roadway, as shown in Table 3.4
applied, and
below. For stopping sight distance, a conserva-
•
tive tail light height of 0,60 m is recommended. If fallen trees or rocks are a real risk, an object
the distance required to stop the vehicle from the instant the brakes are applied
text, research has established that the probability of a collision involving an object of a height of
These are referred to as brake reaction distance
0,15 m or less is infinitesimally small. For pass-
and stopping distance, respectively.
ing sight distance, an object height of 1,30 m will
These two components, using a reaction time of
allow the driver to discern the top of an oncom-
2,5 seconds and a deceleration rate of 3,0 m/s2,
ing car. A zero object height is recommended
result in the relationship
where road washouts are a serious risk. It is
s =
v (0,694 + 0,013v)
s =
stopping sight distance, m
v =
initial speed, km/h
also recommended for pavement markings in
where:
situations such as at intersections or interchanges, where these provide essential guidance. 3-15 Chapter 3: Design Controls
Geometric Design Guide
height of 0,15 m is recommended. In this con-
Stopping sight distances calculated using this
ously stated.
The brake reaction time is
equation are given in Table 3.5, rounded up for
assumed to be the same as for level conditions.
design purposes. Also shown in the table for
The stopping sight distance for design speeds
general interest are the values of stopping sight
from 30 to 130 km/h as corrected for gradient is
distance adopted in the 2000 AASHTO Policy
illustrated in Figure 3.2.
on the Geometric Design of Highways and The sight distance at any point on the highway
Streets, the "Green Book 2000".
Geometric Design Guide
is generally different in each direction, particuIn the measurement of stopping sight distance,
larly on straight roads in rolling terrain. As a
the driver's eye height is taken as being at 1,05
general rule, the sight distance available on
m and the object height is as defined in Table
downgrades is longer than on upgrades, more
3.4.
or less automatically providing the necessary corrections for grade. This is because down
3.5.5
Effect of gradient on stopping
grades are normally followed by sag vertical
sight distance
curves, with the following grade also being visible to the driver
When a highway is on a gradient, the equation
3.5.6
for stopping sight distance becomes
Variation of stopping sight dis-
tance for trucks in which G is the percentage gradient divided by
The recommended minimum stopping sight dis-
100, with upgrades being positive and down-
tance model directly reflects the operation of
grades negative and the other terms as previ-
passenger cars and trucks with antilock braking 3-16
Chapter 3: Design Controls
maximum efficiency under load;
systems. Trucks with conventional braking sys-
• •
tems require longer stopping distances from a given speed than do passenger cars. However, AASHTO suggests that the truck driver is able to
Uneven load between axles; Propensity of truck drivers not to obey posted speed limits;
•
see the vertical features of the obstruction from substantially further because of the higher driv-
Inefficient brakes of articulated trucks, and
•
er eye height. In addition, posted speed limits
Effect of curvature. where some of the
for trucks in South Africa are considerably lower
friction available at the road/tyre inter-
than for passenger vehicles. Separate stopping
face is used to hold the vehicle in a cir-
sight distances for trucks and passenger cars
cular path.
are, therefore, not generally used in highway To balance between the costs and benefits in
design.
designing for trucks, truck stopping sight disThere is, however, evidence to suggest that the
tances should be checked at potentially haz-
sight distance advantages provided by the high-
ardous locations. In general, the deceleration
er driver eye level in trucks do not always com-
rate for trucks is 1,5 m/s2. The driver's eye
pensate for their inferior braking. Some reasons
height is taken as being at 1,8 m and the object
for the longer truck braking distances include:
height is as defined in Table 3.4.
Poor braking characteristics of empty trucks. The problem relates to the sus
The designer should also consider measures
pension and tyres that are designed for
such as additional signs to improve road safety
Figure 3.2: Stopping distance corrected for gradient
3-17 Chapter 3: Design Controls
Geometric Design Guide
•
if stopping sight distance is found to be inade-
It should be pointed out that there are a variety
quate for trucks and it is not possible to improve
of models defined for the overtaking manoeu-
the geometric design. However, it is empha-
vre.
sized that provision of signage is not a substi-
required to enable an overtaking driver to com-
tute for appropriate design practices.
plete or abort a manoeuvre already com-
3.5.7
menced, with safety. In addition to this distance,
Passing sight distance
The distances usually given are those
the Austroads approach introduces a distance On a two-lane rural road, the passing manoeu-
that is needed for the driver to identify a length
vre is one of the most significant yet complex
of road as a potential overtaking zone. This
and important driving tasks. The process is rel-
"establishment" distance is considerably longer
atively difficult to quantify, primarily because of
than the overtaking manoeuvre distance.
Geometric Design Guide
the many stages involved, the relative speed of vehicles and the lengthy section of road needed
Table 3.6 shows the minimum overtaking sight
to complete the manoeuvre.
Road safety,
distances generally used for various design
capacity and service levels are all affected by
speeds. Passing manoeuvres involving trucks,
the passing ability of faster vehicles. This abili-
particularly in South Africa, require longer dis-
ty is influenced by a variety of factors, including
tances than those indicated. Designers must
traffic volumes, speed differentials, road geom-
take this into account for roads where significant
etry and human factors. The minimum sight dis-
percentages of heavy vehicles are expected in
tance required by a vehicle to overtake safely on
the traffic stream.
two-lane single carriageway roads is the distance which will enable the overtaking driver to
As mentioned above, the designer should seek
pass a slower vehicle without causing an
opportunities to introduce passing lanes on two-
oncoming vehicle to slow below the design speed.
lane roads, particularly where the terrain limits 3-18
Chapter 3: Design Controls
sight distance. A report on a review and evalu-
Limiting sight distances to those provided for
ation of research studies concluded that passing
stopping may also preclude drivers from per-
and climbing lane installations reduce collision
forming evasive manoeuvres, which are often
rates by 25 per cent compared to untreated two-
less hazardous and otherwise preferable to
lane sections.
They provide safer passing
stopping. Even with an appropriate complement
opportunities for drivers who are uncomfortable
of standard traffic control devices, stopping sight
in using the opposing traffic lane and for those
distances may not provide sufficient visibility for
who become frustrated when few passing
drivers to corroborate advance warning and to
opportunities exist, owing to terrain or traffic
perform the necessary manoeuvres. It is evi-
density.
dent that there are many locations such as exits from freeways, or where lane shifts or weaving
Sections with adequate passing sight distance
manoeuvres are performed where it would be
should be provided as frequently as possible.
prudent to provide longer sight distances. In
The appropriate frequency is related to operat-
these circumstances, decision sight distance
ing speed, traffic volumes and composition, ter-
provides the greater length that drivers need. If
rain and construction cost. As a general rule, if
the driver can see what is unfolding far enough
passing sight distance cannot be economically
ahead, he or she should be able to handle
provided at least once every 2 km, passing
almost any situation.
lanes should be considered. The 2+1 crosssection currently in vogue in Europe has some
Decision sight distance, sometimes termed
merit.
anticipatory sight distance, is the distance
This three-lane cross-section has two
required for a driver to:
lanes in one direction and a single lane in the
•
opposing direction. At about two to three kilo-
cult-to-perceive information source or
metre intervals, the second lane is allocated to
hazard in a roadway environment that
movement in the opposite direction. A minimum
may be visually cluttered;
shoulder width is required as discussed in
•
Chapter 4.
recognize the hazard or its potential threat;
Decision sight distance
select an appropriate speed and path; and
•
initiate and complete the required safety manoeuvre safely and efficiently.
Stopping sight distances are usually sufficient to allow reasonably competent and alert drivers to
Because decision sight distance gives drivers
stop under ordinary circumstances. However,
additional margin for error and affords them suf-
these distances are often inadequate when:
• • •
Drivers must make complex decisions;
ficient length to manoeuvre their vehicles at the
Information is difficult to perceive, or
same or reduced speed rather than to just stop,
Unexpected or unusual manoeuvres are
it is substantially longer than stopping sight dis-
required.
tance.
3-19 Chapter 3: Design Controls
Geometric Design Guide
• 3.5.8
detect an unexpected or otherwise diffi-
Drivers need decision sight distances whenever
traffic control devices for advance warning.
there is likelihood for error in either information
Although a sight distance is offered for the right
reception, decision-making, or control actions.
side exit, the designer should bear in mind that
Critical locations where these kinds of errors are
exiting from the right is in total conflict with driv-
likely to occur, and where it is desirable to pro-
er expectancy and is highly undesirable. The
vide decision sight distance include:
only reason for providing this value is to allow
•
Approaches to interchanges and inter
for the remote eventuality that a right side exit
sections;
has to be employed.
•
Changes in cross-section such as at toll plazas and lane drops;
• •
In measuring decision sight distances, the 1 050
Design speed reductions, and
mm eye height and 0 mm object height have
Areas of concentrated demand where
been adopted.
there is apt to be "visual noise", e.g where sources of information, such as
3.5.9
roadway elements, opposing traffic, traf-
Headlight sight distance
fic control devices, advertising signs and construction zones, compete for attention.
Headlight sight distance is typically used in
Geometric Design Guide
establishing the rate of change of grade for sag The minimum decision sight distances that
vertical curves. At speeds above 80 km/h, only
should be provided for specific situations are
large, light coloured objects can be perceived at
shown in Table 3.7. If it is not feasible to provide
the generally accepted stopping sight distances.
these distances because of horizontal or vertical
A five-fold light increase is necessary for a 15
curvature or if relocation is not possible, special
km/h increase in speed and a 50 per cent reduc-
attention should be given to the use of suitable
tion in object size.
3-20 Chapter 3: Design Controls
For night driving on highways without lighting,
ensure that two opposing vehicles travelling in
the length of visible roadway is that which is
the same lane should be able to come to a stop
directly illuminated by the headlights of the vehi-
before impact. A logical basis for the determi-
cle. This length is typically shorter than the min-
nation of the barrier sight distance is that it
imum sight distance.
should at least equal twice the stopping distance. Values given in the South Africa Road
When headlights are operated on low beam, the
Traffic
reduced candlepower at the source and the
approach.
Signs
Manual
approximate
this
downward projection angle significantly restrict the visible length of roadway surface.
Barrier sight distance is measured to an object height of 1,3 metre from an eye height of 1,05
For crest vertical curves, the area beyond the
m.
headlight beam point of tangency with the road-
The object height is the height of an
approaching passenger car.
way surface is shadowed and receives only indirect illumination. Also, a general limit of 120 to
Hidden dip alignments are poor design practice,
150 metres sight distance is all that can be safe-
but are found on many rural roads. They typi-
ly assumed for visibility of an unilluminated
cally mislead drivers into believing that there is
This corre-
more sight distance available than actually
sponds to a satisfactory stopping sight distance
exists. In checking vertical alignment, designers
for 80 to 90 km/h or a decision time of about 5
should pay attention to areas where this defi-
seconds at 100 km/h.
ciency exists, and ensure that drivers are made aware of any such inadequacies.
Since the headlight mounting height (typically about 600 mm) is lower than the driver eye
3.5.11 Obstructions to sight distance
height (1 050 mm for design), sight distance is
on horizontal curves
controlled by the height of the vehicle headlights and a one degree upward divergence of the light
Physical features, such as a concrete barrier
beam from the longitudinal axis of the vehicle.
wall, a bridge pier, a tree, foliage, or the back
Any object within the shadow zone must be high
slope of a cutting, can affect available sight dis-
enough to extend into the headlight beam to be
tance. Accordingly, designs need to be checked
directly illuminated.
in both the horizontal and vertical planes for
3.5.10 Barrier sight distance
obstructions.
Barrier sight distance is not a geometric design
Minimum radii of horizontal curvature are deter-
factor, but is rather an operational guide to the
mined by application of vehicle dynamics and
driver to promote safety on two-lane roads.
not through sight distance controls. It is, therefore, possible that the selected radius may not
Barrier sight distance is the limit below which
be adequate to ensure the safe stopping sight
overtaking is legally prohibited, in order to
distance requirements. 3-21
Chapter 3: Design Controls
If the obstructions to
Geometric Design Guide
object on a bitumen surfacing.
.
sight distance are immovable, re- alignment
L
=
lane width (m)
may be necessary.
s
=
stopping sight distance for specific gradient
The problem is illustrated in Figure 3.3. The dri-
and design speed (m)
ver's eye is assumed to be at the centre of the
from Figure 3.1.
nearside lane. The chord AB is the sight line and the curve ACB is the stopping sight dis-
Given the nature of the relationship, a trial-and-
tance. A zero gradient is assumed. It follows
error approach to the solution is required.
that selection of a radius for a given distance of
Figure 3.3: Horizontal restrictions to stopping sight distance obstruction from the inner lane centre line will
3.6
ENVIRONMENTAL FACTORS
Geometric Design Guide
constitute an under-design if the inner lane is on a downgrade.
A road is a key element in the modern environment with wide ranging implications. Planning
A radius to satisfy stopping sight distance crite-
for effective integration is therefore essential. In
ria can be calculated from the following formula;
South Africa, the National Environmental Management Act of 1998 lays down prescriptions for the provision and operation of infrastructure, including roads. The designer should
where C
R
=
=
distance from centre of
be aware of the constraints imposed by this law.
inside lane to obstruc-
For example, constraints may include avoiding a
tion (m)
particular watercourse or wetland area, or
radius of curve (m)
accommodating prescribed mitigatory meas3-22
Chapter 3: Design Controls
ures such as screening berms or sound fences.
ing landform best by avoiding disruption
In carrying out their mandate to plan and design
of major topographical features;
•
road systems, road designers should consider
ing landform to good effect and which
on the one hand, making facilities aesthetically
minimizes the scale of earthworks;
pleasing and being "good neighbours" in the
•
community and, on the other, providing safe and
To design profiles which reflect existing natural slopes;
•
efficient transportation links to users.
3.6.1
To find an alignment that uses the exist-
To retain the least road footprint, by the return of land to its former use;
Land use and landscape integra
•
tion
To use existing landform to minimize noise and visual intrusion: for example, placing a road in a cutting or behind rising
With regard to environmental factors, the objec-
ground to protect settlements;
•
tive of route selection should be to choose a
To develop new landforms, including
route that has both the minimum effect on land-
mounds and false cuttings, to screen the
form and requires the smallest number of large
road from settlements, and
•
earthworks. Integration with the existing land-
To achieve a balance between horizontal and vertical alignment.
form can best be achieved by grading out cuttings and embankments to slopes that reflect the surrounding topography. This in turn may
3.6.2
Aesthetics of design
affect adjacent sites of conservation or heritage interest and, in such cases, a balance needs to
Design aesthetics and attention to landform are
be struck. A major consideration is that non-
very closely related topics. Aesthetic improve-
renewable resources, such as wetlands, should
ments can often be achieved without incurring
be avoided wherever possible.
additional
costs,
provided
the
designer
approaches the subject in a sensitive manner.
Designs should aim to achieve the best possible
In fact, alignments that are visually pleasing are
use of excavated materials, thus minimizing the
usually less hazardous than other alignments.
need for off-site spoil or borrow pits. If off-site
On any roadway, creating pleasing appearance
the same good design principles as those used
is a worthwhile objective. Scenic values can be
on site, achieved by liaison with the appropriate
considered along with safety, utility, economy,
planning authority. Earthworks can only be integrated successfully if the new landform and its
and all the other factors considered in planning
soil structure allow effective strategic rehabilita-
and design. This is particularly true of the many
tion. Restoration to agricultural use can be a
portions of the National Road system situated in
particularly effective strategy.
areas of natural beauty.
The location of the
road, its alignment and profile, the cross section design, and other features should be in harmo-
Design objectives should be:
•
To choose the route least damaging to
ny with the setting. Economy consistent with
the landscape and which respects exist-
traffic needs is of paramount importance, 3-23
Chapter 3: Design Controls
Geometric Design Guide
works are necessary, they should be subject to
although a reasonable additional expenditure
either by absorbing the noise or deflecting it
can be justified to enhance the beauty of the
upwards.
highway.
include, for example, depressing and some-
Strategies addressing noise levels
times covering the roadway or by installing This topic is addressed in detail in Chapter 5.
3.6.3
sound barriers of earth or masonry. However, these may also trap air pollutants.
Noise abatement
Special sound barriers may be justified at cer-
Noise is defined as an unwanted sound, a sub-
tain locations, particularly along ground level or
jective result of sounds that intrude on or inter-
elevated roads through noise-sensitive areas.
fere with activities such as conversation, think-
Concrete, wood, metal, or masonry walls are
ing, reading or sleeping. Motor vehicle noise is
very effective in deflecting noise. One of the
generated by the functioning of equipment with-
more aesthetically pleasing barriers is the earth
in the vehicle, by its aerodynamics, by the action
berm that has been graded to achieve a natural
of tyres on the roadway and, in metropolitan
form that blends with the surrounding topogra-
areas, by short-duration sounds such as braking
phy. The feasibility of berm construction should
squeal, exhaust backfires, hooters and sirens.
be planned as part of the overall grading plan for
The decrease in sound intensity with distance
the roadway. There will be instances where an
from the source is influenced by several factors.
effective earth berm can be constructed within
Measurements taken near roads show that dou-
the normal right-of-way or with a minimal addi-
bling the distance results in a lowering of 3 dBA
tional right-of-way purchase. If the right-of-way
over clean, level ground and 4,8 dBA over lush
is insufficient to accommodate a three metre
growth.
high berm, a lower berm can be constructed in combination with a wall or screen to achieve the desired height.
Some sustained (ambient) noise is always present.
In a quiet residential neighbourhood at Shrubs, trees or ground covers are not very effi-
night, it is in the 32 to 43 dBA range; the urban
cient in shielding sound because of their perme-
Geometric Design Guide
residential daytime limits are about 41 to 53
ability to the flow of air. However, almost all
dBA. In industrial areas the range is 48 to 66
buffer plantings offer some noise reduction, and
dBA, while, in downtown commercial locations
exceptionally wide and dense plantings may
with heavy traffic, it is 62 to 73 dBA. Increases
result in substantial reductions in noise levels.
up to 9 dBA above ambient noise levels bring
Even where the noise reduction is not consid-
only sporadic protests. Protests become wide-
ered significant, the aesthetic effects of the
spread with increases in the 9 to 16 dBA range.
plantings will produce a positive influence.
At increases greater than 16 dBA, there may be community action.
3.6.4
A design objective is to keep noise at or below
The highway air-pollution problem has two
acceptable levels and this can be achieved
dimensions: the area-wide effects of primarily
Air pollution by vehicles
3-24 Chapter 3: Design Controls
reactive pollutants; and the high concentrations
If at all possible, major routes should not tra-
of largely non-reactive pollutants at points or
verse such areas but should rather be located
corridors along or near roads. The motor vehi-
on the higher ground surrounding inversion-
cle is a primary contributor to both forms,
prone valleys, with relatively low-volume road
accounting for an estimated 70 per cent of the
links serving developments in the valley areas.
CO, 50 per cent of the HC, and 30 per cent of
Attention should also be paid to prevailing wind
the NOx.
directions so that routes bypassing local communities are located downwind of these settle-
Area-wide conditions are exacerbated when
ments.
temperature inversions trap pollutants near the ground surface and there is little or no wind, so
3.6.5
that concentrations of pollutants increase. For some individuals, eyes burn and breathing is dif-
Weather and geomorphology
Land shape, on a broad scale, as well as pre-
ficult. It is alleged that lives can be shortened
vailing weather conditions, which could influ-
and some deaths have actually resulted from
ence the design, are factors over which the
these exposures. Also, certain kinds of vegeta-
designer does not have any control. Certain
tion are killed, stunted, or the foliage burned.
areas of the country are prone to misty condi-
The quantity of air pollutants can be reduced by
tions and others subject to high rainfall. Both
judicious design. Exhaust emissions are high
are factors that have to be taken into account in
while vehicle engines are operating at above-
design.
average levels of output, for example while accelerating from a stationary position or when
Mist and rain both cause reduced visibility.
climbing a steep hill. Smooth traffic flow at con-
Where these are a regular occurrence, they
stant speeds, such as in "green wave" condi-
tend to lie in belts, sometimes fairly narrow,
tions on a signalised route, reduces exhaust
across the landscape.
emissions in addition to leading to a reduction in
acquire local knowledge about the quirks of the
noise levels. By way of contrast, speed humps,
weather patterns and seek ways to reduce their
which are popular as speed-reducing devices in
effect.
residential areas, have the dual penalty of increased pollution and increased noise levels.
Where it is not possible to avoid a mist belt, the
In rural areas, vertical alignments should be
designer should pay particular attention to the
designed with a minimum of "false rises".
concept of the "forgiving highway", by providing flat side slopes and avoiding alignments where
In addition to being able to modify the quantity
short radius curves follow each other in quick
of pollutants in the atmosphere, the designer
succession.
can influence the extent to which emissions
short radius horizontal curves are particularly to
impact on local communities.
Temperature
be avoided. A real effort should also be put into
inversions that trap polluted air are typically
avoiding high fills. In conditions of heavy mist,
associated with closed or bowl-shaped valleys.
vehicles will tend to move very slowly but, even 3-25
Chapter 3: Design Controls
Steep downgrades followed by
Geometric Design Guide
Designers should
at speeds significantly below the design speed
If hourly flows are ordered from highest to low-
of the road, the restricted visibility will lead to
est, it is customary, in rural areas, to design for
high levels of stress. Drivers are more likely to
the thirtieth highest hourly flow, i.e that flow
make incorrect decisions when under stress and
which is exceeded in only 29 hours of the year.
designers should thus do everything possible to
This is because rural roads have very high sea-
keep stress levels within manageable limits.
sonal peaks and it is not economical to have a road congestion-free every hour throughout the
3.7
year. In urban areas, seasonal peaks are less
TRAFFIC CHARACTERISTICS
pronounced and the 100th highest hourly flow is
3.7.1
considered a realistic flow level for design pur-
General
poses. Factual information on expected traffic volumes is an essential input to design. This indicates
To predict hourly flows, it is necessary to know
the need for improvements and directly affects
the ADT and the peaking factor, ß. The param-
the geometric features and design.
eter, ß, is a descriptor of the traffic flow on a given road and depends on factors such as the
Traffic flows vary both seasonally and during the
percentage and incidence of holiday traffic, the
day. The designer should be familiar with the
relative sizes of the daily peaks, etc. The peak-
extent of these fluctuations to enable him or her
ing factor can fluctuate between -0,1 and -0,4. A
to assess the flow patterns. The directional dis-
value of -0,1 indicates minimal seasonal peak-
tribution of the traffic and the manner in which its
ing. This value of ß should be used in urban
composition varies are also important parame-
designs. A value of -0,4 suggests very high
ters. A thorough understanding of the manner in
seasonal peaks and would normally be applied
which all of these behave is a basic requirement
to roads such as the N3. As a general rule, a
of any realistic design.
value of -0,2 could be used as being a typical value. Equation 3.1 below can be used to esti-
3.7.2
Traffic volumes
mate flows between the highest and 1030th
Geometric Design Guide
highest hour. Although not a particularly good Traffic flow is measured by the number of vehi-
model, flows beyond the 1030th hour can be
cles passing a particular station during a given
estimated by using a straight line relationship
period of time. Typically, the flow of interest is
from the 1030th flow to zero veh/hr at the
the Average Daily Traffic (ADT). Flows may also
8760th or last hour of the year.
be reported per hour, such as the "hourly
QN
=
0,072ADT(N/1030)ß
observed traffic volume" or the "thirtieth-highest
where QN
=
two-directional flow in
hour" or "hundredth-hour", which are commonly
N-th hour of year (veh/h)
used for design purposes. Very short duration
ADT
=
average daily traffic (veh/day)
flows, such as for a five-minute period, are typically applied to studies of signalised intersec-
N
=
hour of year
tions.
ß
=
peaking factor.
3-26 Chapter 3: Design Controls
It is interesting to note that the peak hour factor,
towards the central business district (with rela-
K, quoted in the Highway Capacity Manual is
tively low outward-bound flows), whereas the
often assumed for design purposes to be 0,15.
afternoon peak is in the reverse direction. It is
Reference is commonly made to the 30th high-
important to realize that the design flow is actu-
est hour of the year as being the design hour.
ally a composite and not a single value. A road
Applying a value of -0,2 to ß, and assuming N to
must be able to accommodate the major flow in
be 30, QN according to Equation 3.1 is 0,146 x
both directions.
ADT for the thirtieth highest hour. The actual distribution to be used for design purDesigners need to estimate future traffic flows
poses should be measured in the field. If an
for a road section. It is recommended that a
existing road is to be reconstructed, the field
design period of 20 years be used for forward
studies can be carried out on it beforehand. For
planning. The 30th or 100th highest flow used
new facilities, measurements should be made
in the design is that occurring in the design year,
on adjacent roads from which it is expected the
typically twenty years hence. Staged construc-
traffic will be diverted and modelling techniques
tion or widening of roads over this period can be
applied.
a feature of an economical design. Directional distribution is relatively stable and The capacity of rural road sections is influenced
does not change materially from year to year.
by the following key characteristics:
Relationships established from current traffic
•
Road configuration - e.g. two-lane two-
movements are normally also applicable to
way, multi-lane divided or undivided;
future movements.
Operating speed; Terrain;
3.7.4
Lane and shoulder width; Traffic composition, and
Vehicles of different sizes and mass have differ-
Gradients.
ent operating characteristics.
Trucks have a
higher mass/power ratio and occupy more road-
In the case of two-lane two- way roads, the fol-
way space than passenger cars. Consequently,
lowing additional factors are important:
• •
Traffic composition
they constitute a greater impedance to traffic
Directional distribution of traffic flow; and Passing opportunities - sight distance,
flow than passenger vehicles, with the overall
overtaking lanes, climbing lanes or slow vehicle
effect that one truck is equivalent to several pas-
turnout lanes.
senger cars. For design purposes, the percentage of truck traffic during the peak hours has to
3.7.3
Directional distribution
be estimated.
Directional distribution of traffic is an indication
For design of a particular highway, data on the
of the tidal flow during the day. In urban areas,
composition of traffic should be determined from
the morning peak traffic is typically inbound
traffic studies. 3-27
Chapter 3: Design Controls
Truck traffic is normally
Geometric Design Guide
• • • • •
expressed as a percentage of total traffic during
It is difficult to define the life of a "road" because
the design hour in the case of a two-lane road;
major segments may have different lengths of
and as a percentage of total traffic in the pre-
physical life. Each segment is subject to varia-
dominant direction of travel in the case of a
tions in estimated life expectancy because of
multi-lane road.
influences not readily subject to analysis such as obsolescence and unexpected changes in
It is not practical to design for a heterogeneous
land use, resulting in changes in traffic volumes,
traffic stream and, for this reason, trucks are
pattern and load. Regardless of the anticipated
converted to equivalent Passenger Car Units
physical life of the various elements of the road,
(PCUs). The number of PCU's associated with
it is customary to use a single value as the
a single truck is a measure of the impedance
"design life". In essence, the road is expected
that it offers to the passenger cars in the traffic
to provide an acceptable level of service for this
stream. This topic is exhaustively addressed in
period. Whether or not any of its various com-
the Highway Capacity Manual and is not dis-
ponents have a longer physical life expectancy
cussed further here.
than this design life is irrelevant. For example,
Geometric Design Guide
the alignment and, in some instances, the surPassenger car unit equivalents have, in general,
facing of roads built during Roman times are still
been derived from observations as illustrated in
in use today without there being any reference
Table 3.8. The values offered serve only as a
to a design life of 2 000 years.
rough indication and the designer should refer
A typical value of design life is twenty years. In
to the Highway Capacity Manual for detailed
the case of major and correspondingly expen-
calculation of PCU's appropriate to the different
sive structures a design life of fifty years may be
environments and circumstances.
assumed. This should not, however, be confused with the concept of a bridge being able to
3.7.5
Traffic growth
withstand the worst flood in fifty years.
3.7.6
The design of new roads or of improvements to existing roads should be based on the future
Capacity and design volumes
The term capacity is used here to define the ability of a road to accommodate traffic under
traffic expected to use the facilities. 3-28
Chapter 3: Design Controls
given circumstances. Factors to be taken into account are the physical features of the road itself and the prevailing traffic conditions
Physical features having considerable influence are the type of intersection, i.e. whether plain, channelised, roundabout or signalised, the number of intersecting traffic lanes and the adequa-
Prevailing road conditions
cy of speed-change lanes.
Capacity figures for uninterrupted flow on highways have to be modified if certain minimum
Unlike the physical features of the highway,
physical design features are not adhered to.
which are literally fixed in position and have def-
Poor physical features that tend to cause a
inite measurable effects on traffic flows, the pre-
reduction in capacity are:
•
Narrow traffic lanes.
vailing traffic conditions are not fixed but vary
Lane widths of
from hour to hour throughout the day. Hence,
3.65 m are accepted as being the mini-
•
mum necessary for heavy volumes of
the flows at any particular time are a function of
mixed traffic, i.e. before capacity of the
the speeds of vehicles, the composition of the
lane is reduced.
traffic streams and the manner in which the
Inadequate shoulders. The narrowness,
vehicles interact with each other, as well as of
or lack of, shoulders alongside a road
the physical features of the roadway itself.
cause vehicles to travel closer to the centre of the carriageway, thereby increasing the medial traffic friction. In
Capacity
addition, vehicles making emergency
The term "capacity" was introduced in the USA
stops must, of necessity, park on the
in the Highway Capacity Manual, in which it is
carriageway. This causes a substantial
defined as "the maximum number of vehicles
reduction in the effective width of the
that can pass a given point on a roadway or in a
road, thereby reducing capacity.
designated lane during one hour without the
Side obstructions. Vertical obstructions
traffic density being so great as to cause unrea-
such as poles, bridge abutments, retain
sonable delay, hazard, or restriction to the driv-
ing walls or parked cars that are located
•
within about 1,5 m of the edge of the
ers' freedom to manoeuvre under the prevailing
carriageway contribute towards a reduc-
roadway and traffic conditions". This definition
tion in the effective width of the outside
gives a reasonable method of approach but, in
traffic lane.
practice, it is necessary to choose one or more
Imperfect horizontal or vertical curva-
arbitrary criteria of what constitutes restriction of
ture. Long and/or steep hills and sharp
traffic movement, or "congestion".
bends result in restricted sight distance. As drivers then have reduced opportuni-
The Highway Capacity Manual procedure must
ties to pass, the capacity of the facility
however be used for specific road capacity
will be reduced.
designs. In addition to the above, the capacities of certain For typical South African conditions and to bal-
rural roads and the great majority of urban roads
ance financial, safety and operational consider-
are controlled by the layouts of intersections. 3-29
Chapter 3: Design Controls
Geometric Design Guide
•
ations it is recommended that the capacity of a
Classification of roads by design types based on
two-lane rural road be taken on average as
the major geometric features (e.g. freeways) is
being between 10 000 and 12 000 vehicles per
the most helpful one for road location and
day while, for freeways, consideration could be
design purposes. Classification by route num-
given to changing from a four-lane to a six-lane
bering is the most helpful for road traffic opera-
freeway when the traffic flow is of the order of 35
tional purposes, whilst administrative classifica-
000 to 40 000 vehicles per day.
tion is used to denote the level of government responsible for, and the method of, financing
3.8
road facilities.
ROAD CLASSIFICATION
Functional classification, the
grouping of roads by the character of service The classification of roads into different opera-
they provide, was developed for transportation
tional systems, functional classes or geometric
planning purposes.
types is necessary for communication between engineers, administrators and the general pub-
3.8.1
lic. Classification is the tool by which a complex
African roads
Classification criteria for South
network of roads can be subdivided into groups having similar characteristics. A single classifi-
Although numerous classification criteria are
cation system, satisfactory for all purposes,
used on road networks world-wide, in South
would be advantageous but has not been found
Africa there are basically three criteria used to
to be practicable. Moreover, in any classifica-
classify road types. These are:
tion system the division between classes is
• • •
often arbitrary and, consequently, opinions differ on the best definition of any class. There are
Geometric Design Guide
various schemes for classifying roads and the
Functional classification; Administrative classification, and Design Type classification, based on traffic usage.
class definitions generally vary depending on
This document uses the third type for design
the purpose of classification.
purposes.
The principal purposes of road classification are
As a result of growing awareness of the interde-
to:-
pendency of the various modes of transport as
•
Establish logical integrated systems
well as the creation of Metropolitan Transport
that, because of their particular service,
Authorities within South Africa's major metropol-
should be administered by the same
itan areas there is considerable overlap
jurisdiction;
• •
between the functional and administrative clas-
Relate geometric design control and other design standards to the roads in
sification criteria.
each class, and
areas, the general public is more dependent on,
Establish a basis for developing long-
and understands, a route numbering or func-
range programmes, improvement priori
tional classification than on an administrative
ties and financial plans.
classification of roads within the area.
3-30 Chapter 3: Design Controls
Within these metropolitan
Although these guidelines are based on a
Another and less comprehensive form of func-
design type classification, the three different
tional classification was developed for the pur-
approaches mentioned above are briefly
poses of road signing as shown in the South
described in order to provide a picture of the
African Road Traffic Signs Manual. As stated in
road system hierarchy in South Africa.
SARTSM, "There are definite limits to the number of ways
3.8.2
Functional classification concept
in which GUIDANCE signs and specifically DIRECTION signs can be made to indicate with
For transportation planning purposes, road are
sufficient immediate recognition potential, the
most effectively classified by function. The func-
different classes into which the road network is
tional classification system adopted for the
divided for signing purposes."
South African road network is illustrated in Table 3.9. This was used for the South African Rural
Classification for signing thus differentiates
Road Needs Study carried out during the early
mainly between numbered and unnumbered
1980s.
routes and, in respect of numbered routes, also draws a distinction between freeways and other
3-31 Chapter 3: Design Controls
Geometric Design Guide
roads.
Roads have two functions: to provide mobility
arate levels of government each have a roads
and to provide land access. However from a
function mandated to them. Despite this sepa-
design standpoint, these functions are incom-
ration of authority for various classes or roads, it
patible. For mobility, high or continued speeds
is essential to bear in mind that roads act as a
are desirable and variable or low speeds unde-
total system or network and that the subdivision
sirable; for land access, low speeds are desir-
of roads into administrative classes bears no
able and high speeds undesirable. For exam-
relation to the functional type of a road under the
ple, freeways provide a high degree of mobility,
control of a specific authority. The administra-
with access provided only at spaced inter-
tive classification approach thus divides the
changes to preserve the high-speed, high-vol-
South African road network into:
• • •
ume characteristics of the facility. The opposite is true for local, low-speed roads that primarily provide local access. The general relationship of functionally classified systems in serving
Local Authority roads.
bears no relation to the design type of a road
3.4.
under the control of a specific authority. Thus, until the turn of the 21st century, there were sec-
Given a functional classification, design criteria
tions of National Roads that were unsurfaced,
can be applied to encourage the use of the road as intended.
with very low traffic volumes, the most heavily
Design features that can convey
trafficked roads in South Africa, also up to the
the level of functional classification to the driver
turn of the century, often being the responsibili-
include width of roadway, continuity of align-
ty of a local authority.
ment, spacing of intersections, frequency of
Furthermore certain
"routes" in the country comprise both National
access points, building setbacks, alignment and
and Provincial roads, and could even include
grade standards, and traffic controls.
Geometric Design Guide
Provincial, and
This administrative classification of roads also
mobility and land access is illustrated in Figure
3.8.3
National,
local authority roads.
Administratively, a
National Road, which is denoted as such by a
Administrative classification
legal proclamation, in general comprises those roads that form the principal avenues of com-
Legislation and, in some instances, the
munication between major regions of the coun-
Constitution, assigns to certain levels of govern-
try, and/or between major population conurba-
ment the responsibilities for providing, regulat-
tions and/or between major regions of South
ing and operating roads and streets for public
Africa and other countries.
use. This concept and the principles of law that support it were developed in Great Britain, and
3.8.4
even earlier by the Romans. Within the limits of
Design type classification
its constitutional powers, a particular road
The most widely accepted design type criteria
authority may delegate its authority for roads to
are those developed by AASHTO which classify
bodies such as a Roads Board or a Toll Road
a road system into:
Concessionaire. In South Africa, the three sep-
•
Freeways;
3-32 Chapter 3: Design Controls
• •
Arterial roads other than freeways, and
Design designations of these specific National
Collector roads.
Roads are as follows;
For each classification, specific design stan-
Class I
Primary Roads
dards and criteria and access and other policies
Class IA
Primary Freeways in rural areas
have been developed and are applied.
•
Illustrative threshold ADT (with more
For use in the present document the following
than 12% heavy vehicles) = 15 000
service or design classifications are proposed,
veh/d.
related to design traffic volumes given in ADT terms.
•
Each classification groups roads with
Average travel distances on links indi-
similar functions. The factors that influence the
cated by at least 45 per cent of the trips
classification of a roadway to a certain group
being more than 2 hours in duration;
•
include;
• • •
Trip purpose (for the majority of users);
purposes, and
•
Trip length; Size and type of population centre
Minimum design speed 130 km/h.
Class IB
served;
• •
May be provided for network continuity
Primary Freeways in metropolitan areas
Traffic characteristics, and
•
Network and system requirements
Illustrative threshold ADT = 20 000 veh/d
3-33 Chapter 3: Design Controls
Geometric Design Guide
Figure 3.4: Relationship of functional road classes
•
Average travel distance on links indicat ed by majority of trips being less than 2 hours in duration
•
Form integral element of Metropolitan Road Network
•
Generally extension of Rural Freeways (Class IA roads)
•
Minimum design speed 130 km/h
Class II
Primary Arterials, 4 lane single carriageway roads
Class IIA
•
Primary Rural Arterials
Generally provided when 2 lane single carriageway road reaches capacity and freeway not financially affordable
•
Ilustrative threshold ADT : 8 000 - 10 000 veh/d, with bottom end of scale applicable where percentage of truck traffic exceed 15 per cent
•
Minimum design speed 120 km/h
Class IIB
•
Design in context in which it operates.
Class III
•
Primary Metropolitan Arterial Secondary Rural Arterial
Provided to address inter-regional travel demands, or providing access to tourist or National resource areas
•
Provided to address inter-regional travel demands, or providing access to tourist or National resource areas
•
Provided to address inter-regional trav-
Geometric Design Guide
el demands, or providing access to tourist or National resource areas
• • • • • • • • • •
Two lane, single carriageway roads Class IIIA Design ADT greater than 4 000 veh/d Minimum design speed 120 km/h Class IIIB Design ADT 500 - 4 000 veh/d Minimum design speed 110 km/h Class IIIC Design ADT less than 500 veh/d Minimum design speed 100 km/h.
3-34 Chapter 3: Design Controls
TABLE OF CONTENTS 4
ROAD DESIGN ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
4.2
HORIZONTAL ALIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2.1 General Controls for horizontal alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1 4.2.2 Tangents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-3 4.2.3 Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6 4.2.4 Superelevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 4.2.5 Transition curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-17 4.2.6 Lane widening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18
4.3
VERTICAL ALIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 4.3.1 General controls for vertical alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-21 4.3.2 Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-22 4.3.3 Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-26
4.4
CROSS-SECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 4.4.1 General controls for cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-30 4.4.2 Basic Lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-32 4.4.3 Auxiliary lanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-33 4.4.4 Kerbing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-39 4.4.5 Shoulders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-40 4.4.6 Medians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-43 4.4.7 Outer separators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45 4.4.8 Boulevards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-45 4.4.9 Bus stops and taxi lay-byes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 4.4.10 Sidewalks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-48 4.4.11 Cycle paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 4.4.12 Slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 4.4.13 Verges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-52 4.4.14 Clearance profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 4.4.15 Provision for utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 4.4.16 Drainage elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-55
LIST OF TABLES Table 4.1: Minimum radii for various values of emax (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8 Table 4.2: Design domain for emax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9 Table 4.3: Values of superelevation for above min radii of curvature (%): emax = 4 % . . . . . . . . . . . . . . . 4-11 Table 4.4: Values of superelevation for above min radii of curvature (%): emax = 6 % . . . . . . . . . . . . . . . 4-12 Table 4.5: Values of superelevation for above min radii of curvature (%): emax = 8 % . . . . . . . . . . . . . . . 4-13 Table 4.6: Values of superelevation for above minradii of curvature (%): emax = 10 %. . . . . . . . . . . . . . . 4-13 Table 4.7: Maximum relative gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-14 Table 4.8: Lane adjustment factors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Table 4.9: Maximum radii for use in spiral transition curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-18 Table 4.10: Lengths of grade for 15 km/h speed reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.11: Maximum gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-24 Table 4.12: Minimum values of k for crest curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.13: Minimum k-values for barrier sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . 4-27 Table 4.14: Minimum k-values for sag curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-29 Table 4.15: Warrant for climbing lanes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-35 Table 4.16: Shoulder widths for undivided rural roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-42 Table 4.17: Warrants for pedestrian footways in rural areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-49 Table 4.18: Cycle lane widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-51 Table 4.19: Typical widths of roadside elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-53 Table 4.20: Scour velocities for various materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-57
LIST OF FIGURES Figure 4.1: Dynamics of a vehicle on a curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7 Figure 4.2: Methods of distributing of e and f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10 Figure 4.3: Attainment of superelevation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-15 Figure 4.4: Superelevation runoff on reverse curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.5: Superelevation runoff on broken-back curves. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-16 Figure 4.6: Typical turning path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-19 Figure 4.7: Truck speeds on grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-23 Figure 4.8: Sight distance on crest curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-25 Figure 4.9: Sight distance on a sag curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-28 Figure 4.10: Cross-section elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-31 Figure 4.11: Verge area indicating location of boulevard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-46 Figure 4.12: Typical layout of a bus stop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-47 Figure 4.13: Bicycle envelope and clearances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-50 Figure 4.14: Collision rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.15: Prediction of utility pole crashes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-54 Figure 4.16: Typical drain profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-58
Chapter 4 ROAD DESIGN ELEMENTS 4.1
INTRODUCTION
4.2
HORIZONTAL ALIGNMENT
Regardless of the philosophy brought to bear on
The horizontal alignment comprises three ele-
the design of a road or the classification of the
ments: tangents, circular curves and the transi-
road in the network, the final design comprises
tions between tangents and curves.
a grouping and sizing of different elements. A vertical curve is an element. The development
Tangents (sometimes referred to as "straights")
of super elevation is an element. The cross-
have the properties of bearing (direction or
section is heavily disaggregated, comprising a
heading) and length. Circular curves also have
large group of elements. Geometric design thus
two properties; radius and deflection (or devia-
comprises:
tion) angle, these two properties directly leading
•
The selection of elements to be incorpo-
to a third property of interest, namely curve
rated in the design;
length. The basic properties of transition curves
The sizing of the selected elements, and
are shape and length. Design of the horizontal
Linking the elements into a three-dimen-
alignment includes selection of the values asso-
sional sequence.
ciated with each of these six properties.
The selected, sized and linked elements in
4.2.1
combination represent the final design of a road
General Controls for horizontal alignment
which, when built, must constitute a network link which will satisfactorily match the criteria of
The horizontal alignment should be as direction-
safe, convenient and affordable transportation
al as possible and consistent with the topogra-
with minimum side effects, simultaneously
phy. However, it is equally important in terms of
addressing needs other than those pertaining
context sensitive design to preserve developed
directly to the movement of people or freight.
properties and areas of value to the community.
A road network comprises links and nodes. The
Winding alignments composed of short curves
intersections and interchanges are the nodes
and tangents should be avoided if at all possible
and the roads connecting them the links.
because they tend to cause erratic operation
Intersections and the elements that constitute
and a high consequent crash rate.
them are discussed in Chapter 6, with interchanges being dealt with in Chapter 7.
Most design manuals recommend that minimum radii should be avoided wherever possible, sug-
In this chapter, the various elements involved in
gesting that flat curves (i.e. high values of
link design are discussed.
radius) should be used, retaining the minimum
4-1 Chapter 4: Road Design Elements
Geometric Design Guide
• •
for the most critical conditions. The concept of
1 000 metres. If a curve radius is such that the
consistency of design, on the other hand, sug-
curve either has a normal camber or a crossfall
gests that the difference between design speed
of 2 per cent, the limitation on curve length falls
and operating speed should ideally be held to a
away and the curve can be dealt with as though
maximum of 10 km/h, with a 20 km/h difference
it were a tangent.
still representing tolerable design. This could be construed as a recommendation that minimum
Broken-back (also referred to as "flat back")
curvature represents the ideal, which is wholly
curves are a combination of two curves in the
at variance with the historic approach to selec-
same direction with an intervening short tan-
tion of curve radius. What is actually intended
gent.
however is that the designer should seek to
grounds of aesthetics and safety. Not only are
employ the highest possible value of design
broken-back curves unsightly but drivers do not
speed for any given circumstance.
always recognize the short intervening tangent
These should be avoided on the twin
and select a path corresponding to the radius of For small deflection angles, curves should be
the first curve approached, hence leaving the
sufficiently long to avoid the appearance of a
road on the inside and part way along the tan-
kink.
A widely adopted guideline is that, on
gent. On roads in areas with restrictive topog-
minor roads, curves should have a minimum
raphy, such as mountain passes, the designer
5O
may have no option but to accept the use of bro-
and that this length should be increased by 30
ken-back curves but should, nevertheless, be
length of 150 metres for a deflection angle of metres for every
1O
decrease in deflection
aware of their undesirability. Where the inter-
angle. On major roads and freeways, the mini-
vening tangent is longer than 500 metres, the
mum curve length in metres should be three
appellation "broken-back" is no longer appropri-
times the design speed in km/h. The increase in
ate.
Geometric Design Guide
length for decreasing deflection angle also applies to these roads. In the case of a circular
Reverse curves are also a combination of two
curve without transitions, the length in question
curves but in opposite directions with an inter-
is the total length of the arc and, where transi-
vening short tangent. These curves are aes-
tions are applied, the length is that of the circu-
thetically pleasant but it is important to note that
lar curve plus half the total length of the transi-
the intervening tangent must be sufficiently long
tions.
to accommodate the reversal of superelevation between the two curves.
South African practice recommends an upper limit to the length of horizontal curves. Curves
Although compound curves afford flexibility in
to the left generally restrict passing opportuni-
fitting the road to the terrain and other ground
ties and, furthermore, dependant on their radius,
controls, their use should be avoided outside
operation on long curves tends to be erratic.
intersection or interchange areas. Once they
For this reason, it is desirable to restrict the
are on a horizontal curve, drivers expect the
length of superelevated curves to a maximum of
radius to remain unaltered hence supporting a 4-2
Chapter 4: Road Design Elements
constant speed across the length of the curve.
Obviously this would be of fairly limited duration
A compound curve is thus a violation of driver
in terms of the season and the likelihood of a
expectancy and can be expected to have a cor-
prolonged gradient of this magnitude, whereas
respondingly high crash rate.
the east-west bearing would be a problem all year round and over an extended period of time
4.2.2
Tangents
during the day. The designer should be aware
Two fundamentally different approaches can be
of this problem and, if possible, avoid selecting
adopted in the process of route determination.
bearings that reduce visibility.
In a curvilinear approach, the curves are located
problem cannot be avoided, warning signage
first and thereafter connected up by tangents.
may be considered.
If the dazzle
This approach can be adopted with advantage in mountainous or rugged terrain. A typical con-
In the second instance, a bearing at right angles
sequence of this approach is that curves tend to
to the prevailing wind direction can cause prob-
be long and tangents short.
Because of the
lems for empty trucks with closed load compart-
local topography, the approach to route determi-
ments, e.g. pantechnicons, or passenger cars
nation most frequently adopted in South Africa
towing caravans. On freight routes or routes
is for the tangents to be located first and the
with a high incidence of holiday traffic, the
curves fitted to these tangents thereafter.
designer should seek to avoid this bearing. If not possible, locating the road on the lee of a hill
Bearing
that could then offer shelter from the wind may be an option to be explored in order to offer
Economic considerations dictate that, where
some relief.
other constraints on route location are absent, roads should be as directional as possible. In
Length
consequence, tangents may be located on bearings that have an adverse impact on driver comfort and safety. Two conditions require consid-
The minimum allowable length of tangent is that
eration.
which accommodates the rollover of superele-
In the first instance, a combination of gradient, direction of travel and time of day may cause the
It has been found that extremely long tangents,
driver to be dazzled by the sun. The most obvi-
e.g. lengths of twenty kilometres or more, have
ous example is the east-west bearing where the
accident rates similar to those on minimum
vehicle would be moving towards the rising or
length tangents, the lowest accident rate occur-
setting sun. No direction of travel is completely
ring in a range of eight to twelve kilometres.
exempt from this problem. For example, when
This range is recommended for consideration in
travelling at midday in mid-winter in a northerly
fixing the maximum length of tangent on any
direction up a gradient steeper than about eight
route. This maximum is based on the assump-
per cent, the sun can present a problem.
tion of a design speed of 120 km/h or more. At
4-3 Chapter 4: Road Design Elements
Geometric Design Guide
vation in a reverse curve situation.
lower design speeds, it is necessary to consider
constitute poor design so that, at a greater level
maximum lengths considerably shorter than
of precision than the proposed rule of thumb,
eight to twelve kilometres as discussed below.
three possibilities arise. These are:
•
Case 1 - The length of tangent is less
As a rough rule of thumb (which is adequate for
than or equal to the distance, Tmin,
planning purposes) the maximum length of tan-
required to accelerate from the operat
gent in metres should not exceed ten to twenty
ing peed appropriate to one curve to that appropriate to the next curve;
times the design speed in kilometres/hour. If the
•
achievable maximum length of tangent across
Case 2 - The length of tangent allows acceleration to a speed higher than that
the length of the route is regularly greater than
appropriate to the next curve but not as
this guideline value, thought should be given to
high as the desired speed remote from
consideration of a higher design speed. Rules
inhibiting curves, and
•
of thumb have their limitations and, in this case,
Case 3 - The length of tangent, Tmax,
application should be limited to design speeds
allows acceleration up to desired speed.
of 100 km/h or less. At a design speed of 120
These cases are intended to be applied in areas where curves are to design
km/h or higher, a maximum tangent length of
speeds of 100 km/h or less.
1 200 to 2 400 metres would clearly be meaningless.
To apply the guideline values of allowable speed
As an example of the application of the rule of
variations and differences, it is necessary to
thumb, a design speed of 80 km/h would sug-
estimate the operating speeds on the curves
gest that the maximum length of tangent should
preceding and following a tangent.
be in the range of 800 to 1 600 metres. In this
absence of information specific to South African
range, drivers would tend to maintain a fairly
conditions, the international value of V85 given
constant speed of about 80 km/h. At greater
in Equation 4.1 will have to be employed.
lengths of tangent, drivers would accelerate to
V85 = 105,31 + 1,62 x 10-5 x B2 - 0,064 x B
some higher speed only to decelerate at the fol-
(4.1) where V85
lowing curve. This oscillation in speed is inher-
85th percentile speed
=
(km/h)
ently dangerous as discussed in more detail
Geometric Design Guide
In the
B
below. Consistency of design dictates that:
•
and
The difference between design speed
=
Bendiness
=
57 300/R (degrees/km)
θ
=
Deviation angle
L
=
Total length of curve (metres).
and 85th percentile speed, and
•
Variations in 85th percentile speeds between successive elements
The general form for bendiness, B, in the case
should be limited as far as possible. Research
where the circular curve is bounded by transi-
has indicated that, ideally, these differences and
tion curves is
variations should be less than 10 km/h but that an acceptable design still results if they are less
B = (Lcl1/2R + Lcr/R +Lcl2/2R) x 180/π x 1000
than 20 km/h. Differences in excess of 20 km/h
L 4-4
Chapter 4: Road Design Elements
(4.2)
where L
=
where
LCl1 + LCr + LCl2
C
=
average passenger car speed (km/h)
with LCl1 and LCl2 being the lengths of the preceding and succeeding transition curves and
Q
=
flow (veh/h)
LCr the length of the circular curve.
G
=
gradient (per cent)
D
=
directional split as a decimal fraction
The length of tangents is to be compared with PT
the values of Tmin and Tmax that, as suggest-
=
number of trucks in the
ed, are a function of the speeds achievable on
traffic stream as a dec
the curves preceding and following these tan-
imal fraction
gents.
PS
The values are calculated using an
acceleration or deceleration rate of 0,85
m/s2
=
number of semi trailers in the traffic stream as a decimal fraction.
(determined by car-following techniques and which also corresponds to deceleration without
In the absence of significant volumes of traffic
braking) TMIN =
2
V851 - V852
2
and on a level grade, the average speed would,
(4.3)
according to this equation, be of the order of 120
22,03
km/h, suggesting that the 85th percentile speed estimated by Eq. 4.1 is very conservative.
and TMax =
2(V85Tmax)2-(V851)2-(V852)2 22,03
If the tangent length is shorter than TMin the tan-
(4.4)
where V85Tmax =
V851
V852
=
=
85th percentile speed
for the operating speeds of the two adjacent
on long tangent, i.e.
curves to be within the difference ranges
V85, (km/h)
described above to constitute good or tolerable
85th percentile speed
design. In essence, acceleration to the operat-
on preceding curve
ing speed of the following curve could take
(km/h)
place on this curve itself. Where it is necessary
85th percentile speed
to decelerate to the operating speed of the fol-
on following curve
lowing curve, it will be necessary for the driver to
(km/h)
brake in order to achieve the appropriate speed
A tangent has a bendiness of zero so that V85
at the start of the curve. It follows that, on a two-
for TMax is, according to Eq. 4.1, 105,31 km/h.
lane two-way road, tangent lengths shorter than
South African research has derived an expres-
TMin are potentially dangerous.
sion for average speed as given in Eq. 4.5. Where the tangent length is just equal to TMax C
=
143,96 - 10,39 ln Q - 0,04 (G2 -
the vehicle will be able to accelerate from the
5,20) - 18,08 D -33,89 PT -
operating speed of the preceding curve to the
54,15 PS
desired speed and then immediately decelerate
(4.5)
to the operating speed of the following curve. In 4-5 Chapter 4: Road Design Elements
Geometric Design Guide
gent is non-independent and it is only necessary
this case, the difference between operating
ceeding curves. These define, in effect, the rel-
speeds on each curve and the desired speed
ative design domain of horizontal curvature on
has to be within the allowable range.
any given road, i.e. the possible range of values of radius of any curve given, the radius of the preceding curve.
In the case of a tangent length falling in the range TMin < T < TMax , it will be necessary to calculate the highest operating speed that can
The safety of any curve is dictated not only by
be reached by accelerating at a rate of 0,85
the external factors described above but also by
m/s2
from the operating speed of the first curve,
factors internal to it, namely radius, supereleva-
allowing for a deceleration at the same rate to
tion, transitions and curve widening. Of these
the operating speed of the second. It is the dif-
factors, the most significant is radius as
ference between this maximum operating speed
research carried out in Washington State shows
and the speeds on the adjacent curves that is
consistently that crash frequency increases as
critical. The maximum operating speed on a
the curve radius decreases. At present, the best
tangent of this length is calculated as
model shows that A = (0,96 L + 0,0245/R - 0,012S) 0,978(3.3 x W - 30)
V85
[11,016(T - TMin) + V8512]0,5
=
for V851 > V852
(4.7)
(4.6)
where A
=
crashes/million vehicles entering from both
4.2.3
Curves
directions
Over the years, various theoreticians have pro-
L
=
curve length (km)
posed a variety of polynomials as the most
R
=
curve radius (km)
desirable forms of horizontal curvature, with
S
=
1,
desirability presumably being determined by the
curves have
aesthetics of the end resultant and usually from
been provided
a vantage point not normally available to the driver. Accident history suggests, however, that
W
drivers have enough difficulty in negotiating sim-
Geometric Design Guide
if transition
=
0,
otherwise
=
roadway width (lanes plus shoulders) (m).
ple circular curves that have the property of providing a constant rate of change of bearing. It is
Using this relationship, the designer would be
recommended that anything more complex than
able to estimate the merits of increasing the
circular arcs should be avoided, the most note-
radius of a curve. This would presumably be of
worthy exception being the loop ramp on inter-
great benefit in the case where an existing road
changes.
is to be upgraded or rehabilitated.
In the preceding section, relationships between
It is necessary to determine the absolute bound-
operating speed and degree of curvature were
aries of the design domain. The upper bound is
offered,
differences
obviously the tangent in the sense that it has a
between the operating speeds observed on suc-
radius of infinite length. The lower bound is the
as
were
acceptable
4-6 Chapter 4: Road Design Elements
minimum radius for the selected design speed
The side friction factor is a function of the condi-
and this is a function of the centripetal force nec-
tion of the vehicle tyres and the road surface
essary to sustain travel along a circular path.
and varies also with speed. For the purposes of
This force is developed in part by friction
design, it is desirable to select a value lower
between the vehicle's tyres and the road surface
than the limit at which skidding is likely to occur
and in part by the superelevation provided on
and the international general practice is to
the curve. The Newtonian dynamics of the situ-
select values related to the onset of feelings of
ation is illustrated in Figure 4.1.
discomfort. Canadian practice suggests that the
Figure 4.1: Dynamics of a vehicle on a curve side friction factor be taken as
The relationship between speed, radius, lateral
f
friction and superelevation is expressed by the
=
0,21 - 0,001xV
(4.9)
=
vehicle speed (km/h).
e+f where e
f
=
V2/127 R
=
superelevation( taken
=
where V
(4.8)
as positive when the
For any given speed, it is thus only necessary to
slope is downward
select the maximum rate of superelevation,
towards the centre of
emax, in order to determine the minimum allow-
the curve)
able radius of horizontal curvature for that
lateral or side friction
speed. This selection is based on considera-
factor
tions of the design domain as discussed in the
V
=
speed of vehicle (km/h)
following section.
In practice, four values of
R
=
radius of curvature (m).
emax are used, being 4, 6, 8, and 10 per cent. The minimum radius of curvature appropriate to
This equation is used to determine the minimum
design speeds in the range of 40 km/h to 130
radius of curvature that can be traversed at any
km/h for each of these values of emax is given in
given speed.
Table 4.1.
4-7 Chapter 4: Road Design Elements
Geometric Design Guide
relationship:
Guidelines are offered in the following section
as 8 per cent, provided that this value of super-
for the selection of emax
elevation is used only between intersections and that the superelevation is sufficiently remote
4.2.4
Superelevation
from the intersections for full run-off to be achieved prior to reaching the intersection area.
The selection of the appropriate value of emax is at the discretion of the designer in terms of the
In rural areas, the range of observed speeds is
design domain concept. The higher values of
relatively limited and adequate distance to allow
emax are typically applied to rural areas and the
for superelevation development and runoff is
lower values to the urban environment.
usually available. Climatic conditions may, how-
Geometric Design Guide
ever, impose limitations on the maximum value The spatial constraints in urban areas will very
of superelevation that can be applied. Icing of
often preclude the development of high values
the road surface is not a typical manifestation of
of superelevation. Because of congestion and
the South African climate but has been known to
the application of traffic control devices, the
occur in various high-lying parts of the country.
speeds achieved at any point along the road
Heavy rainfall reduces the available side friction
can fluctuate between zero and the posted
and relatively light rain after a long dry spell also
speed - or even higher depending on the local
reduces side friction.
level of law enforcement. Negotiating a curve
to areas where the road surface is polluted by
with a superelevation of 10 per cent at a crawl
rubber and oil spills, as is the case in urban
speed can present a major problem to the driv-
areas and the immediately surrounding rural
er. As a general rule, urban superelevations
areas. Where any of these circumstances are
should not exceed 6 per cent although, in the
likely to occur, a lower value of e
case of an arterial, this could be taken as high
mended. 4-8
Chapter 4: Road Design Elements
This applies particularly
max
is recom-
A lower value of emax should also be considered
•
in a road where steep gradients occur with any
applied to sustain lateral acceleration down to
frequency.
A superelevation of 10 per cent
radii requiring fmax followed by increasing e with
would present trucks with some difficulties when
reducing radius until e reaches emax. In short,
they are climbing a steep grade at low speeds.
first f and then e are increased in inverse pro-
As shown in Table 7.3, the combination of a
portion to the radius of curvature;
superelevation of 10 per cent and a gradient of
•
8 per cent has a resultant of 12,8 per cent at
of Method 2 with first e and then f increased in
approximately 45O to the centreline.
inverse proportion to the radius of curvature;
•
Method 2:
Method 3:
Method 4:
Side friction is first
Effectively the reverse
As
for
Method
3,
except that design speed is replaced by aver-
Whatever the value selected for e max, this value
age running speed, and
should be consistently applied on a regional
•
Method 5:
Superelevation
and
basis. Its selection governs the rate of superel-
side friction are in curvilinear relations with the
evation applied to all radii above the minimum.
inverse of the radius of curvature, with values
Variations in emax result in curves of equal
between those of Methods 1 and 3.
radius having different rates of superelevation. Drivers select their approach speeds to curves
These methods of distribution are illustrated in
on the basis of the radius that they see and not
Figure 4.2.
lack of consistency with regard to supereleva-
In terms of the design domain concept, Method
tion would almost certainly lead to differences in
2 has merit in the urban environment. As point-
side friction demand with possibly critical conse-
ed out earlier, provision of adequate superele-
Recommended rates of emax are
vation in an environment abounding in con-
quences.
offered in Table 4.2.
straints such as closely spaced intersections
Distribution of e and f
and driveways is problematic. It is thus sensible
There are a number of methods of distributing e
to make as much use as possible of side friction
and f over a range of curves flatter than the min-
before having to resort to the application of
imum for a given design speed. Five methods
superelevation.
are well documented by AASHTO. These are:
drivers operating at relatively low speeds in an
•
superelevation
urban environment are prepared to accept high-
and side friction are directly proportional to the
er values of side friction than they would at high
inverse of the radius;
speeds on a rural road.
Method 1:
Both
4-9 Chapter 4: Road Design Elements
It also should be noted that
Geometric Design Guide
on the degree of superelevation provided. A
Method 5 is recommended for adoption in the
bution for superelevation over the range of cur-
case of rural and high-speed urban roads. In
vature.
practice it represents a compromise between Methods 1 and 4. The tendency for flat to inter-
Tables 4.3, 4.4, 4,5 and 4,6 provide values of e
mediate curves to be overdriven is accommo-
for a range of horizontal radii and values of emax
dated by the provision of some superelevation.
of 4, 6, 8, and 10 per cent respectively.
The superelevation provided sustains nearly all
Superelevation runoff
lateral acceleration at running speeds (assumed
Geometric Design Guide
to be about 80 per cent of design speed) with considerable side friction available for greater
In the case of a two-lane road, superelevation
speeds. On the other hand, Method 1, which
runoff (or runout) refers to the process of rotat-
Figure 4.2: Methods of distributing of e and f avoids the use of maximum superelevation for a
ing the outside lane from zero crossfall to
substantial part of the range of curve radii, is
reverse camber (RC) thereafter rotating both
also desirable. Method 5 has an unsymmetrical
lanes to full superelevation. Tangent (or crown)
parabolic form and represents a practical distri-
runoff refers to rotation of the outside lane from 4-10
Chapter 4: Road Design Elements
zero crossfall to normal camber (NC). Rotation
drainage resulting in the possibility of storm
is typically around the centreline of the road
water ponding on the road surface. A further
although
driveway
cause of ponding could be where the centreline
entrances or drainage in the urban environment,
gradient is positive and equal to the relative gra-
may require rotation to be around the inside or
dient. In this case, the inner edge of the road
outside edge. These latter alternatives result in
would have zero gradient over the entire length
the distortion of the vertical alignment of the
of the superelevation runoff.
constraints,
such
as
road centreline and a severe slope on the road edge being rotated, with a potentially unaesthet-
Ponding is extremely dangerous for two rea-
ic end result. In the case of dual carriageway
sons. The more obvious danger is that it can
cross-sections, rotation is typically around the
cause a vehicle to hydroplane, causing a total
outer edges of the median island.
loss of traction and steering ability. If the front
The designer should be sensitive to the fact that
straight ahead when the vehicle moves out of
zero crossfall implies a lack of transverse
the ponded water, the sudden availability of fric-
Note:
NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the trav-
elled way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-11 Chapter 4: Road Design Elements
Geometric Design Guide
wheels are pointing in any direction other than
tion can lead to a sharp swerve and subsequent
which suggests that there is a maximum accept-
loss of control. The other possibility is that of
able difference between the gradients of the
one front wheel striking the water before the
axis of rotation and the pavement edge.
other, in which case the unbalanced drag could
Experience indicates that relative gradients of
also lead to the vehicle swerving out of control.
0,8 and 0,35 per cent provide acceptable runoff
The designer should therefore endeavour to
lengths for design speeds of 20 km/h and 130
avoid the combination of zero longitudinal gradi-
km/h respectively. Interpolation between these
ent and zero crossfall. A pond depth of 15 mm
values provides the relative gradients shown in
is sufficient to cause hydroplaning. In the case
Table 4.7.
of worn tyres, a lesser depth will suffice.
Geometric Design Guide
Many States of the United States have opted for The length of the superelevation runoff section
a standard relative gradient of 1:200, whereas
is selected purely on the basis of appearance,
Canada has elected to use a relative gradient of
Note:
NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the travelled
way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-12 Chapter 4: Road Design Elements
NC denotes Normal Camber, i.e. 2 per cent fall from the centreline to either edge of the trav-
elled way RC denotes Reverse Camber, i.e. 2 per cent crossfall from the outer edge of the travelled way to the inner edge 4-13 Chapter 4: Road Design Elements
Geometric Design Guide
Note:
1:400 in the calculation of the length of the
If the relative gradient approach to determina-
superelevation runoff.
tion of runoff length is adopted, this length is
Other widely used
options include adopting the distance travelled
calculated as
in four seconds and previous editions of the
4.10
AASHTO policy suggested the distance travelled in 2 seconds. As can be seen, there is a
where L
=
large degree of arbitrariness attaching to deter-
tion runoff (m)
mination of the length of superelevation runoff.
w
=
The designer can thus vary the relative gradient
Geometric Design Guide
length of supereleva width of one traffic lane, (m)
to accommodate other elements of the design,
n
=
number of lanes rotated
such as the distance between successive
ed
=
superelevation rate (per cent)
curves or the distance to the following intersec∆
tion. It is, however, suggested that the relative
=
relative gradient, (per cent)
gradients offered in Table 4.7 should provide a b
pleasing appearance and the designer should at
=
adjustment factor for number of lanes rotated
least attempt to achieve relative gradients of a similar magnitude.
Adjustment factor for number of lanes The gradient of the tangent runout is simply a continuation of whatever relative gradient was
If the above relationship is applied to cross-sec-
adopted for the superelevation runoff.
tions wider than two lanes, the length of the
4-14 Chapter 4: Road Design Elements
superelevation runoff could double or treble and
portion of the runoff located on the tangent and
there may simply not be enough space to allow
the balance on the curve.
for these lengths. On a purely empirical basis,
shown that having about 2/3 of the runoff on the
it is recommended that the calculated lengths
tangent produces the best result in terms of lim-
be adjusted downwards by the lane adjustment
iting lateral acceleration.
factors offered in Table 4.8.
demand, deviation by about 10 per cent from
Experience has
If circumstances
this ratio is tolerable.
Location of superelevation runoff The superelevation runoff and tangent runout
The two extremes of runoff location are:
are illustrated in Figure 4.3. Figures 4.4 and 4.5
Full superelevation attained at the
show possible treatments for superelevation
beginning of the curve (BC), and
•
runoff on reverse and broken back curves. In
Only tangent runout attained at the BC.
these cases, the superelevation runoff termi-
Both alternatives result in high values of lateral
nates at a crossfall of two per cent rather than
acceleration and are thus considered undesir-
the more customary zero camber on the out-
able. The preferred option would be to have a
side lane.
Figure 4.3: Attainment of superelevation 4-15 Chapter 4: Road Design Elements
Geometric Design Guide
•
Geometric Design Guide
Figure 4.4: Superelevation runoff on reverse curves
Figure 4.5: Superelevation runoff on broken-back curves 4-16 Chapter 4: Road Design Elements
If the circular curve is preceded by a transition
cubic parabola achieves a maximum value and
curve, all of the superelevation runoff should be
then flattens out again and is thus not a true spi-
located on the transition curve.
ral and the lemniscate requires an unacceptable length of arc to achieve the desired radius. The
4.2.5
Transition curves
clothoid, which has the relationship whereby the radius, R, at any point on the spiral varies with
Any vehicle entering a circular curve does so by
the reciprocal of the distance, L, from the start of
following a spiral path. For most curves, this
the spiral, is thus the preferred option.
transition can be accommodated within the lim-
Expressed mathematically, this relationship is R
its of normal lane width. At minimum radii for the
=
A2/L
(4.11)
design speed, longer transition paths are fol-
where A is a constant called the spiral parame-
lowed and, if these occur on narrow lanes, the
ter and has units of length.
shift in lateral position may even lead to encroachment on adjacent lanes. Under these
The length of a transition curve may be based
circumstances, it may be convenient to shape
on one of three criteria. These are:
the horizontal alignment such that it more accu-
•
Rate of change of centripetal accelera-
rately reflects the path actually followed by a
tion, essentially a comfort factor, varying
vehicle entering the circular curve.
between 0,4 m/s3 and 1,3 m/s3;
• •
Various curves can be used to provide a transi-
Relative slope as proposed in Table 4.3; or Aesthetics.
tion from the tangent to the circular curve. Whatever form is used, it should satisfy the con-
Since relative slope is applied to curves where
ditions that:
transitions are not provided, its use also for tran-
• •
It is tangential to the straight;
sitioned curves would be a sensible point of
Its curvature should be zero (i.e. infinite
departure. For the various design speeds, a
radius) on the straight;
radius corresponding to a specified centripetal
The curvature should increase (i.e.
acceleration can be calculated. These radii are
radius decrease) along the transition;
• •
listed in Table 4.9 for an acceleration of 1,3
Its length should be such that, at its junction with the circular curve, the full
m/s2. There is little point in applying transition
super elevation has been attained;
curves to larger radii where the centripetal
It should join the circular arc tangential-
acceleration would be lower.
ly, and
•
The radius at the end of the transition
Setting out of transitions
should be the same as that of the circular curve.
Application of the spiral has the effect that the Candidate curves are the lemniscate, the cubic
circular curve has to be offset towards its centre.
parabola (also known as Froude's spiral), and
It is thus located between new tangents that are
the clothoid (also known as Euler's spiral). The
parallel to the original tangents but shifted from
4-17 Chapter 4: Road Design Elements
Geometric Design Guide
•
4.2.6
them by an amount, s, known as the shift. The
Lane widening
value of the shift is given by When vehicles negotiate a horizontal curve, the s
=
L2/2R
rear wheels track inside the front wheels. In the
(4.12)
case of semi trailers with multiple axles and where L R
= =
selected length of tran
pivot points, this off-tracking is particularly
sition curve (m)
marked. The track width of a turning vehicle,
radius of circular curve (m)
also known as the swept path width, is the sum of the track width on tangent and the extent of
The starting point of the spiral is located at a dis-
off-tracking, with the off-tracking being a func-
tance, T, from the Point of Intersection (PI) of the
tion of the radius of the turn, the number and
original tangents with
location of pivot points and the length of the wheelbase between axles. The track width is
Geometric Design Guide
T
=
(R+s) tan θ/2 + L/2
calculated as
(4.13) where θ
=
U
=
u + R - (R2 - ΣLi2)0,5 (4.15)
deviation angle of the circular curve
where U
=
track width on curve (m)
u
=
track width on tangent (m)
The most convenient way to set out the spiral is
R
=
radius of turn (m)
by means of deflection angles and chords and
Li
=
wheel base of design
the deflection angle for any chord length, l, is
vehicle between suc-
given as
cessive axles and pivot
a
=
l2/6RL
points (m).
x 57,246 (4.14) 4-18 Chapter 4: Road Design Elements
Strictly speaking, the radius, R, should be the
FA
=
[R2 + A(2L + A)]0,5 - R
radius of the path of the midpoint of the front axle.
(4.16)
For ease of calculation, however, the
radius assumed is that of the road centreline.
where FA
=
width of front overhang (m)
The front overhang is the distance from the front
R
=
radius of curve (m)
axle of the vehicle to the furthest projection of
A
=
front overhang (m)
the vehicle body in front of the front axle. In the
L
=
wheel base of single
case of the turning vehicle, the width of the front
unit or tractor (m)
between the path followed by the outer front
The width of the rear overhang is the radial dis-
edge of the vehicle and the tyre path of the outer
tance between the outside edge of the inner
front wheel. The width of the front overhang is
rearmost tyre and the inside edge of the vehicle
calculated as
body. In the case of a passenger car this dis-
Figure 4.6: Typical turning path 4-19 Chapter 4: Road Design Elements
Geometric Design Guide
overhang is defined as the radial distance
WC
tance is typically less than 0,15 m. The width of
=
N (U + C) + FA(N - 1) + Z
truck bodies is usually the same as the wheel-
(4.18)
base width so that the width of the rear overhang is zero.
where N
=
number of lanes
and the other variables are as previously A typical turning path is illustrated in Figure 4.6.
defined.
Turning paths for numerous vehicles are provided in the 2000 edition of the AASHTO Policy on
As a general rule, values of curve widening,
geometric design of highways and streets and
being (WC - W) where W is the width of the trav-
the designer is directed towards Exhibits 2-3 to
elled way on tangent sections, that are less than
2-23 of that document.
0,6 m are disregarded. Lane widening is thus generally not applied to curves with a radius
It is necessary to provide an allowance, C, for
greater than 300 metres, regardless of the
lateral clearance between the edge of the road-
design speed or the lane width.
way and the nearest wheel path, and for the body clearance between passing vehicles.
Widening should transition gradually on the
Typical values of C are:
approaches to the curve so that the full addi-
•
0,60 m for a travelled way width of 6,0
tional width is available at the start of the curve.
m;
Although a long transition is desirable to ensure
0,75m for a travelled way width of 6,6
that the whole of the travelled way is fully
m, and
usable, this results in narrow pavement slivers
0,90 m for a travelled way width of 7,4 m.
that are difficult, and correspondingly expen-
• •
Geometric Design Guide
sive, to construct. In practice, curve widening is A further allowance, Z, is provided to accommo-
thus applied over no more than the length of the
date the difficulty of manoeuvring on a curve
super elevation runoff preceding the curve. For
and the variation in driver operation. This addi-
ease of construction, the widening is normally
tional width is an empirical value that varies with
applied only on one side of the road. This is
the speed of traffic and the radius of the curve.
usually on the inside of the curve to match the
It is expressed as
tendency for drivers to cut the inside edge of the travelled way.
Z
=
0,1(V/R0,5)
where V
=
design speed of the
(4.17)
In terms of usefulness and aesthetics, a tangent transition edge should be avoided. A smooth
road (km/h)
graceful curve is the preferred option and can be adequately achieved by staking a curved
By combining Eqs 4.12, 4.13 and 4.14 with the
transition by eye. Whichever approach is used,
clearance allowances, C and Z, the width of the
the transition ends should avoid an angular
travelled way can be calculated as
break at the pavement edge.
4-20 Chapter 4: Road Design Elements
Widening is provided to make driving on a curve
constant rate of change of bearing. It thus has
comparable with that on a tangent. On older
a certain academic appeal.
roads with narrow cross-sections and low design speeds and hence sharp curves, there
The general equation of the parabola is
was a considerable need for widening on
y = ax2 + bx +c,
curves. Because of the inconvenience attached
from which it follows that the gradient, dy/dx, at
to widening the surfacing of a lane, it follows that
any point along the curve is expressed as 2ax +
the required widening may not always have
b and the rate of change of gradient, d2y/dx2, is
been provided. Where a road has to be rehabil-
2a. This has the meaning of extent of change
itated and it is not possible to increase the
over a unit distance.
radius of curvature, the designer should consid-
express the rate of change in terms of the dis-
er the need for curve widening.
tance required to effect a unit change of gradi-
Normal usage is to
ent. This expression is referred to as the KIn the case of an alignment where curves in
value of the curve and is equal to 1/2a. It fol-
need of widening of the travelled way follow
lows that the length of a vertical curve can be
each other in quick succession, the inconven-
conveniently expressed as being
ience associated with the application of curve L
widening can be avoided by constructing the
=
AxK
(4.19)
entire section of road, including the intervening tangents, to the additional width.
4.3
where L
=
curve length (m)
A
=
algebraic difference
VERTICAL ALIGNMENT
between the gradients on either side of the
Vertical alignment comprises grades (often
curve
referred to as tangents) and vertical curves.
K
4.3.1
Grades have the properties of length and gradient,
=
rate of change
General controls for vertical
senting the height in metres gained or lost over a horizontal distance of 100 metres.
On rural and high-speed urban roads, a smooth grade line with gradual changes, which are con-
Curves may be either circular or parabolic, with
sistent with the class of the road and the char-
South African practice favouring the latter. The
acter of the terrain, is preferable to an alignment
practical difference between the two forms is
with numerous breaks and short lengths of
insignificant in terms of actual roadway levels
grades and curvature. A series of successive,
along the centreline. The parabola has the
relatively sharp crest and sag curves creates a
property of providing a constant rate of change
roller coaster or hidden dip profile which is aes-
of gradient with distance, which is analogous to
thetically unpleasant.
the horizontal circular curve, which provides a
safety concern, although, at night, the loom of 4-21
Chapter 4: Road Design Elements
Hidden dips can be a
Geometric Design Guide
alignment
invariably expressed as a percentage, repre-
approaching headlights may provide a visual
Where the total change of gradient across a ver-
clue about oncoming vehicles.
Such profiles
tical curve is very small, e.g. less than 0,5 per
occur on relatively straight horizontal alignments
cent, the K-value necessary to achieve the min-
where the road profile closely follows a rolling
imum length of curve would be high.
natural ground line.
these circumstances, the vertical curve could be omitted altogether without there being an
A broken-back grade line, which consists of two
adverse visual impact.
vertical curves in the same direction with a short length of intervening tangent, is aesthetically
The vertical alignment design should not be car-
unacceptable, particularly in sags where a full
ried out in isolation but should be properly coor-
view of the profile is possible. A broken plank
dinated with the horizontal alignment as dis-
grade line, where two long grades are connect-
cussed later. In addition to the controls imposed
ed by a short sag curve, is equally unaccept-
on the grade line by the horizontal alignment,
able. As a general rule, the length of a curve (in
the drainage of the road may also have a major
metres) should not be shorter than the design
impact on the vertical alignment. The top of a
speed in km/h. In the case of freeways, the min-
crest curve and the bottom of a sag imply a zero
imum length should not be less than twice the
gradient and the possibility of ponding on the
design speed in km/h and, for preference,
road surface. Where water flow off the road sur-
should be 400 metres or longer to be in scale
face is constrained by kerbs, the gradient
with the horizontal curvature. The broken-back
should be such that longitudinal flow towards
and broken-plank curves are the vertical coun-
drop inlets or breaks in the kerb line is support-
terparts of the horizontal broken-back curve and
ed.
the long tangent/small radius curve discussed
On lower-speed urban roads, drainage
design may often control the grade design.
earlier. The only difference between them is that these forms of vertical alignment are, at
There are, to date, no specific guidelines on
least, not dangerous.
Geometric Design Guide
Under
consistency of vertical alignment in terms of the
In theory, vertical curves in opposite directions
relative lengths of grades and values of vertical
do not require grades between them. In prac-
curvature, as is the case in horizontal alignment.
tice, however, the outcome is visually not suc-
However, where grades and curves are of
cessful. The junction between the two curves
approximately equal lengths, the general effect
creates the impression of a sharp step in the
of the grade line tends to be pleasing.
alignment, downwards where a sag curve fol-
4.3.2
lows a crest and upwards where the crest curve
Grades
follows the sag. A short length of grade between the two curves will create the impression of con-
The convention adopted universally is that a
tinuous, smoothly flowing vertical curvature.
gradient that is rising in the direction of increas-
The length of the intervening grade in metres
ing stake value is positive and a descending
need not be more than the design speed in km/h
gradient negative.
to achieve this effect.
interlinked in that steep gradients have an 4-22 Chapter 4: Road Design Elements
Gradient and length are
adverse effect on truck speeds and hence on
length of grade" typically taken as being the dis-
the operating characteristics of the entire traffic
tance over which a speed reduction of 15 km/h
stream. This effect is not limited to upgrades
occurs. For a given gradient, lengths less than
because truck operators frequently adopt the
the critical length result in acceptable operation
rule that speeds on downgrades should not
in the desired range of speeds.
exceed those attainable in the reverse direction.
desired freedom of operation is to be maintained
Where the
on grades longer than the critical length, it will It is desirable that truck speeds should not
be necessary to consider alleviating measures
decrease too markedly. Apart from the opera-
such as local reductions of gradient or the pro-
tional impact of low truck speeds, it has also
vision of extra lanes.
tion between crash rates and the speed differ-
Local research indicates that the 85th percentile
ential between trucks and passenger cars.
mass/power ratio is of the order of 185 kg/kW.
American research indicates that crash rates for
The performance of the 85th percentile truck is
speed reductions of less than 15 km/h fluctuate
illustrated in Figure 4.7. The critical lengths of
between 1 and 5 crashes per million kilometres
grade for a speed reduction of 15 km/h are
of travel increasing rapidly to of the order of 21
derived from these performance curves and are
Figure 4.7: Truck speeds on grades shown in Table 4.10.
crashes per million kilometres of travel for a speed reduction of 30 km/h. In the absence of South African research, it is presumed that a
As suggested earlier, grades longer than those
similar trend would manifest itself locally,
given in Table 4.10 may require some form of
although probably at higher crash rates. For
alleviating treatment. One such treatment is the
these reasons, reference is made to the "critical
provision of climbing lanes. This is discussed in 4-23
Chapter 4: Road Design Elements
Geometric Design Guide
been established that there is a strong correla-
Section 4.4.2. Stepping the grade line, i.e. by
readily be achieved under the three sets of cir-
inserting short sections of flatter gradient, as an
cumstances. Other factors that should be borne
alleviating treatment is sometimes offered as
in mind in selecting a maximum gradient
relief to heavy trucks at crawl speeds on steep
include:
gradients. In practice, this has proved to be
•
would suggest a reduction in maximum
ineffective because drivers of heavy trucks sim-
gradient in order to maintain an accept
ply maintain the crawl speed dictated by the
able Level of Service;
steeper gradient in preference to going through
•
the process of working their way up and down
costs, being the whole-life cost of the road and not merely its initial construc-
through the gears.
tion cost;
•
Maximum acceptable gradients shown in Table
property, where relatively flat gradients in a rugged environment may result in
4.11 are dictated primarily by the topography
high fills or deep cuts necessitating the
and the classification of the road. Topography is
acquisition of land additional to the nor-
described as being flat, rolling or mountainous
mal road reserve width;
• •
which is somewhat of a circular definition in the
Geometric Design Guide
traffic operations, where high volumes
sense that what is really being described is not
environmental considerations; and adjacent land use in heavily developed or urban areas
the topography itself but the gradients that can
4-24 Chapter 4: Road Design Elements
It is the designer's responsibility to select a max-
should be considered is of the order of 0,5 per
imum gradient appropriate to the project being
cent.
designed. The values offered in Table 4.11 are
kerbing, gradients should not be less than 0,5
thus only intended to provide an indication of
per cent. If the grade is longer than 500 metres,
gradients appropriate to the various circum-
increasing the camber to 2,5 or 3,0 per cent
stances.
should be considered. The latter value of cam-
It is recommended that, even without
ber should only be considered in areas subject Maximum gradients on freeways should be in
to heavy rainfall because it may give rise to
the range of three to four per cent regardless of
problems related to steering and maintaining the
the topography being traversed. Lower order
vehicle's position within its lane.
roads have been constructed to gradients as steep as twenty per cent but it is pointed out that
Crest curves are often in cut and, for the mini-
compaction with a normal 12/14 tonne roller is
mum value of K for a design speed of 120 km/h,
virtually impossible on a gradient steeper than
the gradient would be at a value of less than 0,5
about twelve per cent. It is recommended that
per cent for a distance of 55 metres on either
this be considered the absolute maximum gradi-
side of the crest. Over this distance, channel
ent that can be applied to any road.
grading should be applied to the side drains. On sag curves, the distance over which the longitu-
The minimum gradient can, in theory, be level,
dinal gradient is less than 0,5 per cent is 26
i.e. zero per cent. This could only be applied to
metres on either side of the lowest point at the
rural roads where storm water would be
minimum K-value.
and allowed to spill over the edge of the shoul-
Varying the camber between 2 per cent and 3
der. If used on a road that is kerbed, channel
per cent over a distance of about 80 metres will
grading would have to be employed. Given the
provide an edge grading at 0,5 per cent in the
limits of accuracy to which kerbs and channels
case where the centreline gradient is flat. As an
can be constructed, the flattest gradient that
alternative means of achieving adequate
Figure 4.8: Sight distance on crest curves 4-25 Chapter 4: Road Design Elements
Geometric Design Guide
removed from the road surface by the camber
drainage, this is more useful in theory than in
by the required sight distance is contained with-
practice because shaping and compacting the
in the length of the vertical curve.
road surface to have this wind in it is extremely
If the curve length is shorter than the required
difficult. This method is not unknown but is not
sight distance, lesser values of K can be
recommended because of the construction
employed as indicated by Equation 4.21.
problem. (4.21)
4.3.3
Curves where K
=
Distance required for a 1 % change of gradient (m)
As the parameter, K, has been described as the S
determinant of the shape of the parabolic curve,
=
Stopping sight distance
it follows that some or other value of K can be
for selected design
determined such that it provides adequate sight
speed (m)
distance across the length of the curve. Sight
h1
=
Driver eye height (m)
distance is measured from the driver eye height,
h2
=
Object height (m)
h1, to a specified object height, h2. In the case
A
=
Algebraic difference in
of a crest curve, the line of sight is taken as
gradient between the
being a grazing ray to the road surface, as illus-
approaching and depart-
trated in Figure 4.7.
ing grades (%) The values of K offered in Table 4.12 are based
Vertical curvature for stopping sight distance
on a curve length longer than the required stop-
The required value of K is derived from the
ping distance with a driver eye height of 1,05
equation of the parabola as indicated in
metres and various heights of object as dis-
Equation 4.20.
cussed in Chapter 3. K=
S2 200(h10,5 + h20,5)2
Geometric Design Guide
where K
S
=
=
(4.20)
Vertical curvature for passing sight distance
Distance required for a
Similar calculations can be carried out based on
1 % change of gradient
passing sight distance. High values of K result
(m)
so that, in the situation where the crest of the
Stopping sight distance
curve is in cut, the increase in volumes of exca-
for selected design
vation will be significant. Although the designer
speed (m)
should seek to provide as much passing sight
h1
=
Driver eye height (m)
distance as possible along the length of the
h2 A
= =
Object height (m) Algebraic difference in gradient between the approaching and depart ing grades (%).
road, it may be useful to shorten the crest curve in order to increase the lengths of the grades on either side rather than to attempt achieving passing sight distance over the crest curve itself.
This relationship applies to the condition where4-26
Chapter 4: Road Design Elements
Vertical curvature for barrier sight dis-
specifically in cases where the actual speed dif-
tance
ferentials between the overtaking and the overtaken vehicles are greater than those applied in
On undivided roads, barrier sight distance (also
the derivation of passing sight distances. K val-
referred to as non-striping sight distance) indi-
ues of crest curvature corresponding to barrier
cates whether no-passing pavement markings
sight distance are offered in Table 4.13.
are required. Barrier sight distance is shorter than passing sight distance.
Sag curves
The designer
or more wherever possible because passing
During the hours of daylight or on well-lit streets
manoeuvres can often be completed in less
at night, sag curves do not present any prob-
than the calculated passing sight distance
lems with regard to sight distance. Under these
4-27 Chapter 4: Road Design Elements
Geometric Design Guide
should attempt to provide barrier sight distance
circumstances, the value of K is determined by
existing pedestrian bridges, a clearance of 5,6
considerations of comfort, specifically the
metres may be accepted. This is suggested
degree of vertical acceleration involved in the
because pedestrian overpasses are relatively
change in gradient. The maximum comfortable
light structures that are unable to absorb severe
vertical acceleration is often taken as 0,3
m/s2.
impact and are more likely to collapse in such an event.
The increased vertical clearance
Where the only source of illumination is the
reduces the probability of damage to the struc-
vehicle's headlights, the line of sight is replaced
ture and improves the level of safety for pedes-
by a line commencing at headlight height, taken
trians using it.
as being 0,6 metres, and with a divergence angle of 1O relative to the grade line at the posi-
The criteria of comfort, sight distance and verti-
tion of the vehicle on the curve. This situation is
cal clearance lead to a minimum desirable
illustrated in Figure 4.9.
length of sag curve. A further criterion is that of drainage, whereby a minimum distance with a
Although not a frequent occurrence, sight dis-
gradient of less than 0,5 per cent is desired.
tance on a sag curve may be impaired by a structure passing over the road. Checking the
Where the stopping sight distance is less than
available sight distance at an undercrossing is best made graphically on the profile.
the length of the curve, the value of K is given
In this
by Equation 4.22.
case, the line of sight is a grazing ray to the soffit of the structure. The selected clearance is
K
thus of interest. Clearances are typically taken
=
____ S2___
(4.22)
120 + 3,5S
as 5,2 metres measured at the lowest point on the soffit. In the case of pedestrian bridges, the where S
Geometric Design Guide
clearance is 5,9 metres while, in the case of
=
Figure 4.9: Sight distance on a sag curve 4-28 Chapter 4: Road Design Elements
stopping distance (m)
4.4
The criterion of comfort, as expressed in
CROSS-SECTIONS
Equation 4.23, provides K-values roughly half of those dictated by considerations of stopping
The prime determinants of cross-section design
sight distance.
are:
• K
=
V2
where V
=
design speed (km/h)
/ 395
The function that the road is intended to serve;
(4.23)
•
The nature and volume of traffic to be accommodated; and
•
K-values for sag curves, as determined by
The speed of the traffic.
headlight distance and comfort, for the case where the sight distance is less than the length
Road function refers to a spectrum of needs
of the curve, are given in Table 4.14
ranging
from
accessibility
to
mobility.
Furthermore, a road can be classified on a variIn both crest and sag curves, K-values dictated
ety of different bases. A common feature of
by sight distance, where the curve length is
these considerations is that they largely concern
greater than the sight distance, can be used
addressing the needs of the occupants of a
throughout.
The alternative case, where the
moving vehicle. These needs may find expres-
sight distance is longer than the curve length,
sion in a desire for ready access to or from a
generally offers a lower K-value.
property adjacent to the road, freedom to
K-values appropriate to headlight distance
for high-speed long distance travel. High-speed
should be used in rural areas and also where
traffic requires more space than relatively slow-
street lighting is not provided.
Where street
moving traffic. Space takes the form of wider
lighting is provided, the lower K-values associ-
lanes, wider shoulders and (possibly) the inclu-
ated with the comfort criterion may be adopted.
sion of a median in the cross-section.
4-29 Chapter 4: Road Design Elements
Geometric Design Guide
manoeuvre in a terminal or intersection area or
All these needs have to be met in terms of over-
these lanes or, as a further development, to pro-
all objectives of safety, economy, convenience
vide cycle paths adjacent to or, for preference,
and minimum side effects.
removed from the travelled lanes.
In urban areas, road functions also have to
Although the horizontal and vertical alignments
include considerations of living space. People
are disaggregated in the sense that they are a
enjoy casual encounters, meeting people on
combination of tangents and curves, the cross-
neutral territory, as it were, without the obligation
section is heavily disaggregated, comprising a
of having to act as host or hostess in the home.
multitude of individual elements.
The sidewalk café, the flea market and window-
ments are illustrated in Figure 4.10. Design is
shopping all have to be accommodated within
thus concerned primarily with the selection of
the road reserve. All of these activities impact
elements that have to be incorporated within the
on the cross-section, which has to be designed
cross-section, followed by sizing of these indi-
accordingly.
vidual elements.
Traffic does not exclusively comprise motorised
In spite of this disaggregated approach to
vehicles. In developing areas, it may be neces-
design, there are numerous combinations of
sary to make provision for animal-drawn vehi-
elements that occur frequently. Cases in point
cles and, in this context, developing areas are
are:
not necessarily exclusively rural. The volume of
• • •
motorised vehicles will have an impact on the design of the cross-section with regard to the
Two-lane two-way roads; Two-plus-one roads; Four-lane undivided and divided roads, and
number of lanes that have to be provided. High
•
volumes of moving vehicles will generate a need for special lanes such as for turning, pass-
Four, six (or more) lane freeways.
Each of these composite cross-sections leads to
ing, climbing or parking.
Geometric Design Guide
These ele-
the development of a standard road reserve
In urban areas, the presence of large numbers
width that often is enshrined in legislation. In
of pedestrians will require adequate provision to
this section, the individual elements rather than
be
the composite cross-section will be discussed.
made
in
terms
of
sidewalk
widths.
Pedestrians are also to be found on rural roads.
4.4.1
On rural roads, speeds are high so that crashes
General controls for cross-sections
involving pedestrians are inevitably fatal. It is thus sensible to make at least modest provision
Safety is a primary consideration in the design
for pedestrians on rural roads, even though their
of the cross-section. The safety of the road user
numbers may be low.
refers to all those within the road reserve, whether in vehicles or not.
Cyclists can often be accommodated on the normal travelled lanes but, when the number of
Wide lanes supposedly promote the safety of
cyclists increases, it may be necessary to widen
the occupants of vehicles although current evi4-30
Chapter 4: Road Design Elements
dence suggests that there is an upper limit
other protected land use. During the planning
beyond which safety is reduced by further
process, it is prudent to attempt to acquire addi-
increases in lane width. The reverse side of the
tional road reserve width to allow for improve-
coin is that wide lanes have a negative impact
ments to traffic operations, auxiliary lanes, wider
on the safety of pedestrians attempting to cross
pedestrian areas, cycle paths as well as for pro-
the road or street. South Africa has a particu-
vision of utilities, streetscaping and mainte-
larly bad record in terms of pedestrian fatalities,
nance considerations.
which account for approximately half of the total
not be taken to the extreme, where large tracts
number of fatalities. In devising safe cross-sec-
of land are unnecessarily sterilised in anticipa-
tions, it is therefore necessary to consider the
tion of some or other future eventuality.
This should, however,
needs of the entire population of road users and not just those in vehicles.
The location of existing major utilities, which may be either above or below ground, and diffi-
In urban areas, it is necessary to make provision
cult or costly to relocate is a fairly common
for boarding and alighting public transport pas-
design control in urban areas. In particular the
sengers, disabled persons and other non-vehic-
location of aboveground utilities in relation to
ular users of the facility in addition to accommo-
clear zone requirements should be carefully
dating pedestrians and cyclists. In these areas,
considered.
design speed usually plays a lesser role in the design of the cross-section.
In the discussion that follows, dimensions are
Land availability is of particular importance in
field" designs. Very often, however, the design-
urban areas. Land for the road reserve may be
er does not have this level of freedom of choice.
restricted because of the existence of major
For example, rehabilitation projects often
buildings or high cost of acquisition or some or
require the creation of additional lanes without
Figure 4.10: Cross-section elements 4-31 Chapter 4: Road Design Elements
Geometric Design Guide
essentially discussed in terms of new or "green
the funding or space available for additional
Similarly, a fourth lane on a 13,4 metre wide
construction. The choice then is one of either
cross-section would imply lane widths of 3,1
accepting a lane or shoulder width that is nar-
metres and a zero width of shoulder if the shoul-
rower than would be desired or foregoing the
der rounding of 0,5 metres is to be maintained.
additional lanes.
The traffic volumes necessitating consideration of a four-lane cross-section would render such a
As discussed below, the preferred cross-section
configuration of lanes and shoulders highly
for a two-lane two-way road has a total width of
undesirable.
13,4 metres. This comprises lane widths of 3,7
4.4.2
metres and usable shoulder widths of 2,5 metres plus a 0,5 metre allowance for shoulder
Basic Lanes
Basic lanes are those that are continuous from
rounding. If a cross-section of this width already
one end of the road to the other. The number of
exists and it is deemed necessary to incorporate
lanes to be provided is largely determined by
a climbing lane without incurring extra construc-
traffic flow and the desired Level of Service that
tion costs, this can be achieved by accepting a
the road is to provide. Reference is thus to the
climbing lane width of 3,1 metres with the adja-
Highway Capacity Manual.
cent shoulder having a width of 1,0 metres. The through lanes have to be reduced to a width of
The anticipated traffic speed offers an indication
3,4 metres each and the opposite shoulder to a
of the required width of lane. Lane widths typi-
width of 1,5 metres thus allowing for the shoul-
cally used are 3,1 metres, 3,4 metres and 3,7
der rounding of 0,5 metres on both shoulders.
metres. Research has indicated that the crash rate starts to show a marginal increase above a
Many of the older cross-sections have a total
lane width of 3,6 metres. It is accordingly rec-
paved width between shoulder breakpoints of
ommended that widths significantly greater than
12,4 metres, comprising lanes with a width of
3,7 m should not be employed.
authorities use lane widths of 5,4 metres on
and 0,5 metres of shoulder rounding on both
major routes.
sides. Three lanes with a width of 3,1 metres
Geometric Design Guide
Some local
3,4 metres, shoulders with a width of 2,3 metres
The intention is apparently to
make provision for parking or cycle traffic with-
each would allow for shoulders that are approx-
out demarcating the lanes as such. In practice,
imately 1 metre wide and 1 metre of shoulder
passenger cars are very inclined to use these
rounding. It is suggested that the dimensions
lanes as though they were two unmarked lanes
offered for the three-lane cross-section consti-
2,7 metres in width. Although this is a very eco-
tute an irreducible minimum and can be consid-
nomical method for providing adequate capaci-
ered as a palliative measure only over a rela-
ty, the safety record of such cross-sections may
tively short distance, e.g. to accommodate a
be suspect and their use should be discour-
climbing lane. Accommodating a third lane on
aged.
cross-sections narrower than 12,4 metres between shoulder breakpoints should not be
The narrowest width recommended for consid-
essayed.
eration (3,1 metres) allows for a clear space of 4-32 Chapter 4: Road Design Elements
300 mm on either side of a vehicle 2,5 metres
shoulder, or a cross fall, being a slope from one
wide. This width would only be applied to roads
edge of the travelled way to the other edge.
where traffic volumes and/or speeds are expect-
These slopes are provided to ensure drainage
ed to be low.
of the road surface and are typically at two per cent although, in areas where high rainfall inten-
Where traffic volumes are such that a multi-lane
sities are likely to occur, the slope could be
or divided cross-section is required, 3,7 metres
increased to as much as three per cent.
would be a logical width to adopt. High speeds
4.4.3
would also warrant this width because, on a nar-
Auxiliary lanes
rower lane, momentary inattention by a driver could easily cause a vehicle to veer into the path
Auxiliary lanes are located immediately adjacent
of another. Intermediate volumes and speeds
to the basic lanes. They are generally short and
are adequately accommodated by a lane width
are provided only to accommodate some or
of 3,4 metres.
other special circumstance.
In the case of a rehabilitation or reconstruction
Auxiliary lanes are often used at intersections.
project, it may be necessary to add lanes to the
They can be turning lanes, either to the left or to
cross-section. The cost of the additional earth-
the right, or through lanes. The turning lanes
works may however be so prohibitive that the
are principally intended to remove slower vehi-
funds required to upgrade the road to full 3,7
cles, or stopped vehicles waiting for a gap in
metre lanes and 3,0 metre shoulders may sim-
opposing traffic, from the through traffic stream
ply not be available. The options available to
hence increasing the capacity of the through
the road authority are then either not to upgrade
lanes.
the road at all or to accept some lesser widths of
employed at signalised intersections to match
lanes and/or shoulders. It is suggested that a
the interrupted flow through the intersection
stepwise approach to the problem, first reducing
area with the continuous flow on the approach
the available width of shoulder and thereafter
lanes.
considering reductions in lane width, could be
lanes are discussed in depth in Chapter 6.
adopted by.
Auxiliary lanes are also employed at inter-
The application and design of these
safe, even at a speed of 120 km/h, but is not a
changes.
These are through lanes and are
comfortable solution when applied over an
intended to achieve lane balance where turning
extended distance. A reduction in the posted
volumes are sufficient to warrant multi-lane on-
speed limit may be desirable if the lesser lane
and off-ramps. They may also be used between
width is applied over several kilometres. At still
closely spaced interchanges to support weav-
lower lane widths, it may be advisable to provide
ing, principally replacing a merge-diverge with a
a posted speed limit of 100 km/h even for rela-
Type A weave. Auxiliary lanes at interchanges
tively short sections of road.
are discussed in Chapter 7.
Travelled lanes have either a camber, being a
Climbing lanes and passing lanes are auxiliary
slope from the centreline towards the outside
lanes employed on network links, i.e. other than 4-33
Chapter 4: Road Design Elements
Geometric Design Guide
A 3,4 metre lane is reasonably
Through auxiliary lanes are often
Climbing
last-second manoeuvres occur. In the case of
lanes are often referred to as truck lanes,
merging, such manoeuvres can be extremely
crawler lanes, overtaking lanes or passing
hazardous. Reference should be made to the
lanes. The function of climbing lanes is, howev-
SADC Road and Traffic Signs Manual for the
er, very different from that of passing lanes.
recommended signage and road markings.
at intersections and interchanges.
Climbing lanes remove slower vehicles from the The lane drop should not be located so far from
traffic stream and have the effect of reducing the
the end of the need for the auxiliary lane that
number of passenger car equivalents (PCE's) in the stream.
drivers accept the increase in number of lanes
If the reduction is sufficient, the
and do not expect the reduction. Furthermore,
Level of Service (LOS) on the grade will match
the location should not be such that the lane
that on the preceding and succeeding grades.
drop is effectively concealed from the driver.
Passing lanes also remove slower traffic from
Concealment can arise from location immedi-
the stream but, in this case, the objective is pla-
ately beyond a crest curve. An "architectural"
toon dispersal, thus supporting an increase in
approach which locates the lane drop on a hor-
the capacity of the road. In short, the climbing
izontal curve and selects curve radii that result
lane seeks to match the LOS on a steep grade
in a smooth transition from two lanes to one
to that on the preceding and following flatter
across the length of the curve is a particularly
grades, whereas the passing lane improves the
subtle form of concealment that should be
capacity of the road as a whole. The ultimate
avoided at all costs. To simplify the driving task,
demonstration of the latter circumstance is the
the lane drop should be highly visible to
four-lane road, which could be described as a
approaching traffic and drivers should not be
two-lane road with continuous passing lanes in
subjected to successive decision points too
both directions.
quickly implying that lane drops should be removed from other decision points.
From a safety point of view, it is important that drivers are made aware of the start and, more
It is possible that the shoulder adjacent to an
particularly, the end of an auxiliary lane. The
auxiliary lane could be very narrow. However,
Geometric Design Guide
basic driver information requirements in the latter
for emergency use it is recommended that a
case are:
•
three metre wide surfaced shoulder be provided
Indication of the presence of a lane
along and extending downstream of the taper
drop;
•
for a distance equal to the stopping sight dis-
Indication of the location of the lane
tance for the design speed of the road.
drop; and
•
Indication of the appropriate action to
Climbing lanes
be undertaken. Four types of warrants for climbing lanes are in It has been observed that, without adequate
use. These are:
signposting and road markings indicating the
•
presence of a lane drop to drivers, erratic and
Reduction of truck speed through a given amount or to a specified speed;
4-34 Chapter 4: Road Design Elements
• • •
Reduction in truck speed in association
before the climbing lane is warranted.
The
with a specified volume of traffic;
speed reduction applied is 20 km/h from an ini-
Reduction in LOS through one or more
tial 80 km/h. The volume warrant is given in
levels, and
Table 4.15 below.
Economic analysis. Truck speed reductions without reference to
The first three focus on the performance of
traffic volumes have merits in terms of safety.
trucks and infer some or other impact on pas-
Their principal benefit lies in reduction in the
senger vehicles. The fourth directly quantifies
speed differential in the through lane, thereby
the effect of slow moving vehicles (which need
reducing the probability of the occurrence of a
not necessarily be exclusively truck traffic) on
crash. It is, however, theoretically possible that
the traffic stream in terms of delay over the
a climbing lane would be considered warranted
design life of the climbing lane and compares
merely because it would lead to the required
the benefit of the removal of delay with the cost
truck speed reduction even if total traffic vol-
of providing and maintaining the climbing lane.
umes were very low with virtually no trucks in
Speed reductions adopted internationally vary in
the traffic stream. The addition of a volume war-
the range of 15 km/h to 25 km/h and are usual-
rant increases the likelihood of a reasonable
ly intended to be applied to a single grade. The
economic return on the provision of a climbing
most widely occurring value of speed reduction
lane.
is 15 km/h, based on considerations of safety.
warrants such as those described above are not
The Australian approach bases the need for
intended to be - or are ever likely to be - fully
climbing lanes on examination of a considerable
economic.
It is conceded that performance-based
lanes is based on traffic volume, percentage of
Level of Service is a descriptor of operational
trucks and the availability of passing opportuni-
characteristics in a traffic stream. An important
ties along the road. Speed reduction is to 40
feature is that it is purely a representation of the
km/h and not by 40 km/h.
driver's perception of the traffic environment and
South African practice, as described in TRH17,
is not concerned with the cost of modifying that
uses a combination of the speed and traffic vol-
environment. The warrants suggested by the
ume as a warrant, requiring both to be met
Highway Capacity Manual are: 4-35
Chapter 4: Road Design Elements
Geometric Design Guide
length of road. The justification for climbing
• •
A reduction of two or more Levels of
climbing lanes are variously referred to as pass-
Service in moving from the approach
ing bays, turnouts or partial climbing lanes and
segment to the grade, or
are typically 100 to 200 metres long. Because
Level of Service E existing on the grade.
vehicles entering the turnout do so at crawl speeds, the tapers can be very short, e.g. twen-
The Highway Capacity Manual warrant is
ty to thirty metres long, corresponding to taper
severe and does not, in any event, match the
rates of 1:6 to 1:10 in the case of 3,1 metre wide
speed reduction warrant. The provision of a
lanes.
climbing lane at a specific site is thus dependent on the type of warrant selected. It is according-
Climbing lanes usually have the same width as
ly suggested that, if a designer wishes to apply
the adjacent basic lane. In very broken terrain,
this warrant, the reduction considered should be
a reduction in width to as little as 3,1 metres can
of one or more Levels of Service, and not two or
be considered because of the low speeds of
more.
vehicles in the climbing lane.
On the same
grounds, the shoulder width may also be In view of the economic restraints on new con-
reduced but to not less than 1,5 metres. If the
struction, a compromise between convenience
shoulders elsewhere on the road are three
and cost effectiveness is required. The com-
metres wide, the additional construction width
promise proposed is that, while delay - seen as
required to accommodate the climbing lane and
a major criterion of Level of Service - is
reduced shoulder is thus only 1,6 metres.
Geometric Design Guide
employed in determination of the need for provision of a climbing lane, the delay considered
While the decision whether or not to provide the
would not be that suffered by the individual vehi-
climbing lane could be based on the economics
cle but rather by the entire traffic stream.
of the matter, the location of its terminals is
Commercially available software calculates the
dependent purely on safety based on the oper-
value of the time saved during the design life of
ational characteristics of truck traffic. The war-
the climbing lane and relates this to input from
rant for the provision of climbing lanes in terms
the designer in respect of construction and
of truck speed reduction is set at 20 km/h as
maintenance costs of the lane.
described previously. It is suggested, as a safety measure to allow for variation in the hill climb-
In mountainous terrain, where trucks are
ing capabilities of individual trucks, that, having
reduced to crawl speeds over extended dis-
established that a climbing lane is warranted, a
tances and relatively few opportunities for over-
speed reduction of 15 km/h be used to deter-
taking exist, the cost of construction of climbing
mine the location of the climbing lane terminals.
lanes may be prohibitive. Under these circum-
Table 4.10 shows critical lengths of grade in
stances, an alternative solution to the opera-
terms of a truck speed reduction of 15 km/h for
tional problem may be to construct short lengths
various gradients. It is recommended that the
of climbing lane as opposed to a continuous
full width of the climbing lane be provided at or
lane over the length of the grade. These short
before the end of the critical length, with this 4-36
Chapter 4: Road Design Elements
length being measured on the preceding sag
The terminal may take one of two possible
curve from a point halfway between the Vertical
forms. One option is to provide a right-to-left
Point of Intersection (VPI) and the end of the
taper merging the basic lane with the climbing
vertical curve (EVC). The full width of the climb-
lane, followed by a left-to-right taper back to the
ing lane should be maintained until the point is
two-lane cross-section. The motivation for this
reached where truck speed has once again
layout is that faster-moving vehicles find it easy
increased to be 15 km/h less than the normal
to merge with slower-moving traffic. This is its
speed on a level grade.
only advantage. Should a vehicle not manage to complete the merge before the end of the
The length of the entrance taper should be such
right-to-left taper, its only refuge is the painted
that a vehicle can negotiate the reverse curve
island, thereafter being confronted by an oppos-
path with the benefit of a 2 per cent crossfall on
ing lane situation. Furthermore, in this layout,
the first curve followed by a negative superele-
the basic lane terminates at the end of the
vation of 2 per cent on the second curve with a
climbing lane with the climbing lane thereafter
short intervening tangent to allow for the rever-
becoming the basic lane. This is a contradiction
sal of curvature. Ideally, it should not be neces-
of the fundamental definitions of the basic and
sary for the vehicle path to encroach on the
auxiliary lanes. This option is not recommend-
shoulder. These conditions can be met by a
ed for two-lane roads, although it could possibly
taper which is about 100 metres long, corre-
be used on multilane or divided cross-sections.
sponding to a taper rate of 1 : 27.
This is
approximately half the taper rate applied to
The alternative is to provide a simple taper,
interchange off-ramps, which is appropriate as,
dropping the climbing lane after it has served its
in this case, the taper addresses a reverse path
purpose. A vehicle that cannot complete the
and not a single change of direction.
merging manoeuvre at the end of the climbing lane has the shoulder as an escape route. This is a safer option than that described above.
As stated above, the exit terminal of the climbing lane should be the point at which trucks
As described with regard to the entrance taper,
have accelerated back to a speed that is, at
negotiate a taper that is 100 metres in length.
speeds on the basic lanes. If there is a barrier
However, a flatter taper would allow time to find
line at this point, the lane should be extended to
a gap in the opposing traffic. A taper rate of 1:50
the point at which the barrier line ends. The rea-
is suggested for on-ramps at interchanges
son for this is that a vehicle entering the basic
where vehicles are required to negotiate only a
lane may inadvertently force an overtaking vehi-
single change of direction. The reverse curve
cle into the opposing lane. This is a potentially
path followed in exiting from an auxiliary lane
dangerous situation and the designer must
may require a still flatter taper rate for which rea-
ensure that there is sufficient sight distance to
son a rate of the order of 1:70 is suggested,
support appropriate decisions by the drivers
leading to a taper that is approximately 200
involved.
metres in length. 4-37 Chapter 4: Road Design Elements
Geometric Design Guide
a vehicle exiting from the climbing lane could
most, 15 km/h slower than the truck operating
Passing lanes
there is an absence of passing opportunities. They are aimed at platoon dispersion and local
The procedure followed in the design of the
research has demonstrated that a passing lane
alignment of a road should seek, in the first
length of about one kilometre is adequate for
instance, to provide the maximum possible
this purpose.
passing opportunity. Thereafter, the need for
Numerous short passing lanes
are preferable to few long passing lanes and it is
climbing lanes should be evaluated and deci-
recommended that they be located at two, four
sions taken on where climbing lanes are to be
and eight kilometre spacings. Where traffic vol-
provided and what the lengths of these climbing
umes are low, the longest spacing can be used
lanes should be. At this stage, it is possible to
and, as traffic volumes increase, the intervening
relate the total passing opportunity to the overall
lanes can be added in a logical manner.
length of the road. With one-kilometre long passing lanes provided The analysis of two-lane roads as described in
at two-kilometre intervals, the next level of
the Highway Capacity Manual is based on two
upgrading would be a Two + One cross-section.
criteria, being percentage time spent following
In this case, the road is effectively provided with
and average travel speed. Both of these are
a three-lane cross-section from end to end with
adversely influenced by inadequate passing
the centre lane being alternately allocated to
opportunities. For example, a road with sixty
each of the opposing directions of flow. In keep-
per cent passing opportunity demonstrates a 19
ing with the spacings discussed above, the
per cent increase in time spent following by
switch in the direction of flow in the centre lane
comparison with a road with similar traffic vol-
should be at about two-kilometre intervals.
umes and unlimited passing opportunities. This assumes a 50/50 directional split. The increase
Unlike climbing lanes, passing lanes tend to
in time spent following is greater with unbal-
operate at the speeds prevailing on the rest of
anced flows. If the flow is as low as 800 vehi-
the road. Reductions in lane width are thus not
cles per hour, passing opportunities limited to 60
recommended and passing lanes should have
per cent result in a reduction of the order of
the same width as the basic lanes.
Geometric Design Guide
three per cent in average speeds by comparison with the speeds on roads with unlimited passing
As recommended for climbing lanes, the
opportunity. It is accordingly necessary for the
entrance taper to a passing lane could be 100
designer to carry out an analysis based on the
metres in length and the length of the exit taper
Highway Capacity Manual to establish whether
double this to allow adequate time for merging
or not additional passing opportunities should
vehicles to find a gap in the through flow.
be provided in order to maintain the desired
Seeing that both the entrance and the exit
Level of Service.
tapers signal a change in operating conditions on the road, it is recommended that decision
Passing lanes are normally provided in areas
sight distance should be available at these
where construction costs are low and where
points. 4-38
Chapter 4: Road Design Elements
High occupancy vehicle (HOV) lanes
in the cross-section. The essential point of difference lies in the fact that the passenger car is
As described above, auxiliary lanes are short
usually taken as the design vehicle for basic
and are intended only to deal with a specific cir-
lanes whereas the HOV lanes are designed to
cumstance.
As soon as this circumstance
accommodate buses. Kerb radii at intersections
changes, the auxiliary lane is dropped. HOV
and the width of the turning lanes on bus routes
lanes, on the other hand, form part of the rapid
should be such that buses can negotiate these
transit system of a city and can thus be provid-
curves without encroaching on the adjacent
ed over a substantial distance. Whether they
lanes or, more importantly, on the sidewalks.
should be considered as auxiliary lanes could thus be debated.
Bus routes typically converge on the CBD. The HOV lanes, which, as part of the normal cross-
HOV lanes are typically applied on commuter
section, may have served well in the outlying or
routes with a view to encouraging the use of
suburban areas, could prove inadequate to
public transport or lift clubs hence reducing con-
accommodate the increased volume of bus traf-
gestion. The average occupancy of passenger
fic in the CBD. It may then be necessary to des-
cars is of the order of 1,5 persons per vehicle,
ignate various of the streets in the CBD as
whereas a municipal bus can convey 80 pas-
exclusive bus roads or, alternatively, to consider
sengers effectively replacing 50 or more pas-
a system similar to the O-Bahn routes.
senger cars in the traffic stream. In view of the fact that buses can be 2,6 metres wide, narrow
4.4.4
Kerbing
lane widths are inappropriate to HOV lanes that, ideally, should not be narrower than 3,6 metres,
Kerbs are raised or near-vertical elements that
i.e. allowing a clear space of 0,5 metres
are located adjacent to the travelled way and
between the sides of the vehicle and the lane
are usually used for:
• • •
HOV lanes have to be policed to ensure that only vehicles qualifying for the privilege use
Drainage control; Delineation of the pavement edge; and Reduction in maintenance operations by providing protection for the edge of
them. Signalisation can be employed to give
surfacing.
vehicles in HOV lanes priority over other road
Kerbing is normally only applied in urban areas
users. This is described in the South African
where vehicle speeds are relatively low.
Road Traffic Signs Manual in some detail. The combination of policing and priority usage
In rural areas, the drainage function is normally
underpins the effectiveness of HOV lanes but
accommodated by channels or open drains of
these operational issues are normally outside
various forms. Delineation is usually by means
the terms of reference of geometric design.
of an edge line or a contrasting colour on the
It is important that the designer draws a distinc-
shoulder. Protection of the edge of surfacing
tion between the basic lanes and the HOV lanes
can be by means of buried edge blocks or, more 4-39
Chapter 4: Road Design Elements
Geometric Design Guide
markings.
typically, by means of a thickened edge. A thick-
drainage. Perhaps their widest application is to
ened edge is simply a bitumen-filled V-shaped
be found in residential areas, where vehicles
groove cut into the base course.
can drive off the travelled way to park on the verge.
Kerbs may be barrier or semi-mountable or mountable. Barrier and semi mountable kerbs
Channels are usually about 300 mm wide, thus
normally are accompanied by a channel (or gut-
automatically providing an offset between the
ter) whereas the mountable kerb is, in effect, a
kerb and the edge of the travelled way. Where
channel itself.
channels are not provided, the offset should still be maintained for reasons of safety.
Barrier kerbs are intended primarily to control
4.4.5
drainage as well as access and can inhibit slow-
Shoulders
moving vehicles from leaving the roadway. When struck at high speeds, barrier kerbs can
Shoulders are the usable areas immediately
result in loss of control and damage to the vehi-
adjacent to the travelled way and are a critical
cle. In spite of the name, barrier kerbs are inad-
element of the roadway cross-section.
equate to prevent a vehicle from leaving the
provide:
road after a high-speed impact. In addition, a
• •
barrier kerb can lead to a high-speed errant
A recovery area for errant vehicles; A refuge for stopped or disabled vehicles;
vehicle vaulting over a barrier or guardrail. For
•
this reason, barrier kerbing is not generally used
An area out of the travel lanes for emergency and maintenance vehicles; and
on urban freeways and is considered undesir-
Geometric Design Guide
They
able on expressways and arterials with design
•
speeds higher than 70 km/h. Barrier kerbs are
In addition, shoulders support use of the road by
never used in conjunction with rigid concrete
other modes of transport, for example cyclists
barrier systems.
and pedestrians.
Semi-mountable kerbs have a face slope of 25
Regulation 298 of the regulations promulgated
mm/m to 62,5 mm/m and are considered mount-
in terms of National Road Traffic Act (Act
able under emergency conditions. They are typ-
93/1996) prohibits driving on the shoulder
ically used on urban freeways and arterials and
except that this is permitted:
also in intersections areas as a demarcation of
• • •
raised islands.
Lateral support of the roadway structure.
On a two-lane road; Between the hours of sunrise and sunset, While being overtaken by another vehicle.
Mountable kerbs have a relatively flat sloping
provided this can be done without endangering
face of 10 mm/m to 25 mm/m and can be
the vehicle, other vehicles, pedestrians or prop-
crossed easily by vehicles. They are particular-
erty and if persons and vehicles on the road are
ly useful as a form of lane demarcation on high-
clearly discernable at a distance of at least 150
speed roads but are not effective as a form of
metres. 4-40
Chapter 4: Road Design Elements
Considering the above applications of the shoul-
Between the two extremes of 3,0 metres and
der, a stopped vehicle can be accommodated
1,0 metres, shoulder widths of 1,5 or 2,5 metres
on a shoulder that is three metres wide. There
could be used in the case of intermediate traffic
is no merit in adopting a shoulder width greater
volumes and speeds. These alternative shoul-
than this. The shoulder should, however, not be
der widths would not normally be used for the
so narrow that a stopped vehicle could cause
inner shoulders of a dual carriageway road.
congestion by forcing vehicles travelling in both
Table 4.16 illustrates the application of the vari-
directions into a single lane. A partly blocked
ous shoulder widths on undivided rural roads.
lane is acceptable under conditions of low speed and low traffic volume. Assuming the
Paved widths of between 1,5 and 2,5 metres
narrowest width of lane, i.e. 3,1 metres, it would
should be avoided. The presence of the paving
be possible for two vehicles to pass each other
may tempt a driver to move onto the shoulder to
next to a stopped car if the shoulder were not
allow another vehicle to overtake, but these
less than 1,0 metres wide. Hazards, including
widths cannot accommodate a moving vehicle
the edges of high fills, cause a lateral shift of
with any safety.
vehicles if closer to the lane edge than 1,5 metres. Allowing for shoulder rounding of 0,5
These shoulder widths are recommended for
metres, the usable shoulder is thus 1,0 metres
adoption for new construction. In the case of
wide and this should be considered the irre-
rehabilitation or reconstruction projects, there
ducible minimum width of shoulder.
may not be sufficient width of cross-section to accommodate the desirable widths and some
Where the traffic situation demands a dual-car-
lesser width will have to be considered.
riageway cross-section, the greatest width of
pointed out in Section 4.4.2, it may be advisable
shoulder, i.e. three metres, is called for. This
to first reduce the shoulder width before consid-
width would apply to the outer shoulder. The
ering reductions of lane width.
As
inner shoulder need only be one metre wide: to protect the integrity of the pavement-
The shoulder breakpoint is usually about 500
layers;
• •
mm beyond the edge of the usable shoulder to
to avoid drop-offs at the lane edge; and
allow for shoulder rounding.
provide space for roadmarkings
provided the median island is not kerbed, thus
Where guardrails or other roadside appurte-
allowing a disabled vehicle to be moved clear of
nances have to be provided, these are located
the adjacent lane. If a barrier, such as kerbing
300 to 500 mm beyond the usable shoulder.
or a guardrail, makes the median island inac-
The shoulder breakpoint should be a further 500
cessible, the full shoulder width should be pro-
mm beyond these appurtenances, as a lesser
vided in the case of a six-lane cross section
distance will not provide the support needed by
because negotiating two lanes to reach the
a guardrail when hit by an errant vehicle.
safety of the outside shoulder (with a disabled vehicle) could be difficult. 4-41
Chapter 4: Road Design Elements
Geometric Design Guide
•
The surfacing of shoulders is recommended:
should constitute adequate grounds for full sur-
• • •
facing of the shoulders.
For freeways; In front of guardrails; Where the total gradient, being the
Full surfacing implies continuous surfacing
resultant of the longitudinal gradient and
along the length of the road and not necessarily
the camber or superelevation, exceeds
across the full width of the shoulder, although
six per cent;
•
this is the desirable option.
Where the materials with which the
above that the minimum recommended width of
shoulders are constructed are readily
• • •
It is suggested
erodible, or where the availability of
shoulder is 1,0 metres. If it were considered
material for maintenance of the shoul-
necessary to surface the shoulder at all, there
ders is limited;
would be little or no operational advantage in
Where heavy vehicles would tend to use
surfacing a lesser width than this. In the case of
the shoulder as an auxiliary lane;
new construction, the designer has the option of
In mist belts; or
considering the economic merits of a relatively
Where significant usage by pedestrians
narrow surfaced shoulder vis-à-vis a wide
occurs.
unsurfaced shoulder. In the case of rehabilita-
Geometric Design Guide
tion projects, it may be decided to retain the full A patchwork of surfaced shoulders would be
3,0 metre shoulder but, as a cost-saving meas-
both unsightly and unsafe. Where the interven-
ure, to surface only half of the total width.
ing lengths of unsurfaced shoulders are short, it is suggested that they also be surfaced. As a
The cross fall on surfaced shoulders is normally
guideline, it is proposed that if surfacing sixty or
an extension of that on the travelled lanes.
more per cent of the shoulder is warranted, this
Where shoulders are not surfaced, the cross fall 4-42
Chapter 4: Road Design Elements
is normally one per cent steeper than that on the
maximum stem thickness of 175 to 200 mm,
lane to allow for the rougher surface and the
corresponding to the diameter of a guardrail
consequently slower rate of flow of storm water
post, is recommended.
off the road surface. A further application of medians refers to access
At night or during inclement weather it is impor-
management, where right turns into or out of
tant that the driver should be able to distinguish
local land uses are often discouraged on high-
clearly between the shoulder and the lane. This
speed roads.
can be accomplished by the use of a shoulder material of a contrasting colour or texture. Edge
From the above it can be inferred that medians
marking is a convenient way of indicating the
are typically applied in the case of high speed or
boundary between the lane and the shoulder. Rumble strips can also be used and have been
high volume roads with a basic function of
shown to reduce the rate of run-off-road inci-
mobility.
dents by twenty per cent or more. Rumble strips can be raised or grooved. Being intended to
Median islands can be as narrow as one metre,
provide the driver with an audible warning, the
which is sufficient to contain a median barrier
noise level they generate is unacceptable in
comprising back-to-back guardrails. This sug-
urban areas and should therefore only be used
gests that, including minimum width median
in rural areas.
shoulders, the minimum width of the median should be not less than three metres.
4.4.6
Inner
shoulders are often not provided in the urban
Medians
cross-section but kerbing would require an offThe median is the total width between the inner
set of about 300 mm. The minimum width of an
edges of the inside traffic lanes and includes the
urban median should thus be 1,6 metres.
long ago as the early 1930s it was proposed to
Research has found that few out-of-control vehi-
"separate the up-traffic from the down-traffic", a
cles travel further than nine metres from the
function which the median fulfils to this day.
edge of lane, so that this width of median would
This separation is intended to reduce the proba-
be sufficient to avoid most head-on crashes.
bility of head-on crashes and also to reduce the nuisance of headlight glare (usually by the
In urban areas, medians often contain right-turn-
planting of shrubs on the central island). The
ing lanes.
reduction in head-on crashes is achieved
shoulder is invariably replaced by kerbing so
through selection of a suitable width of median
that the median would be the sum of the lane
or the use of median barriers. Shrubs can also
width plus the width required to provide a pedes-
serve as a barrier to prevent cross-median acci-
trian refuge. Pedestrians do not feel safe on
dents but the stems of the shrubs should not
median islands narrower than about two metres,
grow so thick as to become a further hazard. A
suggesting that the median should have a width 4-43
Chapter 4: Road Design Elements
In intersection designs, the inner
Geometric Design Guide
central island and the median shoulders. As
of the order of 5,5 to 6,0 metres. This width is
piped drainage system is generally available.
adequate to accommodate pedestrians as well
The depressed median allows the roadbed to
as the right-turning lane.
If intersections are
drain into the median, specifically on curves
closely spaced, it may be necessary to apply
where water from the outer carriageway is pre-
this width to the full length of the median, where-
vented from draining across the inner carriage-
as with widely spaced intersections, e.g. 500
way. In the urban context, kerbing includes drop
metres or more between intersections, a lesser
inlets directing storm water into the underground
width can be applied between the intersections
system.
with the median being flared out by means of Urban median islands are usually narrower than
active tapers at the intersections.
their rural counterparts and do not normally Medians with a width of nine metres or more
have barriers. The barriers have to be terminat-
allow for individual grading of the two carriage-
ed at every intersection and at some entrances
ways, which can be useful in rolling terrain. In
so that the safety offered by the barrier is more
addition, these medians lend themselves to
than offset by the hazard of the barrier ends.
landscaping and to the creation of a park like
Kerbing offers a modest degree of protection to
environment. Unfortunately, they create prob-
pedestrians who may be on the median while
lems at intersections by virtue of the long travel
crossing the road. In addition, kerbing can, to a
distances that they impose on turning vehicles.
limited extent, redirect errant vehicles back into
The incidence of crashes at intersections
their own lanes.
Geometric Design Guide
increases with increasing width of median and, at widths of 20 metres, the intersection should
The speeds on rural roads make kerbing inap-
be designed as two intersections back-to-back,
propriate in this environment, as the driver of a
with traffic control on the roadway crossing the
vehicle striking a kerb at high speed would
median. The wide rural median does not trans-
almost certainly lose control of the vehicle, with
late well to the urban environment so that roads
this problem being compounded by the
on the outskirts of urban areas should be
inevitable damage to the front wheels of the
designed with medians appropriate to a future
vehicle.
urban characteristic. Depressed medians generally have flat slopes Medians may be either depressed or raised.
and a gently rounded bottom so that the driver
Depressed medians are normally used in rural
of a vehicle leaving the road has an opportunity
areas and raised medians in urban areas. This
to regain control, minimising occupant injury and
differentiation between rural and urban areas
vehicle damage. Overturning crashes are more
arises for two reasons: drainage and safety.
frequent on slopes steeper than 1 : 4 for median widths of six to twelve metres.
This should,
Storm water drainage in rural areas is generally
therefore, be considered the steepest allowable
above the surface for ease of maintenance
slope, with slopes of 1 : 6 or flatter being pre-
whereas, in urban areas, a well-developed
ferred. 4-44
Chapter 4: Road Design Elements
4.4.7
Outer separators
At an intersection, the frontage road should either be terminated or moved a substantial dis-
The outer separator is the area between the
tance away from the through lanes.
This is
edges of the travelled way of the major road and
intended to safeguard the operation of the inter-
the adjacent parallel road or street. It compris-
section because vehicles attempting to turn
es the left shoulder of the major road, an island
from the through road to a frontage road could
and the right shoulder of the adjacent road or
very easily generate a queue that backs up onto
street. The outer separator serves as a buffer
the through lanes. Not only is this operationally
between through traffic and local traffic on a
undesirable but it could also be unsafe.
frontage or service road. It is typically applied where the corridor has to serve the two func-
Where it is anticipated that a road will have to be
tions of long distance travel and local accessi-
widened at some time in the future, the width of
bility. An arterial passing through a local shop-
the outer separator should be such that it can
ping area is an example of this application.
accommodate the additional lane, hence minimising the extent of damage to the rest of the road cross-section.
If travel on the frontage road is one way and in the same direction as that on the adjacent
4.4.8
through lane, the outer separator can be as nar-
Boulevards
row as three metres or, if barriers are provided, Boulevards are only used in urban areas and
two metres.
are similar to outer separators with regard to At night, drivers on the through lane would find
their function and location. The principal differ-
an opposing direction of flow on the frontage
ence is that they separate a sidewalk and not a
road very confusing, being confronted by head-
frontage
lights both to the right and to the left. Under
Boulevards are a desirable feature because:
these circumstances, the width of the outer sep-
•
road
from
the
through
lanes.
The separation between the sidewalk and the vehicular traffic provides
arator should be substantially increased, prefer-
increased safety for pedestrians and
ably doubled, to minimise the effect of the
•
on non-illuminated sections of the road.
The probability of a pedestrian/vehicle collision is reduced as the sidewalk is
Plantings or dazzle screens on the outer sepa-
placed some distance from the kerb;
•
rator are recommended for the same reason.
Pedestrians are less likely to be splashed by passing vehicles in wet weather;
On rural freeways, the outer separator should
•
be at least nine metres wide, based on the dis-
Space is provided for street furniture and
tance that an out-of-control vehicle is able to
streetscaping as well as for surface and
move away from the edge of the through lane.
underground utilities, and
•
Reference in the literature is to outer separator
Changes to the cross-slope of the side-
widths of twenty to thirty-five metres in rural
walk to provide for appropriate driveway
areas.
gradients are minimised using the 4-45 Chapter 4: Road Design Elements
Geometric Design Guide
children at play;
approaching traffic, particularly headlight glare
boulevard area to effect the gradient
have to be provided across boulevards, the vari-
change.
ation in slope should not be so drastic that vehicles cannot traverse the area without scraping
The verge, showing the location and dimensions
their undersides on the ridge between the
of the boulevard, is illustrated in Figure 4.11.
boulevard and the sidewalk.
Aesthetic considerations in the urban environ-
Boulevards are as wide as the road reserve
ment are important, particularly when major
allows.
streets pass through or are adjacent to parkland and residential areas.
than two to three metres.
Desirably, the positive
aesthetic qualities of the adjacent land use are
4.4.9
carried over into the verge and boulevard areas
Geometric Design Guide
Ideally, they should not be narrower
Bus stops and taxi lay-byes
of the street cross-section. As a feature of the
Bus stops and lay-byes can be located within
urban landscape, boulevards are usually
the width of the boulevard. In this case, grass-
grassed or landscaped. If the boulevards are
ing of the boulevard is discontinued and the
narrower than 1,5 metres, they are surfaced
area surrounding the bus stop is paved as an
rather than grassed because of the mainte-
extension of the sidewalk to provide users of
nance difficulties associated with narrow strips.
public transit with all-weather access to buses.
The entire area from the reserve boundary to
Pedestrian accidents often occur at bus stops.
the road edge is normally sloped towards the
This can be attributed to the fact that buses fre-
road to assist drainage, not only of this area but
quently stop too close to the road edge, thus
of the adjacent development as well. Because
obstructing oncoming drivers' view of pedestri-
Figure 4.11: Verge area indicating location of boulevard of the impedance offered by grass to overland
ans crossing the road.
flow, the slope of the boulevard should be at
stances, a pedestrian stepping out from behind
least four per cent. Local circumstances may
the bus would be moving directly into the path of
require steeper slopes but, where driveways
an oncoming vehicle. 4-46
Chapter 4: Road Design Elements
Under these circum-
Two approaches can be adopted to minimise
The location of bus stops can have an adverse
this problem. If the bus stop is provided with
impact on safety. A bus at a stop located imme-
adequate entrance and exit tapers, it is easy for
diately in advance of intersections would force
buses to move well clear of the travelled way. If
left-turning vehicles into a situation of heavily
space permits, a painted island can be provided
reduced sight distance.
between the bus stop and the travelled way so
pulling out of the stop, the bus could seriously
that the stop is, in effect, a short length of auxil-
influence the operation of the intersection as a
iary lane. In addition to an approach aimed at
whole.
Furthermore, while
the physical dimensions of the bus stop, a further safety measure could be the provision of
Best practice suggests that bus stops should be
barriers preventing bus passengers from cross-
located beyond intersections.
ing the road until they have moved clear of the
should not be located more than about fifty
bus stop itself.
metres
the
nearest
intersection.
Figure 4.12: Typical layout of a bus stop A proposed typical layout of a bus stop is illus-
Destinations for bus passengers may be on the
trated in Figure 4.12. If the frequency of service
bus route itself but are more likely to be to one
on a particular road is high, e.g. where two or
side or the other of the route. The close prox-
more bus routes have converged upstream of
imity of the bus stop to an intersection offers
the bus stop, the length of the bus stop should
passengers a convenient route to their final des-
be increased to 25 metres to accommodate two
tination. However, it is suggested that a bus
buses. If necessary, the tapers can be reduced
stop should not be located closer than about 15
to not less than 1 : 3 for design speeds of 70
metres from the kerb line of the intersecting
km/h or less.
road or street. A lesser spacing would make it
4-47 Chapter 4: Road Design Elements
Geometric Design Guide
from
However, they
difficult for a left-turning bus to enter the bus
driveway entrances may have to have a steeper
stop and, furthermore, may result in encroach-
cross-slope than this to match the gradient of
ment on the sight triangle required by a driver on
the driveways but should not exceed a cross-
the intersecting road or street.
slope of five per cent.
4.4.10 Sidewalks
Kerbs, raised medians and channelising islands can be major obstructions to the elderly and
Pedestrian traffic is not encouraged in the road
people with disabilities, particularly those in
reserves of freeways, expressways or other
wheelchairs.
high-speed arterials and accommodation of
minimising the impact of these obstacles is to
pedestrian traffic is usually handled elsewhere.
provide ramps, also referred to as kerb cuts,
On all other urban streets, pedestrian traffic can
dropped kerbs or pram dips.
be expected and it is necessary to provide side-
have a slope of not more than about six per
walks.
cent. A kerb height of 150 millimetres would
The most common method for
Ramps should
thus require a ramp length of 2,5 metres. There should be a clear sidewalk width of 1,5 metres
In commercial areas or areas where the road
beyond the top end of the ramp so that, where a
reserve width is restricted, sidewalks may
ramp is provided, the overall sidewalk width
extend from the kerb to the road reserve bound-
should be not less than four metres.
ary. As discussed above, there is distinct merit
chairs may be 0,75 metres wide so that two
in placing a boulevard between the sidewalk
wheelchairs passing each other on the ramp
and the travelled way, but pedestrian volumes
would require a ramp width of the order of 2 to
may be so high that the entire available width
2,5 metres. If it is not possible to provide this
would have to be utilised for the sidewalk. The
width, a width of not less than 1,5 metres should
width of a sidewalk should not be less than 1,5
be considered.
metres and a minimum width of two metres should be provided near hospitals and old-age
The designer should be aware that, in accom-
homes where wheelchair traffic could be expect-
modating one group of pedestrians with disabil-
ed. If the sidewalk is immediately adjacent to
Geometric Design Guide
Wheel
ities, a different group might be disadvantaged
the kerbing, the minimum width should be
in the process. Visually impaired pedestrians
increased by about 0,6 to 1 metre. This is to
would have trouble in locating the kerb face in
make provision for fire hydrants, street lighting
the presence of a ramp.
and other road furniture. It also allows for the
As a vertical face
across the sidewalk would be unexpected, even
proximity of moving vehicles and the opening of
by sighted pedestrians, the sides of the ramp
car doors.
should also be sloped.
The normal cross-slope on a sidewalk is 2 per
Sidewalks are not normally provided in rural
cent. Cross-slopes steeper than this present a
areas. It should, however, be noted that approx-
problem to people with walking impairments or
imately half the fatalities on the South African
who are in wheel chairs. Sidewalks crossing
road network are pedestrians, with many of 4-48
Chapter 4: Road Design Elements
these fatalities occurring in rural areas.
pacted regularly to provide pedestrians with a
Provision should therefore be made for pedes-
hard surface to walk on. In high rainfall areas, a
trian safety outside urban areas. Paved foot-
portion at least of the shoulder should be paved,
ways could be considered under the warranting
with this paved area being at least 1,5 metres
conditions listed in Table 4.17.
wide. Furthermore, the road shoulder should be well drained to prevent the accumulation of
Footways can be as little as one metre wide, but
water, which would force pedestrians to walk on
a width of 1,8 metres would allow two people to
the carriageway.
walk side by side.
4.4.11 Cycle paths The safest location for footways is at the edge of the road reserve. This location is not popular
Changes from single to multiple land usage will
with pedestrians because the footway then fol-
result in shorter trip lengths, making the bicycle
lows all the variations in the natural ground
a more popular form of transport. In addition,
level. In rolling or mountainous terrain through
people are becoming conscious of the need for
cuts and fills, such a footway would not make for
exercise and of the bicycle as means of exer-
comfortable walking. Even if this footway were
cise. Finally, unlike the motor vehicle, bicycles
provided, pedestrians would almost certainly
are environmentally friendly.
prefer to walk on the more level surface of the
If adequately
possible, be situated at least three metres away
can play an important role in the transportation
from the travelled way. This corresponds to a
system. It is important to realise that cyclists
location immediately outside the edge of the
need sufficient space to operate with safety and
usable shoulder in the case of a high volume
convenience rather than simply being assigned
high-speed road.
whatever space is left over after the needs of
In cases where footways are not warranted but
vehicular traffic has been accommodated.
where a large number of pedestrians walk alongside the road, the road shoulder should be
The basic requirements of cyclists are:
upgraded to cater for them. The minimum width
• • •
of these shoulders should be three metres. If not surfaced, they should be bladed and com-
Space to ride; A smooth surface; Speed maintenance, and
4-49 Chapter 4: Road Design Elements
Geometric Design Guide
planned, designed and maintained, cycle paths
shoulder. In level terrain, the footway should, if
•
safely. The surface of a cycle path should not
Connectivity.
deviate from a three-metre straightedge by The bicycle design envelope and clearances are
more than 5 mm and should also be shaped to
illustrated in Figure 4.13. The one metre enve-
existing features to within 5 mm.
lope allows for the width of the bicycle as well as Adequate clearances to
For bicycles to be effective as a means of trans-
fixed objects and passing vehicles should be
portation, cyclists must be able easily to main-
provided, in addition to the one metre envelope.
tain a steady speed with ease. Cyclists typical-
for erratic tracking.
Geometric Design Guide
ly travel at speeds of twenty to thirty km/h someBicycles have narrow tyres inflated to high pres-
times reaching 50 km/h on downgrades. Once
sures and have no suspension system to speak
slowed or stopped it takes considerable time
of. A smooth surface is therefore desirable for
and effort to regain the desired operating speed.
bicycles to be used effectively, comfortably and
Bicycle routes should thus be designed for con-
Figure 4.13: Bicycle envelope and clearances 4-50 Chapter 4: Road Design Elements
tinuous movement, avoiding steep gradients,
from the roadway and from which all
rough surfaces, sharp corners, intersections or
motor traffic, with the exception of main-
the need to give way to other road users.
tenance vehicles, is excluded. Cycle paths may be located within the road reserve or in an independent reserve.
It should be possible to undertake and complete journeys by bicycle.
Bicycle routes on road-
Cycle lanes should have the widths indicated in
ways or separate paths should form a connect-
Table 4.18.
ed network on which bicycle trips can be made effectively and conveniently. Connectivity is an
The geometry dictated by motor vehicles is gen-
important aspect of effective bicycle routes and
erally adequate for bicycles except that bicycles
should be given careful consideration during the
have a longer stopping sight distance.
planning process. Facilities for bicycles can take the form of a:
becomes the design vehicle, with dimensions,
•
Shared roadway/cycle lane where motor
performance, stopping sight distance, minimum
vehicles and bicycles travel in a com-
horizontal and vertical curvature, and clear-
mon lane;
•
ances.
Cycle lane, which is part of the travelled way but demarcated as a separate lane;
•
4.4.12 Slopes
Shoulder lane, which is a smooth, paved portion of the shoulder, properly demar-
The slopes of the sides of the road prism are,
cated by pavement markings or traffic
like those of medians, dictated by two different
signs. As the shoulder provides a useful
conditions.
area for cycling with few conflicts with
safety and a slope of 1:4 is the steepest accept-
fast-moving motor vehicles, this facility
•
Shallow slopes are required for
is very useful in rural areas.
able slope for this purpose. The alternative is to
Cycle path, which is physically removed
accept a steeper slope and provide for safety by 4-51
Chapter 4: Road Design Elements
Geometric Design Guide
In the case of separate cycle paths, the bicycle
some other means, such as barriers. In this
to provide the road and its appurtenant works.
case the steepest slope that can be used is dic-
Utilities not directly connected with the road,
tated by the natural angle of repose and erodi-
e.g. telephone or power lines are normally locat-
bility of the construction material.
ed in the verge.
Non-cohesive materials require a slope of 1:2,
In urban areas, the area between the edge of
whereas cohesive soft materials can maintain a
the travelled way and the road reserve bound-
slope of 1:1,5. Cuts in firm cohesive materials
ary provides space for a variety of elements
such as stiffer clays can be built to a slope of
that, for convenience, are summarised in Table
1:1. Rock cuts can be constructed to a slope of
4.19.
1:0,25 (4:1) provided that the material is reaSome of these elements have been discussed
sonably unfissured and stable.
previously.
The intention is to provide the
It is stressed that the slopes suggested are only
designer with a checklist of elements that should
an indication of normally used values.
The
be accommodated.
Some elements will be
detailed design of a project should therefore
mutually exclusive.
For example, the use of
include geotechnical analysis, which will indi-
barrier kerbs indicates that mountable kerbs
cate the steepest slopes appropriate to the con-
cannot be present.
struction or in-situ material. Economic analysis
temporary change in cross-section, for example
will indicate the height of fill above which a slope
the boulevard being replaced by a bus embay-
of 1:4 should be replaced by a steeper slope
ment. Yet others may overlap, for example the
and alternative provision made for safety. As a
driveway approach that crosses a sidewalk.
Others may represent a
rule of thumb, the transition from the flat slopes to slopes dictated by the materials typically
Elements that are most likely to be accommo-
occurs at a fill height of about 3 m.
dated in the verge are;
•
Berms intended to function as barriers protecting the surrounding development
4.4.13 Verges
Geometric Design Guide
from visual intrusion or noise;
• • •
The verge is defined as the area between the longitudinal works and the road reserve boundary. The limit of the longitudinal works in the
Cut and fill slopes; Driveway approaches, and Underground services.
case of the rural cross-section is often at the top
Even in the unlikely event that none of these
of cut or the toe of fill. Where drainage works,
elements have to be provided, there has to be a
such as side drains or catch water drains, are
clear space between the edge of the travelled
required, these form part of the longitudinal
way and the road reserve boundary. This space
works, which may thus be wider than the actual
would provide sight distance in the case of hor-
road prism. In rural areas the verge simply rep-
izontal curvature and also allow for emergency
resents the difference in width between a statu-
stopping. Furthermore, there should be some
tory road reserve and the width actually needed
flexibility to accommodate future unknowns. It 4-52
Chapter 4: Road Design Elements
is suggested that this clear space should ideally
road itself. However, to the driver the presence
be a minimum of five metres wide with an
of a pole is a hazard to be avoided and whether
absolute minimum width of three metres.
the pole is carrying a power line or a streetlight is a matter of indifference. Given this broader
4.4.14 Clearance profiles
approach to utilities, surface utilities typically located in the reserve include:
• • • • •
exclusively reserved for provision of the road. It defines the lowest permissible height of the soffit of any structure passing over the road and also the closest approach of any lateral obstacle
Electrical transmission lines; Telephone lines; Street lighting; Traffic signal poles, and Fire hydrants.
to the road cross-section. Clearance profiles Underground utilities include:
are described in detail in Chapter 10.
• • • • •
4.4.15 Provision for utilities
Both surface and underground utilities are often
Storm and foul water sewers; Water reticulation; Buried telephone lines; Gas pipelines, and Power transmission cables.
located within the road reserve. Utilities convey the sense of services not directly related to the
Most urban authorities have guidelines for the
4-53 Chapter 4: Road Design Elements
Geometric Design Guide
The clearance profile describes the space that is
Geometric Design Guide
Figure 4.14: Collision rate
Figure 4.15: Prediction of utility pole crashes placement of utilities. The use of an integrated
utilities is by manholes and open manholes are
process in the planning and location of road-
an unnecessary hazard to pedestrians. In older
ways and utilities is encouraged in order to
municipal areas, services were sometimes
avoid or at least to minimise conflicts.
located under the roadway itself. This practice should be actively discouraged as it places both
As a rule, underground utilities should be locat-
maintenance workers and passing vehicles at
ed in the verges or boulevards. Access to these
risk. Every time the road is resurfaced, it is nec4-54
Chapter 4: Road Design Elements
4.4.16 Drainage elements
essary to remove the manholes and replace them at the new level. This operation carries an element of risk but, if not carried out, the man-
The process of design of storm water drainage
hole is at a lower level than the road surface and
systems is exhaustively discussed in the South
the drop could be sufficient for a driver to lose
African Roads Agency Drainage Manual, 1986.
control of the vehicle. The lower level of the manhole would, during rainy weather, lead to
In this section, discussion is limited to the ele-
the creation of a pond of water that could slow-
ments that the designer should incorporate in
ly drain into the conduits of the buried utility,
the cross-section to ensure adequate drainage
possibly leading to disruption of the service
of the road reserve and the adjacent land.
being provided. A distinction is drawn between rural and urban drainage. Rural drainage focuses largely on the
The problem with surface utilities is that they are
swift removal of storm water from the travelled
carried on poles that can be hit by errant vehi-
way onto the verge and on its movement to a
cles. Research has indicated that the frequen-
point where it can be taken from the upstream to
cy of crashes is a function of the pole density in
the downstream side of the road. In an urban
poles per kilometre and the average pole offset
environment, the road reserve serves as the
from the travelled way. The crash frequency is
principal conduit of storm water from surround-
typically of the order of 0,1 crashes per kilome-
ing properties and its conveyance to a point
tre per year with a pole spacing of less than 20
where it can be discharged into natural water-
poles per kilometre and an offset of eight
courses.
metres. When the pole density is higher than 30
Rural drainage is, in short, the
removal of water from the road reserve whereas
poles per kilometre and the offset less than one
urban drainage attracts water to the reserve.
metre, the collision rate climbs to a high of 1,5 crashes per kilometre per year. This is illustrat-
In both rural and urban environments, storm
ed in Figure 4.14.
water drainage is aimed both at the safety of the road user and the integrity of the design layers
A nomograph predicting utility pole crashes is
of the road.
given in Figure 4.15.
exclusively directed towards safeguarding the design layers. It was previously common prac-
The example shows that a road with an ADT of
tice to recommend a minimum depth of drain.
11 000 vehicles and a pole density of 40 poles
The safety of the road user dictates rather that a
per kilometre will experience 0,75 crashes per
maximum depth of drain be specified. The rec-
kilometre per year if the pole offset is 1,5
ommended maximum depth is 500 mm.
metres. If the designer were to increase the
volume of water to be conducted by a drain thus
pole offset to 3 metres, the crash rate would
indicates the required width of the drain rather
reduce to 0,5 crashes per kilometre per year, an
than its depth, since the need to keep the design
improvement of 33 per cent.
layers unsaturated has not changed.
4-55 Chapter 4: Road Design Elements
The
Geometric Design Guide
Previously, rural drainage was
Rural drainage
scour is likely to occur are given in Table 4.20. Conventional open-channel hydraulics will, in
Rural drainage is normally by means of unpaved
conjunction with Table 4.20, indicate when
open drains, which may either silt or scour,
either silting or scouring is likely, and hence
depending on the flow speeds in them. Both
whether it is necessary to pave a drain or not.
silting and scouring of a drain increase the hazard to the road user. Scour would lead to the
As a rough guide to longitudinal slopes, it is sug-
creation of a deep channel that would be impos-
gested that unpaved drains should not be steeper
sible to traverse with any degree of safety. It
than 2 per cent, or flatter than 0,5 per cent.
may also cause erosion of the shoulder and ulti-
Paved drains should not be flatter than 0,3 per
mately threaten the integrity of the travelled way
cent. Practical experience indicates that it is dif-
itself. Silting may block the drain, so that water
ficult to construct a paved drain accurately to the
that should have been removed would be dis-
tolerances demanded by a slope flatter than 0,3
charged onto the road surface.
per cent, so that local imperfections may cause silting of an otherwise adequate drain.
The effectiveness of the drain depends on water speed, which is a function of longitudinal slope,
Where the longitudinal slope is so flat that self-
as well as of other variables. There is a range
cleansing water speeds are not achieved, even
of slopes over which the flow velocity of water
with paving, it will be necessary to consider a
on in-situ materials will be so low that silting
piped drainage system.
occurs, and another range where the flow velocOn
As an alternative to lining a material subject to
slopes between these two ranges neither silting
scour, it is possible to reduce flow velocity by
nor scouring will occur and unpaved drains will
constructing weirs across an unpaved drain.
be effective.
The drain will then in effect become a series of
ity will be high enough to cause scour.
Geometric Design Guide
stilling basins at consecutively lower levels. Paving solves some of the problems caused by
While this could be an economical solution in
both silting and scouring. Paving generally has
terms of construction cost, it has the disadvan-
a lower coefficient of roughness than in-situ
tage that an area of deep localised erosion,
materials, so that water speeds are higher in a
immediately followed by a stone-pitched or con-
paved drain than in an unpaved drain with the
crete wall, would confront an errant vehicle. If
same slope. Furthermore, it is possible to force
this alternative is to be considered at all, it
higher speeds in the paved drain by selection of
should be restricted to roads with very low traf-
the channel cross- section. The problem of silt-
fic volumes and the weirs should be spaced as
ing can be resolved, at least partially, by paving
far apart as possible.
the drain. Drains constructed through in-situ materials The flow velocity below which silting is likely to
generally have flat inverts so that, for a given
occur is 0,6 m/s. Flow velocities above which
flow, the flow velocity will be reduced. The flat 4-56
Chapter 4: Road Design Elements
inverts reduce the possibility of scour and are
reduce the likelihood of a vehicle digging its
easy to clear if silting occurs. Paved drains, not
front bumper into the far side of the drain and
being susceptible to scour, have a V-profile.
somersaulting.
Self-cleansing velocities are thus achieved at relatively small flows and the need for mainte-
Typical drain profiles are illustrated in Figure 4.16.
nance is reduced.
The sides of the drain should not be so steep as
(a)
to be dangerous to the road user; a maximum
Side drains are located beyond the shoulder
Ideally, both
breakpoint and parallel to the centre line of the
sides of the drain should be designed to this
road. While usually employed in cuts, they may
slope or flatter. Where space for the provision of
also be used to run water along the toe of a fill
the drain is restricted, the slope closest to the
to a point where the water can conveniently be
road should remain at 1:4 and the outer slope
diverted, either away from the road prism or
steepened. This has the effect of positioning the
through it, by means of a culvert.
drain as far as possible from the path of vehi-
in conjunction with fills, side drains should be
cles. One example of this is a side drain in a
located as close to the edge of the reserve
cut, where the outer slope of the side drain forms an extension of the cut face.
When used
boundary as is practicable to ensure that ero-
These
sion of the toe of the fill does not occur. Side
slopes, in combination with the flat invert, give
drains are intended as collectors of water and
the trapezoidal profile of an unpaved drain.
the area that they drain usually includes a cut face and the road surface.
It is recommended that the bottom of a lined Vprofile and the junctions between the sides and
(b)
Edge drains
bottom of an unlined trapezoidal profile be slightly rounded.
The rounding will ease the
Edge drains are intended to divert water from fill
path of an errant vehicle across the drain, and
slopes that may otherwise erode either because 4-57
Chapter 4: Road Design Elements
Geometric Design Guide
slope of 1:4 is recommended.
Side drains
Geometric Design Guide
Figure 4.16: Typical drain profiles of the erodibility of the material or because they
located almost under a guardrail would heighten
are subjected to concentrations of water and
the possibility that a vehicle wheel might snag
high flow velocities.
under the guardrail.
Guardrail posts tend to
serve as points of concentration of water, so that, as a general rule, edge drains are warrant-
Edge drains are constructed of either concrete
ed when the fill material is erodible or when
or premixed asphalt. Premix berms normally
guardrails are to be installed.
have a height of 75 to 80 mm, and are trapezoidal in profile with a base width of 250 mm and
Edge drains should preferably be raised rather
a top width of 100 mm. Concrete edge drains
than depressed in profile. A depressed drain
are normal barrier kerbs and channels. These 4-58
Chapter 4: Road Design Elements
require a properly compacted backing for stabil-
used, a transverse slope flatter than 1:10 may
ity and are, therefore, not as easy to construct
make it difficult to protect the design layers of
as premix berms.
the road.
Unlike side drains, median drains,
whether lined or not, are generally constructed
(c)
Catch water drains
with a shallow V-profile with the bottom gently rounded.
The catch water drain, a berm located at the top of a cut, is to the cut face what the edge drain is
(e)
Chutes
to a fill. It is intended to deflect overland flow from the area outside the road reserve away
Chutes are intended to convey a concentration
from the cut face. Even if the cut is through
of water down a slope that, without such protec-
material which is not likely to scour, the catch
tion, would be subject to scour. They may vary
water drain serve to reduce the volume of water
in size from large structures to half-round pre-
that would otherwise have to be removed by the
cast concrete products, but they are all open
side drain located at the bottom of the cut face.
channels. Flow velocities are high, so that stilling basins are required if down-stream erosion
Catch water drains are seldom, if ever, lined.
is to be avoided. An example of the application
They are constructed with the undisturbed top-
of chutes is the discharge of water down a fill
soil of the area as their inverts and can readily
slope from an edge drain. The entrances to
be grassed as a protection against scour.
chutes require attention to ensure that water is
Transverse weirs can also be constructed to
deflected from the edge drain into the chute,
reduce flow velocities, since the restrictions pre-
particularly where the road is on a steep grade.
viously mentioned in relation to weirs do not
It is important that chutes be adequately spaced
apply to catch water drains. The cut face and
to remove excess water from the shoulders of
the profile of the drain reduce the probability of
the road. Furthermore, the dimensions of the
a vehicle entering the drain but, should this hap-
chutes and stilling basins should be such that
pen, the speed of the vehicle will probably be
these drainage elements do not represent an
low.
excessive risk to errant vehicles.
they should be as shallow as is compatible with
(d)
their function and depths in excess of 150 mm
Median drains
should be viewed with caution. Median drains do not only drain the median but also, in the case of a horizontal curve, prevent
Because of the suggested shallow depth of
water from the higher carriageway flowing in a
chutes, particular attention should be paid to
sheet across the lower carriageway. The space
their design and construction to ensure that the
available for the provision of median drains
highly energised stream is not deflected out of
makes it possible to recommend that the trans-
the chute. This is a serious erosion hazard that
verse slopes should be in the range of 1:4 to
can be obviated by replacing the chute with a
1:10. If the narrowest median recommended is
pipe. 4-59
Chapter 4: Road Design Elements
Geometric Design Guide
Generally,
(f)
Mitre banks
requirements for these forms of protection are usually conflicting. For major storms, the runoff
As their name implies, these banks are con-
should be retarded to reduce flood peaks and,
structed at an angle to the centre line of the
for minor storms, the runoff is best handled by
road. They are intended to remove water from
rapid removal. The solution is to provide two
a drain next to the toe of a fill and to discharge
separate but allied drainage systems.
it beyond the road reserve boundary. Several
ajor storms involve considerations such as
mitre banks can be constructed along the length
attempting to achieve rates of runoff that do not
of a drain, as the concentration of water in the
exceed pre-development levels.
drain should ideally be dispersed and its speed
achieved in part through the application of
correspondingly reduced before discharge.
detention and retention ponds.
Speed can be reduced not only by reducing the
the layout of road patterns can be coordinated
volume, and hence the depth, of flow but also by
with the run-off requirements of the system to
positioning the mitre bank so that its toe is virtu-
increase the time of concentration, hence reduc-
ally parallel to the natural contours.
ing the risk of flood hazards
The
This is
Furthermore,
upstream face of a mitre bank is usually protected by stone pitching, since the volume and
Accommodation of minor storms is achieved
speed of flow of water that it deflects may cause
with kerbs and channels, drop inlets and under-
scour and ultimately lead to breaching of the
ground reticulation. The runoff is initially col-
mitre bank.
lected in channels until the flooded width of the road reaches a specified limit and then dis-
(g) Rural underground systems
charged into the underground system, which is
Geometric Design Guide
connected to an outfall point - typically a natural The geometric designer is not directly con-
watercourse. In the case of high-speed routes,
cerned with the underground system, except for
such as freeways, expressways and major arte-
its inlets. These should be hydraulically efficient
rials, no encroachment of storm water onto the
and correctly positioned to ensure that water
travelled lanes can be considered. Minor arteri-
does not back up onto the road surface or satu-
als and collectors should have one clear lane of
rate the design layers. To restrict the hazard to
3,0 metres minimum width in each direction and
the road user, inlets that are flush with the sur-
local streets need only have one clear lane of
face drain invert are preferable to raised struc-
3,0 metres minimum width. In all cases, the
tures.
100-year storm should not cause a barrier kerb to be overtopped. The designer should ensure,
Underground reticulation is costly both to pro-
therefore, that the resultant of gradients and
vide and to maintain.
The designer should
crossfall is sufficiently steep and the spacing of
therefore, without violating the principles dis-
drop inlets sufficiently short to ensure that the
cussed above, attempt to reduce the use of
recommended lateral spreads of water are not
underground drainage as far as possible by the
exceeded.
discerning use of surface drainage.
Urban drainage Urban drainage entails the provision of protection from major and minor storms and the basic 4-60 Chapter 4: Road Design Elements
TABLE OF CONTENTS 5. 5.1 5.2 5.3 5.4
5.5
ALIGNMENT DESIGN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 VISION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1 DESIGN APPROACH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-3 FITTING THE ROAD TO THE LANDSCAPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5 5.4.1 . . . Freeway design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5 5.4.2 . . . Single carriageway roads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9 5.4.3 . . . Perspectives in road design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-9 INTEGRATION WITH THE ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-20
LIST OF FIGURES Figure 5-1: Radii for acceptable curve lengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6 Figure 5-2: 1/R diagram for short curve/long tangent alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7 Figure 5-3: 1/R diagram for long curve/short tangent alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7 Figure 5.4:. 1/R diagram for curvilinear alignment with transitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8 Figure 5.5: 1/R diagrams for alternative alignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-8 Figure 5-5: Short sag curve on long tangent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-10 Figure 5-6: Short humps on long horizontal curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11 Figure 5-7: Short vertical curves preceding a long horizontal curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12 Figure 5-8: Distorted alignment at bridge crossing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13 Figure 5-9: Broken-back curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-14 Figure 5-10: Out-of-phase vertical and horizontal alignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15 Figure 5-11: Minor rolling on long horizontal curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-16 Figure 5-12: Break in horizontal alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-17 Figure 5-13: Well-coordinated crest and horizontal curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18 Figure 5-14: Well coordinated sag and horizontal curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-18 Figure 5-15: N2 North: Curvilinear alignment in a valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-19 Figure 5-16: N2 North - Median widening to accommodate stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-19 Figure 5-17: N2 - Combining horizontal and vertical alignment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-20 Figure 5-18: Blending of fills into landscape. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-21 Figure 5.19: Contour plans of cut slopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-22 Figure 5.20: Cuts with constant or varying cut batters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-23
Chapter 5 ALIGNMENT DESIGN 5.1
INTRODUCTION
5.2
VISION
Most of the chapters in this guide assist the
The eye is a truly remarkable instrument sensi-
designer by describing the principles on which
tive enough for the dark-adapted eye to detect a
design parameters are based and by offering
single photon. It is also backed up by enormous
ranges and guidance in the selection of suitable
computing power and a memory bank of visual
standards. In this section an attempt is made to
images. Although it is capable of processing
stand away from the detail and discuss highway
fine detail in real time, it remains subject to the
design holistically.
laws of physics.
South Africans are blessed with a beautiful
Despite the difference in scale, the stars over-
country. There are thus boundless opportunities
head can be brighter than the spark from a
to develop highway location and design as an
campfire. This demonstrates better than words
art form in this country's varied landscape from
that seeing depends on the energy of the light
mountains and plains to deserts and seas.
received by the eye. Also there has to be a rea-
When properly applied, there can be benefits
sonable contrast between the source and the
both to the user and to the landscape.
background.
Stars are not visible during the
Creating a harmonious alignment is architec-
per cent can be perceived. At twilight, much
ture. It requires the ability to visualize the final
greater differences are required. The cones of
form both from the perspective of the driver and
the eye's retina which perceive colour also
the outside observer as well as a grounding in
require significant light energy and do not oper-
engineering principles The ability to capture the
ate at low levels at night. The yellow and green
final form in the mind's eye and translate the
colours at the centre of the spectrum appear
design into two-dimensional representations
significantly brighter than the reds and blues
defining the layout represents creativity at a high
which have longer and shorter wavelengths
level. Most texts on the subject of the aesthet-
respectively.
ics of alignment design stress the role of experience. Undoubtedly, we all learn as we grow in
Although very quick, our visual response is not
the profession. However, in our society at pres-
immediate. Approximately 0,2 of a second is
ent we do not have the luxury of widely avail-
required to fix on an object.
able, experienced engineers. Designers often
changing environment we can at best process
have to tackle difficult problems for the first time
twelve images per second.
and at short notice. In what follows we have
from the dashboard clock to a road sign takes
therefore concentrated on describing what it is
about one second. From there, reaction time
important to know and on giving some guidance
requires 1,3 seconds thus leading to a final safe
as to what is, and what is not, good practice.
reaction/response time of 2,5 seconds. A com5-1
Chapter 5: Alignment Design
However, in a Changing focus
Geometric Design Guide
day. In daylight, a contrast in brightness of five
designers and are further summarized below:
plex environment, where the desired response is not immediately obvious, may require a significantly longer reaction time as discussed in
1.
objects and incidences that must be
Chapter 3.
reacted to increases proportionately.
The driver sees the road and surrounds as if he
Concentration focuses on the approach-
were stationary in a 3D movie. Nearby objects
ing road and traffic in the immediate
speed past in a blur. Objects in the foreground
vicinity.
can only be seen briefly. Only objects at the
tion becomes more and more dangerous. It follows that the driver will only be
Under conditions of good lighting, people with
able to see interesting objects in the
good eyesight can observe an object that sub-
centre of his visual field and therefore
tends an angle of one arc minute or 29 millime-
the road should aim the eye towards
Lines can be perceived
objects of interest and create variety
more acutely. However, they should subtend an
through curvature.Planes that stand per-
angle of at least 4 arc seconds to be visible.
pendicular to the road are prominent
People with good eyesight are not the norm and
while parallel ones are not.
allowance has to be made for lower acuity when
2.
designing objects and messages that are impor-
As speed increases the eyes seem to focus further and further ahead. Drivers
tant to the driver in a high-speed environment
anticipate the distance ahead that they
and a constantly changing visual field.
Geometric Design Guide
Observing irrelevant objects
outside of the necessary area of atten-
infinity focus can be scanned at leisure.
tres at 100 metres.
As speed increases, the number of
will require to respond to emergen-
Because of the ability of both the head and eyes
cies.At 70 km/h the focal point of the eye
to swivel, it is difficult to place boundaries on the
is approximately 400 metres ahead
driver's field of view. However, the sensitive part
while at 120 km/h the focal point can be
of the retina at the centre of the visual field has
up to 1 000 metres ahead. From the
a relative arc of 2,5 degrees. The peripheral
principle of visual acuity, it follows that
vision lies outside this cone where little detail
anything that has to be brought to the
can be perceived. The visual field also dimin-
driver's attention must lie close to the
ishes the more finely we focus.
axis of vision and also be large enough to be recognized at a long distance. 3.
From these general principles of vision, J R
As the level of concentration increases,
Hamilton and L L Thurston (in a paper entitled
the total visual field decreases with the
"Human Limitations in Automobile Driving" pub-
result that the peripheral vision diminishes as
lished in 1937) enunciated five propositions that
speed increases.
are applicable to the highway environment.
referred to as tunnel vision and, unless
These were summarized in "Man Made
the point of concentration is made to
America" by Tunnard and Pushkarev (1963),
move through an arc by means of a
which should be required reading for all highway
curving roadway, driving along a straight 5-2
Chapter 5: Alignment Design
This is sometimes
4.
and uneventful highway can become-
networks, drivers have to concentrate in order to
hypnotic.
survive. They must focus as far ahead as pos-
As speed increases, foreground fea-
sible to anticipate the approaching vehicles and
tures begin to fade because the driver is
changes in alignment.
not able to see clearly except at a dis-
sudden events in the foreground and both their
tant focus. Foreground detail is greatly
peripheral vision and space perception are
diminished at 80 km/h, and beyond 100
impaired.
They cannot cope with
km/h reception of the foreground is negligible.
Thus, only at a distance of
With modern virtual reality techniques it is also
between 50 metres and 100 metres
possible to show that, at 120 km/h, the paved
does vision become adequate at 100
area and median across a 30 metre wide free-
km/h. It follows that emphasizing elabo-
way takes up thirty per cent of the visual field,
rate detail is meaningless for the driver.
the roadside about fifteen per cent, and the sky
Only large simple shapes are usually
dominates at fifty five per cent. When the road-
perceived and particularly the geometry
way is only 15 metres wide at somewhat lower
of the paved road at the centre of the dri-
speeds the roadway takes up about fifteen per
ver's vision. Outside the road, only the
cent, the sky approximately twenty five per cent,
general outline and form of the land
and the roadside the remaining sixty per cent.
together with objects on the horizon are distinguishable. This leads us to the conclusion that our
Space perception also becomes impaired as
approach to aligning a divided freeway should
speed increases. This is a complex sub-
be very different from that for a two lane main
ject and is related to the fact that we can-
road. For freeways, the pavement and its medi-
not perceive small relative changes in objects at long distances.
an dominate and the designer should focus his
A person
attention on the architecture of the alignment as
requires clues from the surrounding land-
it is moulded into the land form. On the other
scape to perceive motion. The movements
hand, for the narrow pavements typical of much
of objects travelling parallel and closest
of our network, the opportunity to use the road
to the axis of vision cannot be perceived
as a platform for viewing the environment can
beyond 250 metres on either side. As speed
be used to dramatic effect in road architecture.
increases the time interval between first discerning movement and passing the object reduces.
5.3
It follows therefore that the
DESIGN APPROACH
highway should offer as many clues as possible to allow the driver to judge his
In engineering terms, the geometric elements of
speed and remain in tune with reality as
the highway are simple. We use tangents, hor-
the space around him changes continuously.
izontal curves, grades and vertical curves.
When taken together, these principles confirm
However, the use of these elements in combi-
that, at the high speeds on main and freeway
nation can be nearly infinite: The result can be 5-3
Chapter 5: Alignment Design
Geometric Design Guide
5.
pleasing or discordant; economical or expen-
11.
sive; safe or dangerous.
The history, competency and funding of road maintenance in the region.
In
The two main factors that dominate alignment
context-sensitive
information
is
required which should be collated and summa-
design are the purpose of the road and the site.
rized on plans and in tables to build up an
The standards for a particular facility are usual-
understanding of the site and used to underpin
ly determined at the early planning stage and
the design trade-offs that will invariably follow.
are based on the highway’s position in the road hierarchy and on the budget available.
short,
Decisions such as which side of a ridge to fol-
This
low, where and at what angle to cross a river,
leads to a definition of standards in terms of
where material can be borrowed to support a
width, design speed, road surface, maximum
grade separation structure, where intersections
gradient, accessibility and drainage standards
or interchanges can be safely located, are all
as input to the initial design.
commonplace when a road alignment is designed. As they are also usually inter-related,
With these standards in mind, it is then the task
comprehensive information is essential to effec-
of the designer to understand the site as holisti-
tive and efficient design.
cally and in as much detail as possible. We need to know about:
With the purpose of the road established and
1.
The landform and how it was sculptured
the opportunities and constraints understood,
by nature;
the next step is to identify broad corridors or
2.
The underlying geomorphology;
avenues for further investigation both from the
3.
The engineering properties of the sur-
information gathered and from on-site recon-
face materials including erodibility;
naissance. Within each corridor, sketch-plan-
The cadastral layout and ownership pat-
ning techniques can be employed, ideally at a
terns;
scale of 1:5 000 to enable the alternatives to
The use of the land and the natural veg-
more closely examined. The ranking of the pre-
etation;
ferred options that flow from this examination
The normal and extreme weather cond -
should be rigorous and scientific. It should deal
ions;
with all issues, including aesthetics, engineer-
The drainage patterns, streams and
ing, socio-political issues and the life cycle cost.
peak discharges of the important catch-
This process, with its plethora of feedback loops
ments including their silt load;
and consultation, invariably leads to the selec-
Sensitive environmental areas that
tion of one preferred corridor, or at most two, in
require protection or preservation;
which an alignment can be designed at a scale
9.
The location of road building material;
suitable for final construction drawings.
10.
The need for access to and through the
process should be followed, even if there is only
road including an understanding of local
one obvious and preferred corridor, as it is part
circulation; and
of the process of understanding the site.
4.
5.
Geometric Design Guide
6. 7.
8.
5-4 Chapter 5: Alignment Design
This
5.4
FITTING THE ROAD TO THE
fall relative to the centreline. The notion is thus
LANDSCAPE
that of an abstract ribbon in space, which the designer should be able to visualise from study
The appearance of the road to the traveller and
of the survey plan, the longitudinal section and
its appearance in the landscape depend on how
the cross-section. As a rule, the most pleasing
we string the tangents, grades and curves
appearance results when the horizontal and ver-
together.
tical curves are of approximately equal length
These elements form an inclined
and in phase with one another.
plane, a simple geometric form common to all engineering design. However, nature did not create the surface of the earth using pencil and
External harmony describes the match or mis-
paper. The inclined plane is not nature's way of
match between the road and its environment.
folding the landscape, forming escarpments and
The achievement of external harmony results in
incising the rivers. A road with its rigid geomet-
the road being an enhancement of the land-
ric form made up of vectors and circular arcs is
scape rather than a scar across it. An example
thus a discordance in nature. It is the designer's
of the latter is a long tangent at right angles to
task to minimise that discord. This can be done
the natural contours and with a succession of
by the judicious use of curvature.
short crest and sag curves closely following the ground line.
This results in a roller coaster
Returning to the earlier discussion on what the
appearance totally at odds with the form of the
eye sees at speed and how vehicles behave, we
landscape.`
concluded that there is a vast scale difference
5.4.1
between a divided freeway and a two-lane road.
Freeway design
while the other is typically 13,7 metres wide or at
In locating a freeway or divided roadway, the
most 15 if there are sidewalks or surfaced
appearance of the road as a ribbon in the land-
drains. On a freeway, the paved ribbon domi-
scape from the viewpoint of the driver is crucial.
nates the driver's view, whereas, on a two-lane
To create a continuous and homogenous
road, the roadside takes up most of the visual
appearance, sudden breaks, kinks or abrupt
field.
It follows, therefore, that the design
changes in the alignment must be avoided. This
approaches for these two cases need to be very
invariably requires the use of above minimum
different indeed.
standards for horizontal and vertical curves and the use of the clothoid spiral to form transitions
This difference in approach is captured in the
between tangents and curves. To create inter-
twin concepts of internal and external harmony
est in a changing landscape the designer should
of the alignment.
strive for the curvilinear or 'splined' alignment. Figure 5-1 illustrates a range of acceptable
Internal harmony describes the drivers' view of
curve lengths for varying deflections and radii
the road itself as the centreline rises and falls or
that are visually desirable. Curve radii of up to
changes direction and the road edges rise and
18 000 metres with lengths in excess of 4 000 5-5
Chapter 5: Alignment Design
Geometric Design Guide
The former can easily be 35 to 40 metres wide
metres have been used successfully in South
the designer must be aware of the optical illu-
Africa.
sion that results from this combination. A crest curve causes a horizontal curve to appear to
Visualising the roadway from the three dimen-
have a larger radius than it has in reality and a
sional viewpoint of the driver is important. The
sag curve causes the horizontal curve to appear
Figure 5-1: Radii for acceptable curve lengths road that flows through the landscape, avoids
sharper than it really is. The lower the K-value
major obstacles and is in scale with the terrain
of the vertical curvature, the more pronounced
should be the designer's objective.
the effect is. If the horizontal curve has a mini-
Geometric Design Guide
mum radius, the crest curve could tempt a drivThe co-ordination of the vertical and horizontal
er to maintain a speed higher than is safe
alignment ensures that the scale of the plan and
whereas the sag curve could lead to unneces-
profile view are in harmony. However, it is not
sary and sharp braking which would not neces-
always possible to ensure that horizontal and
sarily be anticipated by following drivers.
vertical curves coincide. When these elements
Correct phasing of the vertical and horizontal
are out of phase or out of scale the designer
alignment should thus be accompanied by the
should take particular care to avoid unpleasant
use of horizontal radii that are well above the
effects.
minimum for the design speed of the road.
Several examples of good and bad
practice are illustrated and discussed in Section A useful tool for analysing the curvilinear nature
5.4.3.
of an alignment is the 1/R diagram in which the Although the aesthetics of the 3-D alignment of
inverse of the radius is plotted against the cen-
the road are enhanced by having the vertical
treline distance. In the diagrams shown in the
curves contained within the horizontal curves,
following figures tangents have a zero value and 5-6
Chapter 5: Alignment Design
the curves a constant value that is inversely pro-
Figure 5-5 illustrates alternative 1/R diagrams
portional to the radius. Transitions appear as
for a group of successive tangents employing:
sloping lines.
1.
Minimum radius curves;
2.
Curves approximately equal in length to
The examples in Figures 5-2 to 5-4 illustrate
the adjacent tangents; and
how 1/R diagrams are used to visualise the
3.
Long curves with the intervening tan-
curvilinear nature of an alignment. Not only is
gents only long enough to accommo-
the disjointed nature of the alignment shown in
date superelevation development
Figure 5-2 immediately apparent but the total area enclosed by the 1/R line and the horizontal
A fourth 1/R diagram illustrates the effect of
Figure 5-3: 1/R diagram for long curve/short tangent alignment axis in Figure 5-4 is significantly less than the
removing the broken-back curve between SV 1
same areas in Figures 5-2 and 5-3.
800 and SV 6 000. Depending on topographic 5-7 Chapter 5: Alignment Design
Geometric Design Guide
Figure 5-2: 1/R diagram for short curve/long tangent alignment
Figure 5-4: 1/R diagram for curvilinear alignment with transitions restraints, this alignment would be the preferred
ficient for aesthetic design. It is essential that
option. It illustrates two sets of true S-curves,
the road flow with the landscape.
being reverse curves with equal radii. The first
approaching an escarpment from rolling terrain
S-curve has radii of approximately 3 000 metres
the curve radii and curve length should be grad-
and the other radii of approximately 1 100 metres.
ually decreased in the transition zone until the
When
Geometric Design Guide
road enters the pass. This continuity of alignRelating the areas to the deflection angles and
ment enhances both the aesthetic and operating
the remaining tangents by simply making the
characteristics by reducing the element of sur-
curves as long as possible is not, of course, suf-
prise. It is under these conditions that co-ordi-
Figure 5.5: 1/R diagrams for alternative alignments 5-8 Chapter 5: Alignment Design
nating the horizontal and vertical alignment is
As the roadside takes up more than half the dri-
particularly important, as the gradients will also
ver's visual field when driving on a single car-
change in keeping with the horizontal alignment.
riageway road, there is significant benefit to be gained by maintaining the grade line above
Treatment of the median can play an important
ground level for as much of the alignment as
role in the design of a divided highway. Medians
possible. Roadways elevated in this way create
that are less than 10 metres wide should never
greater interest and improve overall visibility in
be narrowed and should usually be treated as
addition to the obvious benefit of having suffi-
unifying the two carriageways. When median
cient height to accommodate drainage struc-
widths of 15 metres or more are used, the car-
tures.
riageways can be treated independently and
5.4.3
some variation of median width or even separation of the carriageways can assist in fitting the
To illustrate the advantages of visualising the
highway to the landscape.
5.4.2
Perspectives in road design
alignment in three dimensions and to guide the design towards good practice, a number of
Single carriageway roads
alignment combinations are shown in Figures 55 to 5-17.
The advantages of curvilinear, co-ordinated alignments apply equally well to single carriageway roads. However, because the carriageway is not as wide as that of a dual carriageway, geometric standards that are well above the minimum are not demanded by the scale of the road but are certainly not precluded.
In designing the road using curves to fit the landscape, the chosen radii should allow, whermay require the day lighting of shallow cuts. With sufficiently long radii, proper sight distance can be achieved.
The alternative method of
achieving an adequate percentage of passing sight distance is by reducing the radius and hence the length of the horizontal curve enabling provision of passing sight distance on the adjacent tangents. The result may provide adequate sight distance but possibly at the cost of aesthetics.
5-9 Chapter 5: Alignment Design
Geometric Design Guide
ever possible, for passing sight distance. This
Figure 5-5 shows the advantage of maintaining
result in a disjointed alignment will be there for
a constant, uniform grade for as long as possi-
the life of the road.
ble.
Local dips to minimise earthworks that
Geometric Design Guide
A: Local dip on long grade
B: Local dip eliminated on long grade
Figure 5-5: Short sag curve on long tangent 5-10 Chapter 5: Alignment Design
Short crests and sags should also be avoided
Maintaining a constant grade is the preferred
on horizontal curves, as shown in Figure 5-6.
option.
B: Removal of humps on horizontal curve
Figure 5-6: Short humps on long horizontal curve 5-11 Chapter 5: Alignment Design
Geometric Design Guide
A: Short humps on long horizontal curve
A short discontinuity or dip in the alignment pre-
advance and following the sag curve improves
ceding a horizontal curve creates a particularly
the appearance, as shown in Figure 5-7.
discordant view. Eliminating the crest curves in
Geometric Design Guide
A: Short hump and dip preceding horizontal curve
B: Long sag curve linking into horizontal curve
Figure 5-7: Short vertical curves preceding a long horizontal curve 5-12 Chapter 5: Alignment Design
A common fault in road alignment is illustrated in
The advantages in the alignment aesthetics of a
Figure 5-8. The roadway is often unnaturally
skew crossing often far outweigh the savings
curved to cross a small stream or grade separa-
deriving from a square crossing
tion at right angles.
A: Distorted alignment to create square river crossing
Figure 5-8: Distorted alignment at bridge crossing 5-13 Chapter 5: Alignment Design
Geometric Design Guide
B: Skew crossing improves horizontal alignment
Figure 5-9A illustrates the broken-back horizon-
back effect. The advantages of using a single
tal curve, or two curves in the same direction
radius curve throughout are illustrated in Figure
separated by a short tangent. The sag curve on
5-9.B.
the separating tangent intensifies the broken-
Geometric Design Guide
A: Broken-back curve
B: Replacement of broken-back curve by single radius long curve
Figure 5-9: Broken-back curve 5-14 Chapter 5: Alignment Design
A sag curve at the start of a horizontal curve has
the horizontal curve would cause it to start earlier.
the effect of enhancing the sharp angle appear-
Applying both remedial measures should result
ance as shown in Figure 5-10, and should be
in a better phasing of the horizontal and vertical
avoided. Raising the preceding grade will move
alignments.
the sag curve downstream. A longer radius on
5-15 Chapter 5: Alignment Design
Geometric Design Guide
Figure 5-10: Out-of-phase vertical and horizontal alignments
Minor changes in grade or rolling of the vertical
avoided on long horizontal curves.
alignment as shown in Figure 5-11 should be
A: Rolling gradeline
B: Elimination of rolling gradeline
Geometric Design Guide
Figure 5-11 B illustrates the advantages of co-ordinating the horizontal and vertical alignments.
Figure 5-11: Minor rolling on long horizontal curve 5-16 Chapter 5: Alignment Design
crest and the continuation of the curve is visible
horizontal curve is hidden by an intervening
in the distance. The road appears disjointed.
Figure 5-12: Break in horizontal alignment
5-17 Chapter 5: Alignment Design
Geometric Design Guide
Figure 5-12 shows the effect when the start of a
Geometric Design Guide
Figure 5-13: Well-coordinated crest and horizontal curves
Figure 5-14: Well coordinated sag and horizontal curves Figure 5-13 and Figure 5-14 illustrate the
vertical alignment.
advantages of co-ordinating the horizontal and
curve is contained within the horizontal curve. 5-18
Chapter 5: Alignment Design
In each case the vertical
Figure 5-15: N2 North: Curvilinear alignment in a valley in practice.
Figure 5-16: N2 North - Median widening to accommodate stream 5-19 Chapter 5: Alignment Design
Geometric Design Guide
Figures 5-15 to 5-17 illustrate examples where excellent aesthetic designs have been achieved
Figure 5-17: N2 - Combining horizontal and vertical alignment
5.5
INTEGRATION WITH THE ENVI-
In locating a road, it is necessary to synthesise
RONMENT
land use planning and transportation planning.
Geometric Design Guide
The traffic engineer sees the road as a conduit A road is never the exclusive preserve of the
for goods and people. Estimates are made from
user as it affects everybody in its surroundings.
the factors governing trip generation and trip ori-
The impacts on the non-user are usually a com-
gins and destinations. These ultimately result in
bination of social, economic and environmental
expected and future traffic volumes. The land
factors that act together in complex ways. They
use planner on the other hand is concerned with
can be negative as in the scarring of a hillside,
the organisation of the areas and their relation-
the diversion of a stream or the closing of an
ship to one another. Although making a neces-
access. They can also be positive:
sary contribution to communication, roads, par-
•
When the space used by the road is
ticularly those of an arterial or freeway nature,
orderly;
are viewed as a barrier. They should never cut
The alignment imaginative;
across homogenous areas of whatever type.
• • •
It appears to belong where it is; and It serves the transportation needs of the
The challenge of reconciling the conflicts
community.
between the disciplines can best be tackled by a 5-20 Chapter 5: Alignment Design
team approach where all participants strive to
guardrails.
understand the viewpoints and constraints of
more readily into the natural contour of the land.
the various disciplines.
Integration with the The general rule is that the lower the cut or fill
social environment should be the primary objective.
Flat, rounded slopes also blend
the flatter the slope should be. On fills with a
Of the social values, the aesthetic and
height of 8 to 10 metres, slopes of 1 : 1,1/2 to1
visual impacts are often the most important. It is
: 2 appear acceptable. For heights of 4 to 5
the detail of embankments, road signs, bridges,
metres slopes of 1 : 4 should be the goal.
the planting of the median and a host of other
Minor cuts and fills should be blended into the
design features that attract the eye. All these
landscape so that they are hardly noticeable.
should be integrated with the immediate envi-
This concept is shown in Figure 5-18. In the first
ronment of the road.
sketch, the continuity of the space is preserved in the cross-section whereas, in the second, the
Of all these, the form of the cross section is the
space is chopped up into discontinuous ele-
main element under the control of designer that
ments.
can be manipulated to soften the impact of the road. Flattening the slopes of cut and fill and rounding
The use of contour plans to grade cut and fill
the changes of slopes to create smooth con-
slopes are particularly useful when designing
tours has many benefits. It reduces soil erosion,
interchanges to ensure a pleasing layout of
minimises the risk of injury when a vehicle
what are often large ground areas.
leaves the road and reduces the warrants for 5-21
Chapter 5: Alignment Design
Geometric Design Guide
Figure 5-18: Blending of fills into landscape
Geometric Design Guide
Figure 5.19: Contour plans of cut slopes
Figures 5-19 and 5-20 illustrate the advantages
through a natural feature whereas the second
of transitioning the slopes into a deep cut. The
illustrates what, at best, would be a scar on the
approach shown in the first figure could lead to
landscape.
the appearance of the road being located
5-22 Chapter 5: Alignment Design
B: Cut with varying cut batters
Figure 5.20: Cuts with constant or varying cut batters
5-23 Chapter 5: Alignment Design
Geometric Design Guide
A: Cut with constant cut batters
TABLE OF CONTENTS 6
INTERSECTION DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 6.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2
DESIGN PRINCIPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.2 Elements affecting design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 6.2.3 Traffic manoeuvres and conflicts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3 6.2.4 Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4 6.2.5 Intersection types and selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5 6.2.6 Location of intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 6.2.7 Spacing of intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-8 6.2.8 Channelisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11
6.3
GEOMETRIC CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.3.1 Angle of intersection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.3.2 Horizontal and vertical alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12 6.3.3 Lane widths and shoulders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13
6.4
SIGHT DISTANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.4.1 General Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13 6.4.2 Sight Triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14 6.4.3 Intersection control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15
6.5
CHANNELISATION ELEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26 6.5.1 Channelising islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26 6.5.2 Turning roadway widths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31 6.5.3 Tapers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32
6.6
ROUNDABOUTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34 6.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34 6.6.2 Operation of roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-34 6.6.3 Design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-36 6.6.4 Sight distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-37 6.6.5 Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-37
LIST OF TABLES Table 6.1: Values of human factors appropriate to intersection design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2 Table 6.2 : Vehicle characteristics applicable to design of channelised intersections . . . . . . . . . . . . . . . . . . 6-3 Table 6.3: Features contributing to accidents at intersections and remedial measures . . . . . . . . . . . . . . . . . 6-4 Table 6.4: Typical maximum traffic volumes for priority intersections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6 Table 6.5: Minimum radii for location of intersections on curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7 Table 6.5: Recommended minimum access separations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 Table 6.5: Recommended sight distances for intersections with no traffic control (Case A) . . . . . . . . . . . . . 6-17 Table 6.6: Adjustment factors for approach sight distances based on approach gradient . . . . . . . . . . . . . . 6-17 Table 6.7: Travel Times Used to Determine the Leg of the Departure Sight Triangle along the Major Road for Right and Left Turns from Stop-Controlled Approaches (Cases B1 and B2). . . . . . . . . 6-19 Table 6.8: Travel times used to determine the leg of the departure sight triangle along the major road . to accommodate crossing manoeuvres at stop-controlled intersections (Case B3). . . . . . . . . . . 6-21 Table 6.9: Leg of approach sight triangle along the minor road to accommodate crossing manoeuvres . from yield-controlled approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-23 Table 6.10:Travel times used to determine the sight distance along the major road to accommodate right turns from the major road (Class F). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-24 Table 6.11:Turning roadway widths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-32 Table 6.12:Taper rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-33
LIST OF FIGURES Figure 6.1: Typical intersection types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1 Figure 6-2: Classes of intersections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10 Figure 6.3: Desirable signal spacings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-11 Figure 6.4: Fitting minor road profiles to major road cross-sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14 Figure 6.5: Sight triangles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-16 Figure 6.6: Effect of skew on sight distance at intersections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-25 Figure 6.7: General types and shapes of islands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-27 Figure 6.8: Typical directional island . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-28 Figure 6.9: Typical divisional (splitter) island . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-29 Figure 6.10:Typical median end treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-31 Figure 6.11: Elements of roundabouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-35 Figure 6.12: Intersection conflict points. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-36
Chapter 6 INTERSECTION DESIGN 6.1
INTRODUCTION
The design of an at-grade intersection requires
physical layout of an intersection is defined by
understanding of the principles of both traffic
its horizontal and vertical alignment, roadway
and highway engineering. The operation of an
cross-sections, surface texture and drainage.
lengths and delays, accident potential, vehicle
The successful integration of all these factors is
operating characteristics and traffic control. The
required for good design, which must overcome
Figure 6.1: Typical intersection types 6-1 Chapter 6: Intersection Design
Geometric Design Guide
intersection is influenced by its capacity, queue
the potential safety and operation conflicts that
of 2,5 seconds is normally used as an input to
are inherent when traffic streams interact at
models determining intersection sight distance.
intersections.
However, because of heightened awareness at controlled intersections, circumstances may
Although some guidance on capacity and traffic
indicate a lower acceptable value of 2,0 sec-
control is offered, the focus of this chapter is on
onds at busy urban intersections.
application of the geometric principles that govern the physical layout and location of an inter-
The concept of driver expectancy is crucial in
section.
the evaluation of drivers' response and tasks within intersections.
6.2
DESIGN PRINCIPLES Vehicle Characteristics
6.2.1
General
Geometric Design Guide
The size and manoeuvrability of vehicles is a The unique characteristic of intersections is that
governing factor in intersection design, particu-
vehicles, pedestrians and bicycles travelling in
larly when channelisation features are being
many directions, must share a common area,
selected. Because of the importance of vehicle
often at the same time. The mitigation of the
characteristics in the operation of an intersec-
resulting conflicts is a major objective of inter-
tion, the selection of an appropriate vehicle, as
section design.
This conflict resolution is, in
described in sub-section 3.4.4, will influence the
turn, influenced by construction and mainte-
elements in the above table. In selecting an
nance costs, environmental factors and the
appropriate vehicle, the designer should careful-
ease of implementation.
ly evaluate the expected traffic mix in context.
6.2.2
Various vehicle characteristics and their influ-
Elements affecting design
ence on the design of channelised intersections Human factors
are described in Table 6.2.
The driver's response, discussed in sub-section
Environmental Influences
3.2.1, is a major factor in intersection design. The type of highway and area, surrounding land
The recommended perception and reaction time 6-2
Chapter 6: Intersection Design
use and the prevailing climate all have an influ-
The type of area and adjacent land use governs
ence on the type of design selected. Flexibility
the selection of an appropriate intersection. In
of approach is essential and the concepts of
urban areas, pedestrian flows, on-street parking
context-sensitive design as outlined in Chapter
and bus and taxi activity are commonplace. In
2 should be applied
residential areas, bicycles and school crossings need to be considered. They are usually absent
Functional classification is a key to applying the
in rural areas, where utility and delivery vehicles
appropriate design standards. Primary arterials
are more common.
speeds and are often used by drivers unfamiliar
Local climate can influence design decisions.
with them. Large trucks and buses are common
Where the presence of mist is a frequent occur-
and there is a driver expectancy for route conti-
rence, sight distance would be reduced. Heavy
nuity and a high level of service. Intersection
rainfalls can obscure signs and road markings
design should reflect and make provision for the
and reduce pavement friction.
operating characteristics of drivers and their
6.2.3
expectations in the various classes of roads. Channelisation
should
accommodate
Traffic manoeuvres and conflicts
the
expected vehicles in a simple and direct man-
Typical manoeuvres that result in vehicle conflict
ner. Decision sight distance is an important ele-
at intersections are:
ment and traffic control devices and pavement
• •
markings should be placed with care.
Crossing; Merging;
6-3 Chapter 6: Intersection Design
Geometric Design Guide
carry high traffic volumes, operate at high
• •
Diverging; and
Traffic volumes are the most important determi-
Weaving.
nant of intersection accidents. This is not surprising as the accident potential caused by con-
Diverging and merging may be either to the left
flict increases as traffic on the approach legs
or right, mutual or multiple. Crossings may be
increases.
direct, if the angle of skew is between 75o and 120o, or oblique if the angle is in the range of
The type of traffic control also influences acci-
60o
Oblique skews should be avoided if
dents. More rear-end and sideswipe accidents
at all possible. If the angle of skew is less than
tend to occur at signalised intersections than at
60o,
the possibility of replacing the skew by a
other types of control. Stop and Yield controls
staggered intersection should be considered.
tend to increase the frequency of crossing acci-
to
75o.
Angles of skew greater than
120o
should be
dents. Table 6.3 lists many of the condition that
replaced by relocation of the intersection to an
can lead to accidents and also the geometric
angle of skew closer to
90o.
and control measures that are used to mitigate
Geometric Design Guide
poor accident experience.
Weaving is a combination of merging and
6.2.4
diverging traffic moving in the same direction. It
Capacity
may be simple or complex. To operate successfully, an intersection must be The conflict at intersections created by the vari-
able to handle peak traffic demands. The analy-
ous manoeuvres leads to a unique set of opera-
sis of capacity is based on the operational char-
tional characteristics. Understanding these is
acteristics of conflicting vehicles separated by
central to intersection designs and the most
the time constraints imposed by traffic control
important characteristics are safety and capacity.
devices. The measuring and forecasting of traf6-4
Chapter 6: Intersection Design
fic flows and capacity analysis is a specialised
Unsignalised intersections
subject and designers should refer to the manuals and references commonly used. The follow-
The capacity of the major road at Stop- and
ing is a brief summary of capacity as it relates to
Yield-controlled intersections is not affected by
design.
the presence of the intersection. The capacity of the minor road is dependent on the distribution of gaps in the major road traffic and the gap
Signalised intersections
acceptance of the minor road traffic. acceptance
The idealised flow rate through an intersection
and vehicle length.
green time. Initial driver reaction, vehicle accel-
described in Section 6.4
signal cycle in accordance with C
=
capacity (veh/h)
s
=
saturated flow
Factors affecting capacity include:
• • • •
rate (veh/h) g/c
=
the ratio of green time to
Operational speed of the major road; Intersection sight distance; Radii of turning roadways; Intersection layout and number of lanes;
• •
signal cycle time.
Type of area; and Proportion of heavy vehicles
The critical factors are intersection sight dis-
The important factors affecting saturation flow
tance and the number and arrangement of traf-
are:
fic lanes.
Number of lanes; Widths of lanes;
6.2.5
Proportion of heavy vehicles;
Intersection types and selection
Gradients in excess of 3%; On-street parking;
There are four basic types or classes of inter-
Pedestrian activity; and
section:
Type and phasing of signals.
three-legged T and Y intersections;
four legged intersections with a defined crossing path; multi-legged intersections; and round-
The critical factors are the total number of lanes
abouts, with the last-mentioned encompassing
and the need for exclusive turning lanes at each
all three of the previous types.
approach.
These are
shown schematically in Figure 6.1. The design-
6-5 Chapter 6: Intersection Design
Geometric Design Guide
• • • • • • •
Gap
to compute intersection sight distance as
al to the green time for that approach within the
where
It is not a function of
are usually somewhat shorter than those used
approach or leg of an intersection is proportion-
s x g/c
the
acceptance times used in determining capacity
The capacity of an
=
on
approach speed on the major road.
eration and the behaviour of following vehicles
C
dependent
reaction/response time, vehicle acceleration
is known as the saturation flow rate per hour of
all affect this flow rate.
is
Gap
er's selection of the basic intersections type is
At intersections carrying light crossing and turn-
normally predicated on the design context, as
ing volumes the capacity figures for uninterrupt-
intersections can vary greatly in scope, shape,
ed flow generally apply. Table 6.4 below is a
degree of channelisation and traffic control
guide to the maximum traffic volumes that these
measures.
intersections can accommodate. When volumes exceed the above, the capacity
Important factors to be considered in the selec-
of the intersection should be analysed in detail.
tion of an intersection type are:
• • • •
Cost of construction;
As safety at intersections is of key importance,
Type of area;
the following summary of the interrelationship
Land use and land availability;
between intersections and accidents can act to
Functional classes of the intersecting
guide the selection of an intersection type and
roads;
• • •
layout:
Approach speeds;
•
Proportion of traffic on each approach;
The U.S. National Safety Council esti-
and
mated that 56 per cent of all urban acci-
Volumes to be accommodated.
dents and 32 per cent of all rural accidents occur at intersections.
•
Careful consideration of these factors, together
to the volume and distribution of traffic
with the warrants for selecting appropriate traffic
on the major and minor roads.
control devices, will lead to an appropriate
•
choice or to a limited number of alternatives
Geometric Design Guide
The number of accidents is proportional
Roundabouts have considerable safety advantages over other types of at grade
from which to make the final selection. The crit-
intersections.
ical factors are cost and capacity.
•
Poor sight distance leads to significantly higher injury and total accident rates.
A worldwide review of intersection design prac-
However, on roundabout approaches,
tice reported that, "typically the cheapest inter-
accidents may actually increase with
section type providing the required level of serv-
increasing sight distance.
ice is chosen". This cost is usually the sum of
•
the design, construction and right-of-way costs.
Medians should be as wide as practical
This view is consistent with South African expe-
at rural, unsignalised intersections but
rience, where the cost to the road authority is
not wider than necessary at signalised
often the governing factor in the choice of inter-
intersections.
section type.
•
Channelisation is usually beneficial but
6-6 Chapter 6: Intersection Design
• 6.2.6
kerbed islands in the major road may be
the alignment of either the major or the minor
hazardous in rural areas.
road, or both, to ensure that adequate sight dis-
The hazard of an intersection increases
tance is available. If this is not possible, the
as the approach speed increases.
options available to the designer are to:
Location of intersections
• • •
relocate the intersection; provide all-way Stop control; or provide a Jug handle or Quarter link
Given the fact that intersections are the most
interchange, as described in Section
dangerous part of any road network, it follows
7.6.4.
that their location deserves serious attention by
Where heavy earthworks, possibly beyond the
the designer. It is necessary to minimise both
normal limits of the road reserve, are required to
the likelihood of crashes occurring and the con-
provide adequate sight distances, relocation
sequences of the crashes that do occur. There
may be an option.
are thus various restraints on the location of intersections that should be considered.
The location of an intersection on a curve can create problems for the drivers on both legs of
The need for drivers to discern and perform the
the minor road. Drivers on the minor road leg on
manoeuvres necessary to pass safely through
the inside of the curve will have difficulty in see-
an intersection demands that decision sight dis-
ing approaching traffic because this traffic will
tance be available on the major road approach-
be partly behind them.
es. The driver on the minor road requires ade-
effect, an artificial angle of skew. Furthermore,
quate intersection sight distance, as well as the
there is a possibility that a portion of the sight tri-
sight triangles described in Section 6.4, in order
angle may fall outside the limits of the road
either to merge with traffic on the major road or
reserve, which could hamper efforts to obtain a
to cross safely. It may be necessary to modify
clear line of sight for the driver on the minor
Chapter 6: Intersection Design
Geometric Design Guide
6-7
This constitutes, in
road. For these, reasons, intersections should
The location of an intersection may require
not be located on horizontal curves with radii
modification to improve the angle of skew
less than those indicated in Table 6.5.
between the intersecting roads, as discussed in Section 6.3.1. If the angle of skew is less than
Drivers on the outside of a curve typically have
60O, the intersection can be replaced by two rel-
little or no difficulty in seeing opposing vehicles
atively closely spaced T-intersections. A vehicle
on the major road. The opposing vehicles are
on the minor road would thus follow the route
partly in front of them and they have the addi-
comprising a right turn onto the major road, fol-
tional height advantage caused by the superel-
lowed by a left turn off it. Any delay to the minor
evation of the curve. However, they have to
road vehicle would occur clear of the high-
negotiate the turn onto the major road against
speed traffic on the major road. If the angle of
an adverse superelevation. The risks involved
skew is greater than 120O, relocation should be
in sharp braking during an emergency should
to a four-legged intersection in preference to the
also be borne in mind when an intersection is
two T-intersections, because these would result
located on a curve. In general, an intersection
in the minor road traffic following the route com-
should not be located on a curve with a superel-
prising a left turn onto the major road and a right
evation greater than 6 per cent.
turn off it with consequent delays to the major road traffic and increased risk to the vehicle
Stopping sight distances increase with steepen-
waiting to complete the right turn off the major
ing negative gradient. Stopping sight distance
route. A right-left stagger or offset is, in short, to
required on a gradient of -3 per cent is approxi-
be preferred to a left-right stagger.
mately 6 per cent longer than that on a level grade, whereas, on a -6 per cent gradient, it is
A further limitations on the location of intersec-
approximately 16 per cent longer. It is suggest-
tions - being the spacing of successive intersec-
ed that, as a safety measure, intersections
tions - is discussed in the following section.
should not be located on gradients steeper than
6.2.7
three per cent. The gradient is more critical on
Spacing of intersections
Geometric Design Guide
the minor road than on the major road because all vehicles on the minor road have to stop or
Designers seldom have influence on the spac-
yield.
ing of roadways in a network as it is largely predicated by the original or developed land
One of the consequences of a collision between
use. Nevertheless, the spacing of intersections
vehicles is that either or both may leave the
impacts significantly on the operation, level of
road. It is therefore advisable to avoid locating
service and capacity of a roadway. It follows
intersections on high fills if at all possible. The
that intersection spacing should, inter alia, be
obstruction of sight distance by bridge parapets
based on road function and traffic volume. The
should also be borne in mind. In the case of the crossing road ramp terminal at a narrow dia-
principles described in the National Guidelines
mond interchange, both these problems may
for Road Access Management in South Africa
arise.
should therefore play a role in the determination 6-8 Chapter 6: Intersection Design
of the location of individual intersections. This is
Along signalised arterials, intersection spacing
of particular concern when the provision of a
should be consistent with the running speed and
new intersection on an existing road is being
signal cycle lengths, which are variables in
considered.
themselves. If the spacing of the intersections is based on acceptable running speeds and
Access management is aimed at maintaining an
cycle lengths, signal progression and an effi-
effective and efficient transportation system for
cient use of the roadway can be achieved.
the movement of people and goods, simultaneously supporting the development of the adja-
In Figure 6.2, these three variables are com-
cent land use.
bined in a chart allowing the selection of a suit-
usage generally leads to demands for improved
able intersection spacing.
road infrastructure and the improved infrastruc-
transpires that the minimum spacing on arterial
ture makes access to it very attractive. Allowing
roads should be at least 400m. Where spacings
access simply on the basis of its meeting some
closer than the minimum exist, a number of
or other minimum geometric requirement results
alternative actions can be considered, for exam-
in increasing traffic conflicts and reduction in
ple two-way flows can be converted to one-way
capacity so that the benefit of the original
operation or minor connecting roads can be
improvement is lost.
closed or diverted, and channelisation can be
This then leads to
From this figure it
demands for further road improvements.
used to restrict turning movements.
This cycle can only be broken by the develop-
Where the crossing road of an interchange is an
ment of a proper Access Management Plan by
arterial, the suggested minimum distance along
the local authority concerned. This plan speci-
the arterial from the ramp terminal to the next
fies where intersections may be located.
intersection is 200m in the case of a collector
Furthermore, it defines the class of intersection
road. If the next intersection is with an arterial
that may be considered. Three classes of inter-
the spacing between the ramp and the intersec-
sections are defined in the National Guidelines.
tion should be increased to 600 metres for a
These are:
Class 3 arterial and 800 metres for a Class 2
•
arterial.
Full access, which allows for all possible movements at an intersection or access;
•
•
Partial access, which allows left-in, left-
The spacing between successive unsignalised
out and right-in movements to and from
intersections is measured by the separation
a development or access road; and
between them with separation being defined as
Marginal access, which allows only left-
the distance between their reserve boundaries.
in and/or left-out movements to and from
Recommended access separations are provid-
a development or access road.
ed in Table 6-5. The left-in left-out class of access would typical-
These are illustrated in Figure 6.2.
ly only be applied under circumstances where 6-9 Chapter 6: Intersection Design
Geometric Design Guide
Increasingly intensive land
speeds or traffic volumes or both are high. This
and deceleration lanes using taper rates as list-
is in order to minimise the disruption that would
ed in Table 6-12 and lengths listed in Table 7-5
be caused by right-turning vehicles. As such, it
or 7-6
Geometric Design Guide
would normally be provided with acceleration
Figure 6-2: Classes of intersections 6-10 Chapter 6: Intersection Design
6.2.8
•
Channelisation
Undesirable or wrong-way movements should be discouraged or prohibited;
• •
The purpose of channelisation is to achieve safe and efficient operation by managing the conflicts that are inherent to intersections.
Vehicle paths should be clearly defined; The design should encourage safe vehicle speeds;
NCHRP
Figure 6.3: Desirable signal spacings
•
Report 279 reports that the objectives of good
•
Reduction of the number of points of
right angles and merge at flat angles;
potential conflict to the minimum com-
•
patible with efficient operation;
•
•
Limitation of the frequency of actual con-
The design should be in the context of the traffic control scheme;
•
flicts; and
•
High priority flows should have the greater degree of freedom;
Reduction of the complexity of conflict areas whenever possible;
•
Traffic streams should cross at close to
Decelerating, slow-moving or stopped
Limitation of the severity of those con-
vehicles should be separated from high-
flicts that do occur.
er-speed through lanes; and
•
Refuge for pedestrians and the handi-
To achieve these objectives there are nine prin-
capped should be provided where
ciples of channelisation design:
appropriate. 6-11 Chapter 6: Intersection Design
Geometric Design Guide
of conflict whenever possible;
intersection design are:
•
Channelisation should separate points
The tools available to apply these principles are:
have difficulty at this angle of skew in seeing
• • • • •
Defining and arranging traffic lanes;
vehicles approaching from their left.
Traffic islands of all sizes and types;
designer should be able to specifically justify
Median islands;
using an angle of skew less than 75°. In the
Corner radii;
remodelling of existing intersections, the acci-
Horizontal and vertical approach geom-
dent rates and patterns will usually indicate
etry;
whether a problem exists and provide evidence
Pavement tapers and transitions; and
on any problems related to the angle of skews
• •
The
Traffic control devices.
6.3.2
Horizontal and vertical alignment
The first six elements are a range of physical features while traffic control devices are an inte-
The horizontal and vertical alignments through
gral part of any intersection. These six elements
and approaching an intersection are critical fea-
are discussed in Section 6.5.
tures. Simple alignment design allows for early recognition of the intersection and timely focus
6.3
on the intersecting traffic and manoeuvres that
GEOMETRIC CONTROLS
must be prepared.
6.3.1
Angle of intersection The following are specific operational requirements at intersections:
The angle of intersection of two roadways influ-
•
ences both the operation and safety of an inter-
required sight distance;
section. Large skews increase the pavement
•
area and thus the area of possible conflict.
The alignments should allow for the fre-
Operationally they are undesirable because:
quent braking and turning associated
•
with intersections; and
Crossing vehicles and pedestrians are
•
exposed for longer periods;
•
Geometric Design Guide
The alignments should not restrict the
•
The driver's sight angle is more con-
undue direct attention to be detracted
strained and gap perception becomes
from the intersection manoeuvres and
more difficult;
conflict avoidance.
Vehicular movements are more difficult
As a general guide, horizontal curve radii at
and large trucks require more pavement
intersections should not be less than the desir-
area; and
•
The alignments should not require
able radius for the design speed on the
Defining vehicle paths by channelisation
approach roads.
is more difficult. For high-speed roads with design speeds in
For new intersections the crossing angle should
excess of 80km/h, approach gradients should
preferably be in the range 75° to 120°. The
not be greater than - 3 per cent. For low-speed
absolute minimum angle of skew is 60° because
roads in an urban environment this can be
drivers, particularly of trucks with closed cabs,
increased to - 6 per cent. 6-12
Chapter 6: Intersection Design
distance at the intersection to above-minimum
Stopping sight distance should be provided con-
requirements.
tinuously on all roadways including at the
For new intersections, the gradient on the minor
approaches to intersections. However, in rural
roadway is normally adjusted to form a smooth
areas or when approach speeds are in excess
profile, as suggested in Figure 6.4.
of 80 km/h, the decision sight clearance set out in Section 3.5.8 should be provided on all approaches to intersections for safe operation,
Where major roadways intersect, the profiles of
particularly where auxiliary lanes are added to
both roads are usually adjusted in approximate-
the intersection layout to accommodate the turn-
ly equal manner. When significant channelisa-
ing movements.
tion is introduced in association with complex gradients, intersections should be designed on an elevation plan to avoid discontinuities and
In addition to these forms of sight distance, it is
ensure free drainage.
necessary
to
Distance (ISD).
6.3.3
Lane widths and shoulders
provide
Intersection
Sight
This is the sight distance
required by drivers entering the intersection to enable them to establish that it is safe to do so
Where intersecting roadways have shoulders or
and then to carry out the manoeuvres neces-
sidewalks, the main road shoulder should be
sary either to join or to cross the opposing traf-
continued through the intersection. Lane widths
fic streams.
should be 3,7 m for through lanes and 3,6
derived from elaborate models based on
metres for turning lanes. Where conditions are
assumptions of reaction times, speeds and
severely constrained, lane widths as low as 3,3
acceleration rates of turning vehicles and the
metres can be considered provided that
deceleration rates of the opposing vehicles, etc.
approach speeds are below 80 km/h. In con-
The distances offered in sub-section 6.4.3 are
stricted urban conditions on low speed-road-
derived from research into gap acceptance as
ways, lane widths of 3,0 metres should be the
reported in NCHRP Report 383 "Intersection
minimum adopted. Offsets from the edge of the
Sight Distance".
Previously, values of ISD were
1,0 metres.
6.4
SIGHT DISTANCE
6.4.1
General Considerations
The provision of adequate sight distances and appropriate traffic controls is essential for safe intersection operation.
6-13 Chapter 6: Intersection Design
Geometric Design Guide
turning roadway to kerb lines should be 0,6 to
Geometric Design Guide
Figure 6.4: Fitting minor road profiles to major road cross-sections
6.4.2
Sight Triangles
Two different forms of sight triangle are required.
In the first instance, reference is to
Each quadrant of an intersection should contain
approach sight triangles. The approach triangle
a clear sight triangle free of obstructions that
will have sides with sufficient lengths on both
may block a driver's view of potentially conflict-
intersecting roadways such that drivers can see
ing vehicles on the opposing approaches.
any potentially conflicting vehicle in sufficient 6-14
Chapter 6: Intersection Design
time to slow, or to stop if need be, before enter-
truck driver is considerably greater than that
ing the intersection. For the departure sight tri-
required by the driver of a passenger car. For
angle, the line of sight described by the
design purposes, the eye height of truck drivers
hypotenuse of the sight triangle should be such
is taken as 1,8 metres for checking the avail-
that a vehicle just coming into view on the major
ability of sight distance for trucks.
road will, at the design speed of this road, have
6.4.3
a travel time to the intersection corresponding to
Intersection control
the gap acceptable to the driver of the vehicle on the minor road. Both forms of sight triangle
The recommended dimensions of the clear sight
are required in each quadrant of the intersec-
triangles vary with the type of traffic control used
tion.
at an intersection because different types of control impose different legal constraints on
The line of sight assumes a driver eye height of
drivers resulting in different driver behaviour.
1,05 metres and an object height of 1,3 metres.
Sight distance policies for intersections with the following types of traffic control are presented
The approach and departure sight triangles are
below:
illustrated in Figure 6.5. The areas shown shaded should be kept clear of vegetation or any
• •
other obstacle to a clear line of sight. To this end, the road reserve is normally splayed to
Intersections with no control (Case A); Intersections with Stop control on the minor road (Case B);
ensure that the entire extent of the sight triangle
o
is under the control of the road authority.
Right turn from the minor road (Case B1);
Furthermore, the profiles of the intersecting
o
Left turn from the minor road (Case B2);
roads should be designed to provide the o
required sight distance. Where one or other of
Crossing manoeuvre from the minor road (Case B3);
the approaches is in cut, the affected sight tri-
•
angles may have to be "day lighted", i.e. the nat-
Intersections with Yield control on the minor road (Case C);
ural material occurring within the sight triangles
o
may have to be excavated to ensure intervisibil-
Crossing manoeuvre from the
o
Left or right turn from the minor road (Case C2);
•
Sight distance values are based on the ability of
Intersections with traffic signal control (Case D); and
the driver of a passenger car to see an
•
approaching passenger car. It is also necessary
Intersections with all-way Stop control (Case E).
to check whether the sight distance is adequate for trucks. Because their rate of acceleration is lower than that of passenger cars and as the
A sight-distance policy for stopped vehicles turn-
distance that the truck has to travel to clear the
ing right from a major road (Case F) is also pre-
intersection is longer, the gap acceptable to a
sented.
6-15 Chapter 6: Intersection Design
Geometric Design Guide
minor road (Case C1);
ity between the relevant approaches.
Figure 6.5: Sight triangles
Geometric Design Guide
Intersections with no control (Case A)
If sight triangles of this size cannot be provided, the lengths of the legs on each approach can be
Uncontrolled intersections are not used in con-
determined from a model that is analogous to
junction with the main road network, but are
the stopping sight distance model, with slightly
common in rural networks and access roads to
different assumptions.
rural settlements. In these cases, drivers must be able to see potentially conflicting vehicles on
Field observations indicate that vehicles
intersecting approaches in sufficient time to stop
approaching uncontrolled intersections typically
safely before reaching the intersection. Ideally,
slow down to approximately 50 per cent of their
sight triangles with legs equal to stopping sight
normal running speed. This occurs even when
distance should be provided on all the
no potentially conflicting vehicles are present,
approaches to uncontrolled intersections.
typically at deceleration rates of up to 1.5m/s. 6-16
Chapter 6: Intersection Design
Braking at greater deceleration rates, which can
from a speed less than the normal running
approach those assumed in the calculation of
speed.
onds after a vehicle on the intersecting
Table 6.5 shows the distance travelled by an
approach comes into view. Thus, approaching
approaching vehicle during perception, reaction
vehicles may be travelling at less than their nor-
and braking time as a function of the design
mal running speed during all or part of the per-
speed of the roadway on which the intersection
ception-reaction time and can brake to a stop
approach is located. These distances should be
Note: Based on ratio of stopping sight distance on specified approach grade to stopping sight distance on level terrain. 6-17 Chapter 6: Intersection Design
Geometric Design Guide
stopping sight distances, begins up to 2,5 sec-
used as the legs of the sight triangles shown in
intersections because all minor-road vehicles
Figure 6.4.
should stop before entering or crossing the major road.
Where the gradient of an intersection approach exceeds three per cent, the leg of the clear sight
Vehicles turning right from the minor road have
triangle along that approach should be adjusted
to cross the stream of traffic approaching from
by multiplying the sight distance in Table 6.5 by
the right and then merge with the stream
an adjustment factor in Table 6.6.
approaching from the left. Left-turning vehicles need only merge with the stream approaching
If these sight distances cannot be provided,
from the right.
As the merging manoeuvre
advisory speed signing to reduce speeds or
requires that turning vehicles should be able to
installing Stop signs on one or more approach-
accelerate approximately to the speed of the
es should be investigated.
stream with which they are merging, it necessitates a gap longer than that for the crossing manoeuvre.
Uncontrolled intersections do not normally require departure sight triangles because they typically have very low traffic volumes.
Right turn from the minor road (Case B1)
If a
motorist finds it necessary to stop at an uncon-
A departure sight triangle for traffic approaching
trolled intersection because of the presence of a
from the left, as shown in Figure 6.4(B), should
conflicting vehicle, it is unlikely that another
be provided for right turns from the minor road
potentially conflicting vehicle will be encoun-
onto the major road for all Stop-controlled
tered as the first vehicle departs the intersec-
approaches.
tion.
Field observations of the gaps accepted by the
Intersections with Stop control on the minor road
drivers of vehicles turning to the right onto the
(Case B)
major road have shown that the values in Table 6.7 provide sufficient time for the minor-road
Geometric Design Guide
Departure sight triangles for intersections with
vehicle to accelerate from a stop and merge
Stop control on the minor road should be con-
with the opposing stream without undue inter-
sidered for three situations:
•
ference. These observations also revealed that
Right turns from the minor road (Case
major-road drivers would reduce their speed to
B1);
• •
Left turns from the minor road (Case
some extent to accommodate vehicles entering
B2); and
from the minor road. Where the gap accept-
Crossing the major road from the minor-
ance values in Table 6.7 are used to determine
road approach (Case B3).
the length of the leg of the departure sight triangle along the major road, most major-road driv-
Approach sight triangles, as shown in Figure
ers need not reduce speed to less than 70 per-
6.4(A), need not be provided at Stop-controlled
cent of their initial speed. 6-18
Chapter 6: Intersection Design
Table 6.7 applies to passenger cars. However,
If the sight distances along the major road
for minor-road approaches from which substan-
based on Table 6.7 (including the appropriate
tial volumes of heavy vehicles enter the major
adjustments) cannot be provided, consideration
road, the values for single-unit trucks or semi-
should be given to the installation of advisory
trailers should be applied.
speed signs on the major-road approaches.
Table 6.7 includes adjustments to the accept-
Dimension "a" in Figure 6.4 (b) depends on the
able gaps for the number of lanes on the major
context within which the intersection is being
road and for the approach gradient of the minor
designed. In urban areas, drivers tend to stop
road. The adjustment for the gradient of the
their vehicles immediately behind the Stop line,
minor-road approach need be made only if the
which may be located virtually in line with the
rear wheels of the design vehicle would be on
edge of the major road. A passenger car driver
an upgrade steeper than 3 per cent when the
would, therefore, be located about 2,4 metres
vehicle is at the stop line of the minor-road
away from the Stop line. In rural areas, vehicles
approach.
usually stop at the edge of the shoulder of the major road. In the case of a three metre wide
The length of the sight triangle along the major
shoulder the driver would thus be approximate-
road (distance "b" in Figure 6.4) is the product of
ly 5,4 metres away from the edge of the trav-
the design speed of the major road in
elled way.
metres/second and the critical gap in seconds
6-19 Chapter 6: Intersection Design
Geometric Design Guide
as listed in Table 6.7.
Where the major road is a dual carriageway, two
be provided, it should be kept in mind that field
departure sight triangles have to be considered:
observations indicate that, in making left turns,
a sight triangle to the right, as for the crossing
drivers generally accept gaps that are slightly
movement (Case B3) and one using the accept-
shorter than those accepted in making right
able gap as listed in Table 6.7 for vehicles
turns.
approaching from the left.
This presupposes
decreased by 1,0 to 1,5 seconds for left turn
that the width of the median is sufficient to pro-
manoeuvres, where necessary, without undue
vide a refuge for the vehicle turning from the
interference with major-road traffic. When the
minor road. If the median width is inadequate,
recommended sight distance for a left-turn
the adjustment in Table 6.7 for multilane major
manoeuvre cannot be provided, even with a
roads should be applied with the median being
reduction of 1,0 to 1,5 seconds, consideration
counted as an additional lane.
should be given to the installation of advisory
The travel times in Table 6.7 can be
speed signs and warning devices on the majorThe departure sight triangle should be checked
road approaches.
for various possible design vehicles because the width of the median may be adequate for
Crossing manoeuvre from the minor road (Case
one vehicle type and not for another so that two
B3)
different situations have to be evaluated. In most cases it can be assumed that the deparLeft turn from the minor road (Case B2)
ture sight triangles for right and left turns onto
Geometric Design Guide
the major road, as described for Cases B1 and A departure sight triangle for traffic approaching
B2, will also provide more than adequate sight
from the right, as shown in Figure 6.4, should be
distance for minor-road vehicles crossing the
provided for left turns from the minor road. The
major road. However, it is advisable to check
lengths of the legs of the departure sight triangle
the availability of sight distance for crossing
for left turns should generally be the same as
manoeuvres:
those for the right turns used in Case B1.
•
Where right and/or left turns are not per
Specifically, the length of the leg of the depar-
mitted from a particular approach and
ture sight triangle (dimension "b") along the
crossing is the only legal manoeuvre;
•
major road should be based on the travel times
Where the crossing vehicle has to cross four or more lanes; or
in Table 6.7, including appropriate adjustment
•
factors.
Where substantial volumes of heavy vehicles cross the highway and where there are steep gradients on the depar-
Dimension "a" depends on the context of the
ture roadway on the far side of the inter
design and can vary from 2,4 metres to 5,4
section that might slow the vehicle while
metres.
its rear is still in the intersection.
Where sight distances along the major road
Table 6.8 presents travel times and appropriate
based on the travel times from Table 6.7 cannot
adjustment factors that can be used to deter6-20
Chapter 6: Intersection Design
mine the length of the leg of the sight triangle
or cross the major road without stopping. The
along the major road to accommodate crossing
sight distances needed by drivers on Yield-con-
manoeuvres.
trolled approaches exceed those for Stop-controlled approaches because of the longer travel time of the vehicle on the minor road.
At divided highway intersections, depending on the width of the median and the length of the design vehicle, sight distance may be needed
For four-legged intersections with Yield control
for crossing both roadways of the divided high-
on the minor road, two separate sets of
way or for crossing the near lanes only and
approach sight triangles as shown in Figure
stopping in the median before proceeding.
6.5(A) should be provided: one set of approach
a For minor-road approach gradients that exceed +3 per cent, multiply by the appropriate adjustment fac- t o r from Table 6.6.
Intersections with Yield control on the minor
sight triangles to accommodate right and left
road (Case C)
turns onto the major road and the other for crossing movements. Both sets of sight trian-
Vehicles entering a major road at a Yield-con-
gles should be checked for potential sight
trolled intersection may, because of the pres-
obstructions.
ence of opposing vehicles on the major road, be Crossing manoeuvres (Case C1)
required to stop. Departure sight triangles as described for Stop control must therefore be provided for the Yield condition. However, if no
The lengths of the leg of the approach sight tri-
conflicting
drivers
angle along the minor road to accommodate the
approaching Yield signs are permitted to enter
crossing manoeuvre from a Yield-controlled
vehicles
are
present,
6-21 Chapter 6: Intersection Design
Geometric Design Guide
b Travel time applies to a vehicle that slows before crossing the intersection but does not stop.
approach (distance "a" in Figure 6.5(A) are
These equations provide sufficient travel time
given in Table 6.9. The distances in Table 6.9
for the major road vehicle, during which the
are based on the same assumptions as those
minor-road vehicle can:
for Case A control except that, based on field
(1)
observations, minor-road vehicles that do not
intersection, while decelerating at the rate of
stop are assumed to decelerate to 60 per cent
1.5m/s² to 60 per cent of the minor-road design
of the minor-road design speed rather than to 50
speed; and then
per cent. The distances and times in Table 6.9
(2)
should be adjusted for the gradient of the minor
same speed.
Travel from the decision point to the
Cross and clear the intersection at the
road approach, using the factors in Table 6.6. Field observations did not provide a clear indi-
The length of the leg of the approach sight tri-
cation of the size of the gap acceptable to the
angle along the major road to accommodate the
driver of a vehicle located at the decision point
crossing manoeuvre (distance "b" in Figure
on the minor road. If the required gap is longer
6.5(A)) should be calculated using the following
than that indicated by the above equations, the
equations:
driver would, in all probability, bring the vehicle to a stop and then select a gap on the basis of
6.1
Case B. If the acceptable gap is shorter than that indicated by the above equations, the sight
6.2
distance provided would, at least, provide a margin of safety.
where: tc
b
Geometric Design Guide
ta
=
= =
travel time to reach and clear the
If the major road is a divided highway with a
major road in a crossing manoeu-
median wide enough to store the design vehicle
vre (sec)
for the crossing manoeuvre, then only crossing
length of leg of sight triangle along
of the near lanes need be considered and a
the major road (m)
departure sight triangle for accelerating from a
travel time to reach the major road
stopped position in the median should be pro-
from the decision point for a vehi-
vided, based on Case B1.
cle that does not stop (sec) (use Left and right-turn manoeuvres (Case C2)
appropriate value for the minorroad design speed from Table 6.9, adjusted for approach grade,
To accommodate left and right turns without
where appropriate)
stopping (distance "a" in Figure 6.5(A)), the
width of intersection to be crossed
length of the leg of the approach sight triangle
(m)
along the minor road should be 25 metres. This
length of design vehicle (m)
distance is based on the assumption that drivers
Vminor =
design speed of minor road (km/h)
making right or left turns without stopping will
Vmajor =
design speed of major road (km/h)
slow to a turning speed of 15 km/h. The length
w La
= =
6-22 Chapter 6: Intersection Design
of the leg of the approach sight triangle along
Since approach sight triangles for turning
the major road (distance "b" in Figure 6.5(B)) is
manoeuvres at Yield-controlled are larger than
similar to that of the major-road leg of the depar-
the departure sight triangles used at Stop-con-
ture sight triangle for Stop-controlled intersec-
trolled intersections, no specific check of depar-
tions in Cases B1 and B2.
ture sight triangles at Yield-controlled intersections should be necessary.
For a Yield-controlled intersection, the travel times in Table 6.7 should be increased by 0,5
Intersections with traffic signal control (Case D)
seconds. The minor-road vehicle requires 3,5 intersection. These 3,5 seconds represent addi-
In general, approach or departure sight triangles
tional travel time that is needed at a Yield-con-
are not needed for signalised intersections.
trolled intersection (Case C).
However, the
Indeed, signalisation may be an appropriate
acceleration time after entering the major road is
accident countermeasure for higher volume
3,0 seconds less for a Yield sign than for a Stop
intersections with restricted sight distance and a
sign because the turning vehicle accelerates
history of sight-distance related accidents.
from 15 km/h rather than from a stop. The net 0,5 seconds increase in travel time for a vehicle
However, traffic signals may fail from time to
turning from a Yield-controlled approach is the
time. Furthermore, traffic signals at an intersec-
difference between the 3,5 second increase in
tion are sometimes placed on two-way flashing
travel time on approach and the 3,0 second
operation under off-peak or night time condi-
reduction in travel time on departure explained
tions. To allow for either of these eventualities,
above.
the appropriate departure sight triangles for 6-23 Chapter 6: Intersection Design
Geometric Design Guide
seconds to travel from the decision point to the
Case B, both to the left and to the right, should
required by a stopped vehicle, the need for sight
be provided for the minor-road approaches.
distance design should be based on a right turn by a stopped vehicle.
Intersections with all-way Stop control (Case E) The sight distance along the major road to At intersections with all-way Stop control, the
accommodate right turns is the distance that
first stopped vehicle on each approach would be
would be traversed at the design speed of the
visible to the drivers of the first stopped vehicles
major road in the travel time for the appropriate
on each of the other approaches. It is thus not
design vehicle given in Table 6.10. This table
necessary to provide sight distance triangles at
also contains appropriate adjustment factors for
intersections with All-way Stop control. All-way
the number of major-road lanes to be crossed
Stop control may be an option to consider where
by the turning vehicle.
the sight distance for other types of control cannot be achieved. This is particularly the case if
If stopping sight distance has been provided
signals are not warranted.
continuously along the major road and, if sight distance for Case B (Stop control) or Case C (Yield control) has been provided for each
Right turns from a major road (Case F)
minor-road approach, sight distance should generally be adequate for right turns from the
Geometric Design Guide
Right-turning drivers need sufficient sight dis-
major road. However, at intersections or drive-
tance to enable them to decide when it is safe to
ways located on or near horizontal or vertical
turn right across the lane(s) used by opposing
curves on the major road, the availability of ade-
traffic. At all locations, where right turns across
quate sight distance for right turns from the
opposing traffic are possible, there should be
major road should be checked. In the case of
sufficient sight distance to accommodate these
dual carriageways, the presence of sight
manoeuvres. Since a vehicle that turns right
obstructions in the median should also be
without stopping needs a gap shorter than that
checked. 6-24
Chapter 6: Intersection Design
At four-legged intersections, opposing vehicles
area within each sight triangle should be clear of
turning right can block a driver's view of oncom-
sight obstructions, as described above.
ing traffic. If right-turn lanes are provided, offsetting them to the right, to be directly opposite
At skew intersections, the length of the travel
one other, will provide right-turning drivers with
paths
a better view of oncoming traffic.
increased. The actual path length for a crossing
for
crossing
manoeuvres
will
be
manoeuvre can be calculated by dividing the total width of the lanes (plus the median width,
Effect of skew on sight distance
where appropriate) to be crossed by the sine of the intersection angle and adding the length of
When two highways intersect at an angle out75O
120O
and where
the design vehicle. The actual path length divid-
realignment to increase the angle of intersection
ed by the lane width applied to the major road
side the range of
to
is not justified, some of the factors for determi-
cross-section gives the equivalent number of
nation of intersection sight distance will need
lanes to be crossed. This is an indication of the
adjustment.
number of additional lanes to be applied to the adjustment factor shown in Table 6.8 for Case B3.
Each of the clear sight triangles described above is applicable to oblique-angle intersections. As shown in Figure 6.6, the legs of the
The sight distances offered for Case B can,
sight triangle will lie along the intersection
regardless of the form of control, also accom-
approaches and each sight triangle will be larg-
modate turning movements from the minor road
er or smaller than the corresponding sight trian-
to the major road at skew intersections. In the
gle would be at a right-angle intersection. The
obtuse angle, drivers can easily see the full 6-25
Chapter 6: Intersection Design
Geometric Design Guide
Figure 6.6: Effect of skew on sight distance at intersections
sight triangle and, in addition, often accelerate
flicts that are inherent in any intersection. In that
from the minor road at a higher rate than when
sub-section, nine principles of channelisation
they have to negotiate a ninety-degree change
are listed.
of direction. In the acute-angle quadrant, driv-
process whereby a vehicle can be guided safe-
ers are often required to turn their heads con-
ly through the intersection area from an
siderably to see across the entire clear sight tri-
approach leg to the selected departure leg.
angle. For this reason, it is suggested that Case
Guidance is offered by lane markings that clear-
A should not be applied to oblique-angle inter-
ly define the required vehicle path and also indi-
sections.
Stop or Yield control should be
cate auxiliary lanes for turning movements. A
applied and the sight distances appropriate to
variety of symbols is also used as road mark-
either Case B or Case C provided. Even in a
ings to indicate inter alia that turns, either to the
skew intersection it is usually possible for driv-
left or to the right, from selected lanes are
ers to position their vehicles at approximately
mandatory. At intersections that are complex or
90O to the major road at the Stop line, offering
have high volumes of turning traffic, it is usually
added support for the application of Case B for
necessary to reinforce the guidance offered by
skew intersections.
road markings by the application of:
• • • • • •
When driving through a deflection angle greater than 120O, the right turn to the minor road may be undertaken at crawl speeds.
Allowance
could be made for this by adding the time,
Geometric Design Guide
equivalent to that required for crossing an addi-
In essence, channelisation is the
Channelising islands; Medians and median end treatments; Corner radii; Approach and departure geometry; Pavement tapers and transitions; Traffic control devices including signs and signals; and
tional lane, to the acceptable gap.
•
Arrangement and position of lanes
6.5
6.5.1
Channelising islands
CHANNELISATION ELEMENTS
At-grade intersections with large paved areas,
Islands are included in intersections for one or
such as those with large corner radii or with
more of the following purposes:
angles of skew differing greatly from 90O, permit
• • • •
unpredictable vehicle movements, require long pedestrian crossings and have unused pavement areas. Even at a simple intersection there
Control of angle of conflict; Reduction of excessive pavement areas; Regulation of traffic and indication of the proper use of the intersection;
may be large areas on which vehicles can wander from natural or expected paths.
Separation of conflicts;
•
Under
Arrangement to favour a predominant turning movement;
these circumstances it is usual to resort to chan-
• •
nelisation.
Protection of pedestrians; Protection and storage of turning vehicles; and
•
As stated in sub-section 6.2.8, the fundamental
Location of traffic control devices.
function of channelisation is to manage the con6-26 Chapter 6: Intersection Design
The three main functions of channelising islands
Islands may be kerbed, painted or simply non-
are thus:
paved. Kerbed islands provided the most posi-
•
Directional - to control and direct move-
tive traffic delineation and are normally used in
ments, usually turning;
urban areas to provide some degree of protec-
•
Division - which can be of opposing or
tion to pedestrians and traffic control devices.
same direction, usually through, move-
Painted islands are usually used in suburban
ments; and
•
areas where speeds are low, e.g. in the range of
Refuge - either of turning vehicles or of
50 km/h to 70 km/h and space limited. In rural
pedestrians.
areas, kerbs are not common and, at the Typical island shapes are illustrated in Figure
speeds prevailing in these areas, typically 120
6.7.
km/h or more, they are a potential hazard. If it
The designer should bear in mind that islands
section, the use of mountable kerbing should be
are hazards and should be less hazardous than
considered. As an additional safety measure, a
whatever they are replacing.
kerbed island could be preceded by a painted
Figure 6.7 General types and shapes of islands 6-27 Chapter 6: Intersection Design
Geometric Design Guide
is necessary to employ kerbing at a rural inter-
island. Non-paved islands are defined by the
metres in area to ensure that they are easily vis-
pavement edges and are usually used for large
ible to approaching drivers.
islands at rural intersections.
These islands
Directional islands are typically triangular with
may have delineators on posts and may be
their dimensions and exact shape being dictat-
landscaped.
ed by:
Islands are generally either long or triangular in
• •
shape, with the circular shape being limited to
The corner radii and associated tapers; The angle of skew of the intersection; and
•
application in roundabouts. They are situated in
The turning path of the design vehicle.
areas not intended for use in vehicle paths.
Geometric Design Guide
A typical triangular island is illustrated in Figure Drivers tend to find an archipelago of small
6.8. The approach ends of the island usually
islands confusing and are liable to select an
have a radius of about 0,6 metres and the offset
incorrect path through the intersection area. As
between the island and the edge of the travelled
a general design principle, a few large islands
way is typically 0,6 to 1,0 metres to allow for the
are thus to be preferred to several small islands.
effect of kerbing on the lateral placement of
Islands should not be less than about 5 square
moving vehicles.
Figure 6.8: Typical directional island 6-28 Chapter 6: Intersection Design
Where the major road has
shoulders, the nose of the island is offset about
teardrop shape. They are often employed on
one metre from the edge of the usable shoulder,
the minor legs of an intersection where these
the side adjacent to the through lane being
legs have a two-lane, two-way or four-lane undi-
tapered back to terminate at the edge of the
vided cross-section. The principle function of a
usable shoulder, thus offering some guidance
dividing island is to warn the driver of the pres-
and redirection. A kerbed cross-section on the
ence of the intersection. This can be achieved
major road suggests that the nose of the island
by the left edge of the island being, at the widest
should be offset by about 1,6 metres from the
point of the island, in line with the left edge of the
edge of the travelled way, with the side adjacent
approach leg.
to the through lane being tapered back to termi-
would thus appear as though the entire lane had
Figure 6.9: Typical divisional (splitter) island
nate 0,6 metres from the edge of the through
been blocked off by the island. If space does
lane.
not permit this width of island, a lesser blocking
Dividing, or splitter, islands usually have a
width would have to be applied but it is doubted 6-29
Chapter 6: Intersection Design
Geometric Design Guide
To the approaching driver, it
whether anything less than half of the approach
right, both from the minor road to the major road
lane width would be effective. The taper that
and from the major road to the minor.
can be employed to achieve this effect safely is discussed in Section 6.5.3. A typical dividing
Median islands are discussed in Section 4.4.6
island is illustrated in Figure 6.9.
and outer separator islands in Section 4.4.7. At intersections, the end treatment of median
The shape of the splitter island discussed in
islands is important. The width of the opening
Section 6.6.5 is derived from the need to redi-
between two median ends should match the
rect vehicles entering a roundabout through an
width of the minor road, including its shoulders,
angle of not more than 30 . Although it serves
or, where the minor road is kerbed, the opening
to create the illusion of about a half of the
should not be narrower than the surfaced width
approach lane being blocked off, its true func-
of the minor road plus an offset of 0,6 to 1,0
tion is to achieve the desired extent of deflec-
metres.
O
tion.
Furthermore, this form of splitter island
does not accommodate vehicles turning from
The median end treatment is determined by the
the left. In effect, it has a teardrop shape, albeit
width of the median. Where the median is three
distorted by its abutting a curving roadway with
metres wide or less, a simple semicircle is ade-
a relatively short radius rather than a straight
quate. For wider medians, a bullet nose end
road.
treatment is recommended. The bullet nose is formed by arcs dictated by the wheel paths of
The kerb height should ensure that the island
turning vehicles and an assumed nose radius of
would be visible within normal stopping sight
0,6 to 1,0 metres. It results in less intersection
distance. However, it may be advisable to draw
pavement area and a shorter length of opening
the driver's attention to the island by highlighting
than the semicircular end.
the kerbs with paint or reflective markings.
width of five metres, the width of the minor road
Above a median
Geometric Design Guide
controls the length of the opening. A flattened As in the case of the triangular island, the nose
bullet nose, using the arcs as for the conven-
of the dividing island should be offset by about
tional bullet nose but with a flat end as dictated
one metre but, in this case, to the right of the
by the width of the crossing road and parallel to
centreline of the minor road. Dividing islands
the centreline of the minor road, is recommend-
are usually kerbed to enhance their visibility and
ed.
the offset between the kerbing and the edge of
Figure 6.10.
These end treatments are illustrated in
the travelled way should thus be 0,6 metres as discussed above. For the sake of consistency,
The bullet nose and the flattened bullet nose
the radius of the nose should be of the order of
have the advantage over the semicircular end
0,6 metres.
treatment that the driver of a right turning vehicle has a better guide for the manoeuvre for
The balance of the shape of the island is defined
most of the turning path. Furthermore, these
by the turning paths of vehicles turning to the
end treatments result in an elongated median, 6-30
Chapter 6: Intersection Design
which is better placed to serve as a refuge for
envisaged. Reference is typically to three types
pedestrians crossing the dual carriageway road.
of operation, being:
An additional disadvantage of the use of the
Case 1
One-lane one-way travel with
semicircular end treatment for wide medians is
no provision for passing
that, whereas the bullet nose and the flattened
stopped vehicles; One-lane one-way travel with
left of the centreline of the minor road, the semi-
provision for passing a stopped
circular end treatment tends to direct the vehicle
vehicles; and Case 3
into the opposing traffic lane of the minor road.
Two-lane one-way operation
Figure 6.10: Typical median end treatments 6.5.2
Turning roadway widths
Three traffic conditions should also be considered, being: Condition A
Directional islands are bounded by the major
Insufficient SU vehicles in the
and minor roads and by a short length of one-
turning traffic stream to influ-
way, typically one-lane, turning roadway. The
ence design; Condition B
width of the turning roadway is defined by the
Sufficient SU vehicles to influence design; and
swept area of the design vehicle for the selectCondition C
ed radius of curvature and the type of operation 6-31
Chapter 6: Intersection Design
Sufficient
semi-trailers
to
Geometric Design Guide
Case 2
bullet nose both guide the vehicle towards the
way compared to the 6,4 metre lane width of the
influence design
single curve. The three-centred curve is particTurning roadways are short so that design for
ularly useful for Case C conditions because
Case 1 is usually adequate. It is reasonable to
semi-trailers require an inordinate width of turn-
assume, even in the absence of traffic data, that
ing roadway. For example, the required width of
there will be enough trucks in the traffic stream
turning roadway for Case 1, Condition C and a
to warrant the application of Condition B to the
design speed of 20 km/h is 7,9 metres whereas,
design. Turning roadway widths are listed in
under the same circumstances, passenger cars
Table 6.11
require only 4,0 metres. Drivers of passenger cars could thus quite easily perceive the turning
Three-centred curves are an effective alterna-
roadway as being intended for two-lane opera-
tive to the single radius curves listed in Table
tion.
Geometric Design Guide
6.11.
These curves typically have a ratio of
3:1:3 between the successive radii. However,
It is not possible to list all the possible alterna-
asymmetric combinations, e.g. 2:1:4, have also
tive three-centred curve combinations. When
proved very useful in the past. These curves
three-centred curves are considered, the
closely follow the wheel path of a vehicle nego-
designer should determine the required road-
tiating the turn thus enabling the use of narrow-
way width by the use of templates.
er lanes than with a single radius curve. In addition, three-centre curves allow the use of small-
6.5.3
Tapers
er central radii than do the equivalent single curves. Under Case C conditions, a 55:20:55
There are two types of taper, each with different
metre radius three-centred curve is the equiva-
geometric requirements. These are:
lent of a thirty metre single radius curve and per-
•
mits the use of a 6,0 metre wide turning road-
The active taper, which forces a lateral transition of traffic; and
6-32 Chapter 6: Intersection Design
•
The passive taper, which allows a later-
should not fall outside the range of 75O to 120O,
al transition of traffic.
it is important to note that a very short active taper may result in the creation of a local angle
The active taper is used to narrow a roadway or
of skew of less than 75O. This would make it
lane or as a lane drop, i.e. when two lanes
very difficult for the driver on the turning road-
merge into one.
The passive taper either
way to observe opposing traffic on the through
widens or adds a lane. Active tapers constitute
lane. At an angle of skew of less than 5O the
a hazard insofar as that a driver that fails to per-
driver should, using the rear view mirror, be able
ceive the change in circumstances, may either
to observe opposing traffic comfortably.
drive off the travelled way or hit the adjacent
short, tapers in the range between 1:10 and 1:
kerbing. Passive tapers, on the other hand, cre-
0,3 should be avoided (the latter corresponding
ate space on the travelled way and, thus, are
to an angle of skew of 75O).
not hazards.
In
Consequently, active tapers
should be long and passive tapers may be short.
Acceptable tapers rates are suggested in Table 6.12.
lane to allow for deceleration or followed by an
In entering a deceleration lane, a vehicle follows
auxiliary lane allowing for acceleration, these
a reverse or S-curve alignment, which is effec-
lanes will be added or dropped by means of
tively a passive taper immediately followed by
passive and active tapers respectively. In the
an active taper. Four different combinations of
absence of the auxiliary lanes, the turning road-
taper can be employed. These are:
• • • •
way can, in the case of a left turn, be created by a passive taper from the left edge of the through lane to the left edge of the turning roadway.
A straight-line taper; A partial tangent taper; A symmetrical reverse curve; or An asymmetrical reverse curve.
The turning roadway may be terminated by an
In urban areas, short straight-line tapers appear
active taper connecting its left edge to the left
to offer better targets for the approaching driver.
edge of the through lane. Apropos the sugges-
Urban intersections operate at slow speeds dur-
tion that the angle of skew of the intersection
ing peak periods and, particularly for right-turn6-33
Chapter 6: Intersection Design
Geometric Design Guide
If a turning roadway is preceded by an auxiliary
ing traffic, the need for storage may be more
vehicles already in the circle had to give way to
important than the ability to enter the decelera-
those wishing to enter it. Not surprisingly, grid-
tion lane at relatively high speeds. Tapers could
lock resulted at heavy flow rates. Ultimately,
therefore be as sharp as 1:2, which is about the
traffic circles fell into disfavour and were
limit of manoeuvrability of a passenger car at
replaced by conventional three- and four-legged
crawl speeds.
intersections.
As speeds are higher in rural than in urban
Modern roundabouts differ from traffic circles in
areas, the other forms of taper listed above may
their uniform characteristics and operation.
warrant consideration. The partial tangent taper
Internationally, roundabouts operate on the
is a straight line taper preceded by a short
"Yield on entry" rule so that, where vehicles
curved section with a radius such that the
drive on the left, vehicles yield to the right and
desired taper rate is achieved at a point about
vice versa. South Africa applies the same rule
one third of the way across the width of the aux-
except that, in the case of the mini-roundabout,
iliary lane. The symmetrical reverse curve taper
the rule is slightly modified by the use of the
has curved sections of equal radius at either
R2.2 sign which "....indicates to the driver of a
end. The guideline suggested for the partial tan-
vehicle approaching a traffic circle that he shall
gent, i.e. the curve traversing one third of the
yield right of way to any vehicle which will cross
lane width, applies to the entry and the exit
any yield line at such junction before him and
curves of the taper. The asymmetrical reverse
which, in the normal course of events, will cross
curve usually has an entry curve radius about
the path of such driver's vehicle."
twice that of the exit curve. Drivers are thus inclined to adopt the approach
6.6
that the rule of first-come-first-served applies at
ROUNDABOUTS
mini-roundabouts except that, in the case of
6.6.1
simultaneous arrivals at the mini-roundabout,
Introduction
drivers will yield to the vehicle on the right.
Geometric Design Guide
Traffic circles constructed in the 1930s and
A number of geometric elements are incorporat-
1940s were intended to operate in a weaving
ed in the design of roundabouts and all ele-
mode. As such, the diameters of the circles were large.
ments appear in all roundabouts. These ele-
No clear guidance was offered
ments are illustrated in Figure 6.11.
regarding priority of one vehicle over another and, in consequence, accident rates at traffic
6.6.2
circles tended to be higher than those at con-
Operation of roundabouts
ventional intersections. In an effort to rectify this
Roundabouts operate by deflecting the vehicle
situation, it was decided (in the situation of driv-
path so as to slow traffic and promote yielding.
ing on the left) that vehicles should yield to
The roadway entry is usually flared to increase
those on their left.
capacity.
In effect, this meant that
6-34 Chapter 6: Intersection Design
Delays at roundabouts are usually less than at
In spite of their undoubted advantages, round-
conventional intersections and, in consequence,
abouts are not appropriate to every situation.
capacity is higher. The proviso is that the com-
They may be inappropriate:
bined intersection flow should be less than 3
•
500 veh/h.
Where spatial restraints (including cost of land), unfavourable topography or
Reduced delays improve vehicle
high construction costs make it impossi-
operating costs.
ble to provide an acceptable geometric design;
Roundabouts have less potential conflict points
•
than conventional intersections. In the case of
Where traffic flows are unbalanced, with
Figure 6.11: Elements of roundabouts are replaced by 8, as illustrated in Figure 6.12.
•
In both roundabouts and conventional intersec-
At intersections of major roads with minor roads, where roundabouts would
tions, the diverge is also counted as being a
cause serious delays to traffic on the
conflict point. The safety performance of round-
major roads;
abouts is often superior to that of most conven-
•
tional intersections and the reduced number of
Where there are substantial pedestrian flows;
conflict points at roundabouts result in an
•
observable reduction in accident rates.
As an isolated intersection in a network of linked signalised intersections;
6-35 Chapter 6: Intersection Design
Geometric Design Guide
high flows on one or more approaches;
the four-legged intersection, 32 conflict points
• •
In the presence of reversible lanes;
the intersection flow is greater
Where semi-trailers and/or abnormal
than 1 500 veh/h or; o
vehicles are a significant proportion of
greater than 2 000 veh/h;
the total traffic passing through the inter-
•
section and where there is insufficient
•
on four-legged intersections, is
One major flow has a predominant
space to provide the required layout; and
through movement that is:
Where signalised traffic control down
o
Between 50 and 80 per cent of the approach volume; or
stream could cause a queue to back-up o
through the roundabout.
Between 25 and 40 per cent of the intersection volume; and
o
Roundabouts can be considered when:
•
High volumes of right-turning movements, i.e. more than 25
Intersection volumes do not exceed 3 000
Figure 6.12: Intersection conflict points veh/h at three-legged or 4 000 veh/h at
per cent of the approach vol-
Geometric Design Guide
four-legged intersections;
•
ume, occur and which experi-
The proportional split between the vol-
ence long delays (e.g. 15 sec-
umes on the major and minor road does
onds per vehicle) and a high
not exceed 70/30; o
incidence of right-angled acci-
where, on three-legged inter-
dents.
sections, the intersection flow is less than 1 500 veh/h or, o
6.6.3
on four-legged intersections, is less than 2 000 veh/h;
•
Design speed
The proportional split between the vol
The design speed within the roundabout should
umes on the major and minor road does
ideally
not exceed 60/40 where;
Unfortunately, this suggests a radius of between
o
60 and 80 metres hence requiring an overall
on three-legged intersections,
range
6-36 Chapter 6: Intersection Design
between
40
to
50
km/h.
diameter of the roundabout of the order of 150
intersection at all times. This requirement sug-
metres. Very often, the space for this size of
gests that the elaborate landscaping schemes
intersection will simply not be available and
sometimes placed on the central islands of
some lesser design speed will have to be
roundabouts are totally inappropriate to the
accepted.
intended function of the layout.
Where the design speeds on the approaches
Decision sight distance for intersections as
are high, e.g. more than 15 km/h faster than the
described in Table 3.5 should be provided on
design speed within the facility, it may be nec-
each approach to the roundabout to ensure that
essary to consider forcing a reduction in vehicle
drivers can see the nose of the splitter island. It
speed. This could be by means of horizontal
follows that roundabouts should not be located
reverse curvature. The ratio between the radii
on crest curves.
of successive curves should be of the order of 1,5:1.
6.6.5
Components
Speed humps should not be employed as
The various components of a roundabout are
speed-reducing devices on major roads or on
illustrated in Figure 6.11.
bus routes. Where design speeds are of the Deflection
order of 100 km/h or more, the speed hump would have to be long and the height low to ensure that the vertical acceleration caused by
A very important component is the deflection
the speed hump does not cause the driver to
forced on vehicles on the approach to the
lose control. It is suggested that, in practice, a
roundabout.
suitable profile would be difficult to construct.
speed of vehicles so that, within limits, the
Bus passengers, particularly those sitting in the
greater the deflection the better. The limit is that
rear overhang of the vehicle, would find travers-
the minimum acceptable angle of skew at an
ing a speed hump distinctly uncomfortable.
intersection is 60O, as discussed previously in
Speed humps should thus only be used at
this chapter. This corresponds to a deflection
roundabouts in residential areas or where the
on entry of 30O. The approach radius should not
intention is to apply traffic calming.
exceed 100 metres, which corresponds to the recommended design speed of 40 to 50 km/h.
6.6.4
Sight distance Entries and exits
Site visibility is important in the design of roundabouts. Specifically, approaching drivers should
The widths of single lane approaches to round-
have a clear view of the nose of the splitter or
abouts are typically of the order of 3,4 to 3,7
separator island. At the yield line and while tra-
metres.
versing the roundabout, they should have an
important factors in increasing the capacity of
uninterrupted view of the opposing legs of the
the roundabout and can be increased above the
The entry width is one of the most
6-37 Chapter 6: Intersection Design
Geometric Design Guide
The intention is to reduce the
width of the approach by flaring, i.e. by provid-
side and inside kerbs. For a vehicle with an
ing a passive taper with a taper rate of 1:12 to
overall width of 2,6 metres, the offset should
1:15. The recommended minimum width for a
thus be not less than 1,6 metres with 2,0 metres
single-lane entry is 5 metres.
being preferred. To ensure that vehicles do not travel faster than the design speed, the maxi-
If demanded by high approach volumes, the
mum radius on the vehicle path should be kept
flaring could add a full lane to the entry to
to 100 metres or less.
increase capacity. The width of a two-lane entry should be of the order of 8 metres. A variation
As a general guideline, the circulatory roadway
on the two-lane entry is to have, in effect, a sin-
should be sufficiently wide to allow a stalled
gle-lane circulatory roadway with an auxiliary
vehicle to be passed but without sufficient trucks
lane provided for the benefit of vehicles turning left at the roundabout.
in the traffic stream to influence design (normal-
The auxiliary lane is
ly described as Case 2, Condition A in the case
shielded from traffic approaching from the right
of turning roadways). The minimum roadway
by moving the end of the splitter island forward
width for single-lane operation under these cir-
to provide a circulatory road width adequate
cumstances would be of the order of 6,5 metres
only for single-lane travel. This approach could
between kerbs.
be adopted with advantage when left-turning
Two-lane operation would
require a roadway width of about 8 metres. If
traffic represents 50 per cent or more of the
trucks are present in the traffic stream in suffi-
entry flow or more than 300 veh/h during peak
cient numbers to influence design, the circulato-
hours.
ry road width should be increased by 3 metres both in the single-lane and in the two-lane situ-
Circulatory roadway
ation. A significant proportion of semi-trailers would require the width of the circulatory road
The circulatory roadway width is a function of
width to be increased to 13 metres and 16
the swept path of the design vehicle and of the
metres in the single-lane and the two-lane situ-
layout of the exits and entries and generally
ation respectively.
should be either equal to or 1,2 times the width
Geometric Design Guide
of the entries. The width should be constant A circulatory road width of 13 metres would
throughout the circle.
make it possible for passenger cars to traverse In the construction of the swept path of the
the roundabout on relatively large radius curves
design vehicle, it should be noted that drivers
and at correspondingly high speeds. To avoid
tend to position their vehicles close to the out-
this possibility, the central island should be mod-
side kerbs on entry to and exit from the round-
ified as discussed below.
about and close to the central island between these two points. The vehicle path, being the
The cross-slope on the roadway should be
path of a point at the centre of the vehicle,
away from the central island and equal to the
should thus have an adequate offset to the out-
camber on the approaches to the intersection.
6-38 Chapter 6: Intersection Design
cyclists and a place to mount traffic
Central island
signs. The central island consists of a raised non-traversable area, except in the case of mini-round-
The sizes of splitter islands are dictated by the
abouts where the central island may simply be a
dimensions of the central island and inscribed
painted dot. The island is often landscaped but
circle. As a general guideline, they should have
it should be ensured that the landscaping does
an area of at least 10 square metres so as to
not obscure the sight lines across the island.
ensure their visibility to the oncoming driver.
Historically, central islands were often square or,
The length of splitter islands should be equal to
if they had more than four entries, polygonal.
the comfortable deceleration distance from the
Negotiating the right angle bends was only pos-
design speed of the approach to that of the
sible at crawl speeds and this led to substantial
roundabout.
delays and congestion. It is now customary to Ideally, the nose of the splitter island should be
provide circular islands.
offset to the right of the approach road centreWhile, for semi-trailers, the width of the circula-
line by about 0,6 to 1 metre.
tory road between kerbs would have to be 13 metres in a single-lane configuration, all other vehicles could be served by a road width of 9,5 metres. A mountable area or apron could thus be added to the central island to accommodate this difference. The apron should have crossfall steeper than that of the circulatory road, principally to discourage passenger vehicles from driving on it and a crossfall of 4 to 5 per cent is recommended.
Splitter islands should be provided on the approaches to roundabouts to:
•
Allow drivers to perceive the upcoming roundabout and to reduce entry speed;
•
Provide space for a comfortable deceleration distance;
•
Physically separate entering and exiting traffic;
•
Prevent deliberate and highly dangerous wrong-way driving;
• •
Control entry and exit deflections; and Provide a refuge for pedestrians and 6-39 Chapter 6: Intersection Design
Geometric Design Guide
Splitter islands
TABLE OF CONTENTS 7.
INTERCHANGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1 7.1.2 Design principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1
7.2
INTERCHANGE WARRANTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3 7.2.1 Traffic volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3 7.2.2 Freeways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4 7.2.3 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4 7.2.4 Topography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4
7.3
WEAVING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4
7.4
LOCATION AND SPACING OF INTERCHANGES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6
7.5
BASIC LANES AND LANE BALANCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9
7.6
AUXILIARY LANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 7.6.1 The need for an auxiliary lane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10 7.6.2 Auxiliary lane terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 7.6.3 Driver information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13
7.7
INTERCHANGE TYPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 7.7.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-13 7.7.2 Systems interchanges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-15 7.7.3 Access and service interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17 7.7.4 Interchanges on non-freeway roads. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23
7.8
RAMP DESIGN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 7.8.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 7.8.2 Design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-24 7.8.3 Sight distance on ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-25 7.8.4 Horizontal alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-26 7.8.5 Vertical alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-27 7.8.6 Cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-28 7.8.7 Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-29
7.9
COLLECTOR - DISTRIBUTOR ROADS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35
7.10
OTHER INTERCHANGE DESIGN FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35 7.10.1Ramp metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-35 7.10.2Express-collector systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-36
LIST OF TABLES Table 7.1: Interchange spacing in terms of signage requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7 Table 7.2: Ramp design speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-24 Table 7.3: Maximum resultant gradients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-27 Table 7.4: K-Values of crest curvature for decision sight distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-28 Table 7.5: Length of deceleration lanes (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-31 Table 7.6: Length of acceleration lanes (m) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-31 Table 7.7: Taper rates for exit ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32
LIST OF FIGURES Figure 7.1: Type A weaves: (a) ramp-weave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 . . (b) major weave with crown line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 Figure 7.2: Type B weaves: (a) major weave with lane balance at exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 . . (b) major weave with merging at entrance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5 . . (c) major weave with merging at entrance and lane balance at exit . . . . . . . . . . . . . . . . . . . . . . 7-5 Figure 7.3: Type C weaves: (a) major weave without lane balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6 . . (b) two-sided weave . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6 Figure 7.4: Weaving distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7 Figure 7.5: Relationship between interchange spacing and accident rate. . . . . . . . . . . . . . . . . . . . . . . . . . . 7-9 Figure 7.6: Coordination of lane balance with basic number of lanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11 Figure 7.7: Four-legged systems interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-16 Figure 7.8: Three-legged systems interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-17 Figure 7.9: Diamond interchanges. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-20 Figure 7.10: Par-Clo A interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-21 Figure 7.11: Par-Clo B interchanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-22 Figure 7.12: Par-Clo AB interchanges and rotaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-22 Figure 7.13: Jug Handle interchange. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-23 Figure 7.14: Single lane exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32 Figure 7.15: Two-lane exit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-32 Figure 7.16: One lane entrance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-33 Figure 7:17: Two-lane entrance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-33 Figure 7.18: Major fork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-34
Chapter 7 INTERCHANGES 7.1
INTRODUCTION
7.1.1
General
The various types of interchange configuration are illustrated in Section 7.6. Each basic form can be divided into sub-types. For example, the Diamond Interchange is represented by the nar-
The principal difference between interchanges
row diamond, the wide diamond and the split
and other forms of intersection is that, in inter-
diamond. The most recent development in the
changes, crossing movements are separated in
Diamond interchange form is the Single Point
space whereas, in the latter case,they are sep-
Diamond Interchange. This form is also referred
arated in time. At-grade intersections accom-
to as the Urban Interchange.
modate turning movements either within the limitations of the crossing roadway widths or through the application of turning roadways
Historically, the type of interchange to be
whereas the turning movements at interchanges
applied at a particular site would be selected as
are accommodated on ramps.
The ramps
an input into the design process. In fact, like the
replace the slow turn through an angle of skew
cross-section, the interchange is the aggrega-
that is approximately equal to 90 by high-speed
tion of various elements.
merging and diverging manoeuvres at relatively
approach is thus to select the elements appro-
flat angles.
priate to a particular site in terms of the topog-
O
A more sensible
raphy, local land usage and traffic moveGrade separations, discussed further in Chapter
ments and then to aggregate them into some or
10, provide spatial separation between the
other type of interchange.
crossing movements but do not make provision
7.1.2
for turning movements. They, therefore, do not
Design principles
qualify for consideration as a form of intersection.
high speeds close to the freeway and over relaThe first interchange ever built was at
tively short distances. It is therefore important
Woodbridge, New Jersey, and provided (in the
that drivers should experience no difficulty in
context of driving on the right) loops for all left
recognising their route through the interchange
turns and outer connectors for all right turns
irrespective of whether that route traverses the
thus creating the Cloverleaf Interchange. Since
interchange on the freeway or diverts to depart
then, a variety of interchange forms has been
from the freeway to a destination that may be to
developed. These include the:
the left or the right of the freeway. In following
• • •
Diamond;
their selected route, drivers should be disturbed
Par-Clo (from Partial Cloverleaf); and the
as little as possible by other traffic.
Directional
requirements can be met through the applica7-1 Chapter 7: Interchanges
These
Geometric Design Guide
Manoeuvres in an interchange area occur at
tion of the basic principles of interchange
Route continuity substantially simplifies the nav-
design.
igational aspects of the driving task. For example, if a driver simply wishes to travel on a free-
The driver has a number of tasks to execute
way network through a city from one end to the
successfully to avoid being a hazard to other
other it should not be necessary to deviate from
traffic. It is necessary to:
one route to another.
• •
select a suitable speed and accelerate or decelerate to the selected speed
Uniformity of signing practice is an important
within the available distance;
aspect of consistent design and reference
select the appropriate lane and carry out
should be made to the SADC Road Traffic Signs
the necessary weaving manoeuvres to
Manual.
effect lane changes if necessary; and
•
diverge towards an off-ramp or merge
Ideally, an interchange should have only a sin-
from an on-ramp with the through traffic.
gle exit for each direction of flow with this being located in advance of the interchange structure.
To maintain safety in carrying out these tasks,
The directing of traffic to alternative destinations
the driver must be able to understand the oper-
on either side of the freeway should take place
ation of the interchange and should not be sur-
clear of the freeway itself. In this manner, driv-
prised or misled by an unusual design charac-
ers will be required to take two binary decisions,
teristic. Understanding is best promoted by con-
(Yes/No) followed by (Left/Right), as opposed to
sistency and uniformity in the selection of types
a single compound decision. This spreads the
and in the design of particular features of the
workload and simplifies the decision process,
interchange.
hence improving the operational efficiency of the entire facility. Closely spaced successive
Interchange exits and entrances should always
off-ramps could be a source of confusion to the
be located on the left.
driver leading to erratic responses and manoeu-
Right-hand side
entrances and exits are counter to driver
vres.
Geometric Design Guide
expectancy and also have the effect of mixing high-speed through traffic with lower-speed
Single entrances are to be preferred, also in
turning vehicles. The problem of extracting turn-
support of operational efficiency of the inter-
ing vehicles from the median island and provid-
change. Merging manoeuvres by entering vehi-
ing sufficient vertical clearance either over or
cles are an interruption of the free flow of traffic
under the opposing freeway through lanes is not
in the left lane of the freeway. Closely spaced
trivial. The application of right-hand entrances
entrances exacerbate the problem and the
and exits should only be considered under
resulting turbulence could influence the adja-
extremely limiting circumstances. Even in the
cent lanes as well.
case of a major fork where two freeways are diverging, the lesser movement should, for pref-
From the standpoint of convenience and safety,
erence, be on the left.
in particular prevention of wrong-way move-
7-2 Chapter 7: Interchanges
•
ments, interchanges should provide ramps to
The crossing road ramp terminals may
serve all turning movements. If, for any reason,
include right and left turn lanes, traffic
this is not possible or desirable, it is neverthe-
signals and other traffic control devices.
less to be preferred that, for any travel move-
Not being obstructed by bridge piers
ment from one road to another within an inter-
and the like, these would be rendered
change, the return movement should also be
more visible by taking the crossing road
provided.
over the freeway. The other design principles, being continuity of
Provision of a spatial separation between two
basic lanes, lane balance and lane drops are
crossing streams of traffic raises the problem of
discussed in Section 7.5 as matters of detailed
which to take over the top - the perennial Over
design.
versus Under debate. The choice of whether the crossing road should be taken over or under the freeway depends on a number of factors, not the least of which is the matter of terrain and
7.2
INTERCHANGE WARRANTS
7.2.1
Traffic volumes
construction costs. There are, however, a num-
With increasing traffic volumes, a point will be
ber of advantages in carrying the crossing road
reached where all the options of temporal sepa-
over the freeway. These are:
ration of conflicting movements at an at-grade
•
Exit ramps on up-grades assist deceler-
intersection have been exhausted. One of the
ation and entrance ramps on down-
possible solutions to the problem is to provide
grades assist acceleration and have a
an interchange.
•
•
•
•
Rising exit ramps are highly visible to
The elimination of bottlenecks by means of
drivers who may wish to exit from the
interchanges can be applied to any intersection
freeway.
at which demand exceeds capacity and is not
The structure has target value, i.e. it
necessarily limited to arterials. Under these cir-
provides advance warning of the possi-
cumstances, it is necessary to weigh up the
bility of an interchange ahead necessi-
economic benefits of increased safety, reduced
tating a decision from the driver whether
delay and reduced operating and maintenance
to stay on the freeway or perhaps to
cost of vehicles against the cost of provision of
change lanes with a view to the impend-
the interchange. The latter includes the cost of
ing departure from the freeway.
land acquisition, which could be high, and the
Dropping the freeway into cut reduces
cost of construction. As the construction site
noise levels to surrounding communi-
would be heavily constricted by the need to
ties and also reduces visual intrusion.
accommodate traffic flows that were sufficiently
For the long-distance driver on a rural
heavy to justify the interchange in the first
freeway, a crossing road on a structure
instance, the cost of construction could be sig-
may represent an interesting change of
nificantly higher than on the equivalent green
view.
field site. 7-3 Chapter 7: Interchanges
Geometric Design Guide
beneficial effect on truck noise.
7.2.2
Freeways
7.2.4
The topography may force a vertical separation
The outstanding feature of freeways is the limi-
between crossing roads at the logical intersec-
tation of access that is brought to bear on their
tion location. As an illustration, the through road
operation. Access is permitted only at designat-
may be on a crest curve in cut with the crossing
ed points and only to vehicles travelling at or near freeway speeds.
Topography
road at or above ground level. If it is not possi-
As such, access by
ble to relocate the intersection, a simple Jug-
means of intersections is precluded and the only
handle type of interchange as illustrated in
permitted access is by way of interchanges.
Figure 7.13 may be an adequate solution to the
Crossing roads are normally those that are high
problem.
in the functional road hierarchy, e.g. arterials, although, if these are very widely spaced, it may
7.3
be necessary to provide an interchange serving
WEAVING
a lower order road, for example a collector. The Highway Capacity Manual (2000) defines It follows that the connection between two free-
weaving as the crossing of two or more traffic
ways would also be by means of an inter-
streams travelling in the same general direction
change, in which case reference is to a systems
without the aid of traffic control devices but then
interchange as opposed to an access inter-
goes to address the merge-diverge as a sepa-
change.
rate issue. However, the merge-diverge operation, associated with successive single-lane on-
7.2.3
Safety
and off-ramps where there is no auxiliary lane, does have two streams that, in fact, are cross-
Some at-grade intersections exhibit high crash
ing. Reference to weaving should thus include
rates that cannot be lowered by improvements
the merge-diverge.
to the geometry of the intersections or through the application of control devices. Such situa-
Three types of weave are illustrated in Figures
tions are often found at heavily travelled urban
7.1, 7.2 and 7.3. A Type A weave requires all
Geometric Design Guide
intersections. Crash rates also tend to be high
weaving vehicles to execute one lane change.
at the intersections on heavily travelled rural
Type B weaving occurs when one of the weav-
arterials where there is a proliferation of ribbon
ing streams does not have to change lanes but
development.
the other has to undertake at most one lane change. Type C weaving allows one stream to
A third area of high crash rates is at intersec-
weave without making a lane change, whereas
tions on lightly travelled low volume rural loca-
the other stream has to undertake two or more
tions where speeds tend to be high. In these
lane changes.
cases, low-cost interchanges such as the Jughandle layout may be an adequate solution to
The Type B weave is, in essence, a Type A
the problem.
weave but with the auxiliary lane extending 7-4 Chapter 7: Interchanges
Figure 7.1: Type A weaves: (a) ramp-weave
Figure 7.2: Type B weaves:
(a) major weave with lane balance at exit (b) major weave with merging at entrance (c) major weave with merging at entrance and lane balance at exit 7-5 Chapter 7: Interchanges
Geometric Design Guide
(b) major weave with crown line
Figure 7.3: Type C weaves: (a) major weave without lane balance (b) two-sided weave either up- or downstream of the weaving area
Rural interchanges are typically spaced at dis-
and with an additional lane being provided
tances of eight kilometres apart or more. This
either to the on- or to the off-ramp. It follows that
distance is measured from centreline to centre-
a Type A weaving section can be easily convert-
line of crossing roads.
ed into a Type B weave. At any site at which a Type A weave appears, it would thus be prudent
The generous spacing applied to rural inter-
to check the operation at the site for both types
changes would not be able to serve intensively
of weave. Type C weaves rarely occur in South
developed urban areas adequately. As an illus-
Africa.
tration of context sensitive design, trip lengths are shorter and speeds lower on urban free-
Geometric Design Guide
7.4
LOCATION AND SPACING OF
ways than on rural freeways. As drivers are
INTERCHANGES
accustomed to taking a variety of alternative actions in rapid succession a spacing of closer than eight kilometres can be considered.
The location of interchanges is based primarily on service to adjacent land. On rural freeways
At spacings appropriate to the urban environ-
bypassing small communities, the provision of a
ment, reference to a centreline-to-centreline dis-
single interchange may be adequate, with larg-
tance is too coarse to be practical. The point at
er communities requiring more.
The precise
issue is that weaving takes place between inter-
location of interchanges would depend on the
changes and the available distance is a function
particular needs of the community but, as a gen-
of the layout of successive interchanges. For a
eral guide, would be on roads recognised as
common centreline-to-centreline spacing, the
being major components of the local system.
weaving length available between two diamond 7-6
Chapter 7: Interchanges
interchanges is significantly different from that
conditions on the freeway. The third criterion is
between a Par-Clo-A followed by a Par-Clo-B.
that of turbulence, which is applied to the
Weaving distance is defined in the Highway
merge-diverge situation.
Capacity Manual 2000 and other sources as the distance between the point at which the separa-
(1)
tion between the ramp and the adjacent lane is
Road Traffic Signs Manual to provide adequate
The distance required by the SADC
Figure 7.4: Weaving distance 0,5 metres to the point at the following off-ramp
sign posting which, in turn, influences the safe
at which the distance between ramp and lane is
operation of the freeway, is used to define the
3,7 m as illustrated in Figure 7.4.
minimum distance between ramps. The minimum distances to be used for detailed design
If this definition is adopted, the weaving length
purposes, as measured between Yellow Line
becomes a function of the rates of taper applied
Break Points, for different areas and inter-
to the on- and off-ramps.
change types should be not less than the values
Yellow Line Break Point (YLBP) distance is total-
stated in Table 7.1.
ly unambiguous and is the preferred option.
(2)
Three criteria for the spacing of interchanges
possible to meet the above requirements, relax-
can be considered. In the first instance, the dis-
In exceptional cases, where it is not
ation of these may be considered. It is, howev-
tance required for adequate signage should ide-
er, necessary to ensure that densities in the
ally dictate spacing of successive interchanges.
freeway left hand lane are not so high that the
If it is not possible to achieve these distances, consideration can be given to a relaxation
flow of traffic breaks down. Densities associat-
based on achieving Level Of Service (LOS) D
ed with LOS E would make it difficult, if not actu7-7
Chapter 7: Interchanges
Geometric Design Guide
Reference to the
ally impossible, for drivers to be able to change
merge, stacking of vehicles will occur on the on-
lanes. Formulae according to which densities
ramp with the queue possibly backing up on to
can be estimated are provided in the Highway
the crossing road itself. Stacking can also occur
Capacity Manual (2000). Drivers need time to
on an off-ramp if the crossing road ramp termi-
locate a gap and then to position themselves
nal cannot accommodate the rate of flow arriv-
correctly in relation to the gap while simultane-
ing from the freeway. The queue could conceiv-
ously adjusting their speed to that required for
ably back up onto the freeway, which would cre-
the lane change. The actual process of chang-
ate an extremely hazardous situation.
ing lanes also requires time. In the case of the merge-diverge
It should be realised that relaxations below the
manoeuvre, turbulence caused on the left lane
distances recommended under (1) above will
of the freeway by a close succession of entering
result in an increase in the driver workload.
and exiting vehicles becomes an issue.
Failure to accommodate acceptable levels of
According to Roess and Ulerio this turbulence
driver workload in relation to reaction times can
manifests itself over a distance of roughly 450
be expected to result in higher than average
metres upstream of an off-ramp and down-
crash rates. Twomey et al demonstrate that, at
stream of an on-ramp. A spacing of 900 metres
spacings between noses of greater than 2 500
would suggest that the entire length of freeway
metres, the crash rate is fairly constant, i.e. the
between interchanges would be subject to tur-
presence of the following interchange is not a
bulent flow. The likelihood of breakdown in the
factor in the crash rate. At spacings of less than
traffic flow would thus be high and the designer
2500 m between noses, the crash rate increas-
should ensure that space is available for one
es until, at about 500 m between noses, the
area of turbulence to subside before onset of
crash rate is nearly double that of the 2500 m
the next.
spacing. This is illustrated in Figure 7.2 below
(3)
Geometric Design Guide
(4)
In off-peak periods, vehicles would be
moving between interchanges at the design
The question that must be addressed is the ben-
speed or higher. The geometry of the on- and
efit that the community can expect to derive in
off-ramps should be such that they can accom-
exchange for the cost of the higher accident
modate
speeds.
rate. By virtue of the fact that freeway speeds
Increasing the taper rates or reducing the length
tend to be high, there is a high probability that
of the speed change lanes purely to achieve
many of the accidents would be fatal. It is there-
some or other hypothetically acceptable Yellow
fore suggested that the decision to reduce the
Line Break Point distance does not constitute
interchange spacing below those listed in Table
good design.
7.1 should not be taken lightly.
(5)
manoeuvres
at
these
The spacing between successive inter-
changes will have an impact on traffic opera-
It would be necessary to undertake a full-scale
tions on the crossing roads and vice versa. If
engineering analysis of the situation that would
the crossing road can deliver vehicles to the
include:
freeway faster than they can carry out the
• 7-8
Chapter 7: Interchanges
estimation of future traffic volumes at a
Figure 7.5: Relationship between interchange spacing and accident rate
• •
ten to twenty year time horizon, com-
are not achievable and an interchange is
prising weaving and through volumes in
absolutely vital for service to the community and
the design year;
adjacent land uses, relaxations may be consid-
calculation of traffic densities;
ered but then, to minimise the risk of crashes,
assessment of the local geometry in
the density calculated according to the above-
terms of sight distances, and horizontal
mentioned formulae should not exceed 22 vehi-
and vertical alignment;
• •
cles/kilometre/lane, which corresponds to LOS D.
development of a sign sequence; and a form of benefit/cost analysis relating
7.5
community benefits to the decrease in
BASIC LANES AND LANE BALANCE
traffic safety. Density offers some indication of the level of
an extended length of a route, irrespective of
exposure to risk and, for want of any better
local changes in traffic volumes and require-
measure, it is suggested that a density higher
ments for lane balance. Alternatively stated, the
than 22 vehicles/kilometre/lane, corresponding
basic number of lanes is a constant number of
to LOS D, would not result in acceptable design.
lanes assigned to a route, exclusive of auxiliary
It would be necessary to pay attention to reme-
lanes.
dial actions to prevent interchange constraints, such as inadequate ramp capacity, signalling or
The number of basic lanes changes only when
crossroad volumes, causing back up onto the
there is a significant change in the general level
freeway
of traffic volumes on the route. Short sections of the route may thus have insufficient capacity,
In summary: Spacings of interchanges in terms
which problem can be overcome by the use of
of their YLBP distances should desirably be in
auxiliary lanes. In the case of spare capacity,
accordance with Table 7.1. If these spacings
reduction in the number of lanes is not recom7-9
Chapter 7: Interchanges
Geometric Design Guide
Basic lanes are those that are maintained over
mended because this area could, at some future
This can be used to drop a basic lane to match
time, become a bottleneck.
anticipated
Unusual traffic
flows
beyond
the
diverge.
demands, created by accidents, maintenance or
Alternatively, it can be an auxiliary lane that is
special events, could also result in these areas
dropped.
becoming bottlenecks. Basic lanes and lane balance are brought into harmony with each other by building on the
The basic number of lanes is derived from con-
basic lanes, adding or removing auxiliary lanes
sideration of the design traffic volumes and
as required.
capacity analyses. To promote the smooth flow
The principle of lane balance
should always be applied in the use of auxiliary
of traffic there should be a proper balance of
lanes. Operational problems on existing road-
lanes at points where merging or diverging
ways can be directly attributed to a lack of lane
manoeuvres occur. In essence, there should be
balance and failure to maintain route continuity.
one lane where the driver has the choice of a change of direction without the need to change
The application of lane balance and coordina-
lanes.
tion with basic number of lanes is illustrated in Figure 7.6
At merges, the number of lanes downstream of the merge should be one less than the number
7.6
of lanes upstream of the merge. This is typified
AUXILIARY LANES
by a one-lane ramp merging with a two-lane car-
As in the case of the two-lane two-way road
riageway that, after the merge, continues as a
cross-section with its climbing and passing
two-lane carriageway as is the case on a typical
lanes, and the intersection with its right- and left-
Diamond Interchange layout.
This rule pre-
turning lanes, the auxiliary lane also has its role
cludes a two-lane ramp immediately merging
to play in the freeway cross-section and the
with the carriageway without the addition of an
interchange. In a sense, the application of the
auxiliary lane.
auxiliary lane in the freeway environment is
Geometric Design Guide
identical to its application elsewhere. It is added At diverges, the number of lanes downstream of
to address a local operational issue and, as
the diverge should be one more than the num-
soon as the need for the auxiliary lane is past, it
ber upstream of the diverge. The only exception
is dropped.
to this rule is on short weaving sections, such as
Important features to consider in the application
at Cloverleaf Interchanges, where a condition of
and design of the auxiliary lane are thus:
this exception is that there is an auxiliary lane through the weaving section.
When two lanes diverge from the freeway, the above rule indicates that the number of freeway lanes beyond the diverge is reduced by one.
• • •
The need for an auxiliary lane;
7.6.1
The need for an auxiliary lane
The terminals; Driver information
Auxiliary lanes are normally required on free7-10
Chapter 7: Interchanges
ways either as:
• • •
Ideally, maximum gradients on freeways are in
climbing lanes; or
the range of three to four per cent ensuring that
to support weaving; or
most vehicles can maintain a high and fairly
to support lane balance.
constant speed.
The climbing lane application is similar to that
However, in heavily rolling
country it is not always possible to achieve this
discussed in Chapter 4 in respect of two-lane
ideal without incurring excessive costs in terms
two-way roads whereas the weaving and lane
of earthworks construction.
balance applications are unique to the freeway
Because of the
heavy volumes of traffic that necessitate the
situation.
provision of a freeway, lane changing to overtake a slow-moving vehicle is not always easy
Climbing lanes
and, under peak flow conditions, may actually 7-11 Chapter 7: Interchanges
Geometric Design Guide
Figure 7.6: Coordination of lane balance with basic number of lanes
be impossible. Speed differentials in the traffic
ensure that the freeway is not unduly congested
stream are thus not only extremely disruptive
because of this practice, an auxiliary lane can
but may also be potentially dangerous. Both
be provided between adjacent interchanges
conditions, i.e. disruption and reduction in safe-
resulting in Type A weaving as described in
ty, require consideration.
Section 7.3.
If a gradient on a freeway is steeper than four
If a large number of vehicles are entering at the
per cent, an operational analysis should be car-
upstream interchange, it may be necessary to
ried out to establish the impact of the gradient
provide a two-lane entrance ramp.
on the Level of Service. A drop through one
these vehicles may exit at the following inter-
level, e.g from LOS B through LOS C to LOS D,
change but those wishing to travel further will
would normally suggest a need for a climbing
have to weave across traffic from still further
lane.
upstream that intends exiting at the following
Some of
interchange and then merge with through traffic As discussed in Chapter 4, crash rates increase
on the freeway.
The auxiliary lane is then
exponentially with increasing speed differential.
extended beyond the downstream interchange
For this reason, international warrants for climb-
to allow a separation between the two manoeu-
ing lanes normally include a speed differential in
vres. Similarly, a large volume of exiting vehi-
the range of 15 to 20 km/h. South Africa has
cles may necessitate a two-lane exit, in which
adopted a truck speed reduction of 20 km/h as
case the auxiliary lane should be extended
its speed-based warrant for climbing lanes. If,
upstream.
on an existing freeway, the measured truck
being. The desired length of the extension of
speed reduction in the outermost lane is thus 20
the auxiliary lane beyond the two interchanges
km/h or higher, the provision of a climbing lane
is normally assessed in terms of the probability
should be considered. In the case of a new
of merging vehicles locating an acceptable gap
design, it will be necessary to construct a speed
in the opposing traffic flow.
Type B weaving thus comes into
Geometric Design Guide
profile of the truck traffic to evaluate the need for a climbing lane.
Lane balance
Weaving
As discussed in Section 7.5, lane balance requires that:
•
In the urban environment, interchanges are fair-
In the case of an exit, the number of
ly closely spaced and local drivers are very
lanes downstream of the diverge should
inclined to use freeways as part of the local cir-
be one more than the number upstream; and
culation system - a form of rat-running in
•
reverse and as undesirable as the normal form
of lanes downstream of the merge
of rat-running where the higher order road is
should be one less than the number
bypassed through the use of local residential streets as long-distance urban routes.
In the case of an entrance, the number
upstream
To 7-12
Chapter 7: Interchanges
7.6.2
This is illustrated in Figure 7.6. Single-lane on- and off-ramps do not require
Auxiliary lane terminals
An auxiliary lane is intended to match a particu-
auxiliary lanes to achieve lane balance in terms
lar situation such as, for example, an unaccept-
of the above definition. It should be noted that,
ably high speed differential in the traffic stream.
unless two-lane on- and off-ramps are provided,
It follows that the full width of auxiliary lane must
the Type A weave is actually a violation of the
be provided over the entire distance in which the
principles of lane balance.
situation prevails.
The terminals are thus
required to be provided outside the area of need To achieve lane balance at an exit, three lanes
and not as part of the length of the auxiliary
upstream of the diverge should be followed by a
lane.
two-lane off-ramp in combination with two basic lanes on the freeway. The continuity of basic
ntering and exiting from auxiliary lanes require a
lanes requires that the outermost of the three
reverse curve path to be followed. It is thus sug-
upstream lanes should be an auxiliary lane.
gested that the taper rates discussed in Chapter 4.4.3 be employed rather than those normally
If all three upstream lanes are basic lanes, it is
applied to on- and off-ramps.
possible that traffic volumes beyond the off-
The entrance
taper should thus be about 100 metres long and
ramp may have reduced to the point where
the exit taper about 200 metres long.
three basic lanes are no longer necessary. Provision of a two-lane exit would thus be a
7.6.3
Driver information
convenient device to achieve a lane drop while simultaneously maintaining lane balance. The
The informational needs of drivers relate specif-
alternative would be to provide a single-lane off-
ically to needs with regard to the exit from the
ramp, carrying the basic lanes through the inter-
auxiliary lane and include an indication of:
the additional construction costs involved, this
• • •
approach is not recommended.
These are fully discussed in Chapter 4.4.3 and
If a two-lane on-ramp is joining two basic lanes,
reference should be made to that chapter for
lane balance will require that there be three
further detail.
tance beyond the on-ramp terminal. In view of
the presence of a lane drop; the location of the lane drop; and the appropriate action to be undertaken
lanes beyond the merge. The outside lane of the three could become a new basic lane if the
7.7
INTERCHANGE TYPES
7.7.1
General
increase in traffic on the freeway merits it. On the other hand, it is possible that the flow on the ramp is essentially a local point of high density and that two basic lanes are all that are required downstream of the on-ramp. In this case, the
There is a wide variety of types of interchanges
outside lane could be dropped as soon as con-
that can be employed under the various circum-
venient.
stances that warrant the application of inter7-13 Chapter 7: Interchanges
Geometric Design Guide
change and dropping the outside lane some dis-
changes. The major determinant of the type of
between a major and a local road, as suggested
interchange to be employed at any particular
above in the case where local topography may
site is the classification and characteristics of
force a grade separation between the two roads.
the intersecting road.
Intersecting roads are
typically freeways or urban arterials but may
In addition to the classification and nature of the
also be collectors.
intersecting road, there are a number of controls guiding the selection of the most appropriate
In the case of freeways as intersecting roads,
interchange form for any particular situation. In
reference is made to systems interchanges.
the sense of context sensitive design, these
Systems interchanges exclusively serve vehi-
include;
cles that are already on the freeway system.
• • •
Access to the freeway system from the surrounding area is via interchanges on roads
Safety; Adjacent land use; Design speed of both the freeway and the intersecting road;
other than freeways, for which reason these
•
interchanges are known as access inter-
Traffic volumes of the through and turning movements;
changes. Service areas, providing opportunities
• • • • • • •
to buy fuel, or food or simply to relax for a while are typically accessed via an interchange. In some instances, the services are duplicated on either side of the freeway, in which access is via a left-in/left-out configuration. The requirements in terms of deflection angle, length of ramp and spacing that apply to interchange ramps apply
Traffic composition; Number of required legs; Road reserve and spatial requirements; Topography; Service to adjacent communities; Environmental considerations, and Economics.
equally to left-in/left-out ramps. In effect, this sitThe relative importance of these controls may
uation could be described as being an inter-
vary from interchange to interchange. For any
change without a crossing road.
particular site, each of the controls will have to
The primary difference between systems and
be examined and its relative importance
Geometric Design Guide
access/service interchanges is that the ramps
assessed. Only after this process will it be pos-
on systems interchanges have free-flowing ter-
sible to study alternative interchange types and
minals at both ends, whereas the intersecting
configurations to determine the most suitable in
road ramp terminals on an access interchange
terms of the more important controls.
are typically in the form of at-grade intersections.
While the selection of the most appropriate type Interchanges can also be between non-freeway
and configuration of interchange may vary
roads, for example between two heavily traf-
between sites, it is important to provide consis-
ficked arterials.
tent operating conditions in order to match driv-
In very rare instances there
may even be an application for an interchange
er expectations.
7-14 Chapter 7: Interchanges
7.7.2
Systems interchanges
Partially directional interchanges allow the number of levels to be reduced. The Single Loop
As stated above, at-grade intersections are
Partially-directional Interchange, illustrated in
inappropriate to systems interchanges and their
Figure 7.7 (ii), and the Two Loop arrangement,
avoidance is mandatory. For this reason, hybrid
illustrated in Figure 7.7 (iii) and (iv), require
interchanges, in which an access interchange is
three levels. The difference between Figures
contained within a systems interchange, are to
7.7 (iii) and (iv) is that, in the former case, the
be avoided.
freeways cross and, in the latter, route continuity dictates a change in alignment. Loop ramps
Hybrid interchanges inevitably lead to an unsafe
are normally only used for lighter volumes of
mix of high and low speed traffic. Furthermore,
right-turning traffic. A three-loop arrangement
signposting anything up to six possible destina-
is, in effect, a cloverleaf configuration, with one
tions within a very short distance is, at best, dif-
of the loops being replaced by a directional
ficult. Selecting the appropriate response gen-
ramp and is not likely to occur in practice, large-
erates an enormous workload for the driver so
ly because of the problem of weaving discussed
that the probability of error is substantial. Past
below.
experience suggests that these interchange The principal benefit of the cloverleaf is that it
configurations are rarely successful.
requires only a simple one-level structure, in Directional interchanges provide high-speed
contrast to the complex and correspondingly
connections to left and to right provided that the
costly structures necessary for the directional
ramp exits and entrances are on the left of the
and partially directional configurations. The
through lanes. Where turning volumes are low
major weakness of the cloverleaf is that it
or space is limited, provision of loops for right
requires weaving over very short distances.
turning traffic can be considered.
Directional
Provided weaving volumes are not high and suf-
interchanges that include one or more loops are
ficient space is available to accommodate the
referred to as being partially-directional. If all
interchange, the cloverleaf can, however, be
right turns are required to take place on loops,
considered to be an option.
the cloverleaf configuration emerges. Various
required to take place on the main carriage-
forms of systems interchanges are illustrated
ways, the turbulence so created has a serious
below.
effect on the flow of traffic through the interchange area. The cloverleaf also has the characteristic of confronting the driver with two exits
Four-legged interchanges
from the freeway in quick succession.
Both
The fully directional interchange illustrated in
these problems can be resolved by providing
Figure 7.7 (i) provides single exits from all four
collector-distributor roads adjacent to the
directions and directional ramps for all eight
through carriageways.
turning movements.
The through roads and
ramps are separated vertically on four levels. 7-15 Chapter 7: Interchanges
Geometric Design Guide
If weaving is
Geometric Design Guide
Figure 7.7: Four-legged systems interchanges
Three-legged interchanges
It is also possible with this layout to slightly reduce the height through which vehicles have
Various fully-directional and partially-directional
to climb. Figure 7.8 (iii) illustrates a fully-direc-
three legged interchanges are illustrated in
tional interchange that requires only two but
Figure 7.8. In Figure 7.8 (i), one single structure
widely separated structures.
providing a three-level separation is required.
assumed as being at the top of the page, vehi-
Figure 7.8 (ii) also requires three levels of road-
cles turning from West to South have a slightly
way but spread across two structures hence
longer path imposed on them so that this
reducing the complexity of the structural design.
should, ideally be the lesser turning volume. 7-16
Chapter 7: Interchanges
If North is
Figures 7.8 (iv) and (v) show semi-directional
vehicles entering from the crossing road may be
interchanges. Their names stem from the loop
doing so from a stopped condition, so that it is
ramp located within the directional ramp creat-
necessary to provide acceleration lanes to
ing the appearance of the bell of a trumpet. The
ensure that they enter the freeway at or near
letters "A" and "B" refer to the loop being in
freeway speeds.
Advance of the structure or Beyond it.
The
should be provided with deceleration lanes to
smaller of the turning movements should ideally
accommodate the possibility of a stop at the
be on the loop ramp but the availability of space
crossing road.
Similarly, exiting vehicles
may not always make this possible. As previously discussed, there is distinct merit
7.7.3
Access and service interchanges
in the crossing road being taken over the freeway as opposed to under it. One of the advan-
In the case of the systems interchange, all traf-
tages of the crossing road being over the free-
fic enters the interchange area at freeway
way is that the positive and negative gradients
speeds. At access and service interchanges,
respectively support the required deceleration 7-17
Chapter 7: Interchanges
Geometric Design Guide
Figure 7.8: Three-legged systems interchanges
and acceleration to and from the crossing road.
crossing road, at the time of construction, stops
The final decision on the location of the crossing
immediately beyond the interchange.
road is, however, also dependent on other controls such as topography and cost.
Diamond Interchanges
Access interchanges normally provide for all
There are three basic forms of Diamond, being:
turning movements.
• • •
If, for any reason, it is
deemed necessary to eliminate some of the turning movements, the return movement, for
The Simple Diamond; The Split Diamond, and the Single Point Interchange.
any movement that is provided, should also be provided. Movements excluded from a particu-
The Simple Diamond is easy for the driver to
lar interchange should, desirably, be provided at
understand and is economical in its use of
the next interchange upstream or downstream
space. The major problem with this configura-
as, without this provision, the community served
tion is that the right turn on to the crossing road
loses amenity.
can cause queuing on the exit ramp. In extreme cases, these queues can extend back onto the
There are only two basic interchange types that
freeway, creating a hazardous situation. Where
are appropriate to access and service inter-
the traffic on the right turn is very heavy, it may
changes. These are the Diamond and the Par-
be necessary to consider placing it on a loop
Clo interchanges. Each has a variety of possi-
ramp. This is the reverse of the situation on sys-
ble configurations.
tems interchanges where it is the lesser vol-
Geometric Design Guide
umes that are located on loop ramps. It has the Trumpet interchanges used to be considered
advantage that the right turn is converted into a
suitable in cases where access was to provided
left-turn at the crossing road ramp terminal. By
to one side only, for example to a bypass of a
the provision of auxiliary lanes, this turn can
town or village. In practice, however, once a
operate continuously without being impeded by
bypass has been built it does not take long
traffic signals.
before development starts taking place on the
The Simple Diamond can take one of two con-
other side of the bypass.
figurations: the Narrow Diamond and the Wide
The three-legged
interchange then has to be converted into a
Diamond.
four-legged interchange. Conversion to a ParClo can be achieved at relatively low cost.
The Narrow Diamond is the form customarily
Other than in the case of the Par-Clo AB, one of
applied. In this configuration, the crossing road
the major movements is forced onto a loop
ramp terminals are very close in plan to the free-
ramp. The resulting configuration is thus not
way shoulders to the extent that, where space is
appropriate to the circumstances. In practice,
heavily constricted, retaining walls are located
the interchange should be planned as a
just outside the freeway shoulder breakpoints.
Diamond in the first instance, even though the
Apart from the problem of the right turn referred
7-18 Chapter 7: Interchanges
to above, it can also suffer from a lack of inter-
The Split Diamond can also take one of two
section sight distance at the crossing road ramp
forms: the conventional Split and the transposed
terminals. This problem arises when the cross-
Split. This configuration is normally used when
ing road is taken over the freeway and is on a
the crossing road takes the form of a one-way
minimum value crest curve on the structure. In
pair.
addition, the bridge balustrades can also inhibit
queues backing up are not normally experi-
sight distance. In the case where the crossing
enced on Split Diamonds and the most signifi-
road ramp terminal is signalised, this is less of a
cant drawback is that right-turning vehicles have
problem, although a vehicle accidentally or by
to traverse three intersections before being
intent running the red signal could create a dan-
clear of the interchange. It is also necessary to
gerous situation.
construct frontage roads linking the two one-
The problems of sight distance and
way streets to provide a clear route for rightturning vehicles.
The Wide Diamond was originally intended as a form of stage construction, leading up to conversion to a full Cloverleaf Interchange. The
The transposed Split has the ramps between the
time span between construction of the Diamond
two structures. This results in a very short dis-
and the intended conversion was, however, usu-
tance between the entrance and succeeding
ally so great that, by the time the upgrade
exit ramps, with significant problems of weaving
became necessary, standards had increased to
on the freeway. Scissor ramps are the extreme
the level whereby the loop ramps could not be
example of the transposed Split. These require
accommodated in the available space.
The
either signalisation of the crossing of the two
decline in the popularity of the Cloverleaf has
ramps or a grade separation. The transposed
led to the Wide Diamond also falling out of
Split has little to recommend it and has fallen
favour.
into disuse, being discussed here only for completeness of the record.
The Wide Diamond has the problem of imposing The Single Point Interchange brings the four
a long travel distance on right-turning vehicles
interchange is required where space is at a pre-
road ramp terminals are located at the start of
mium or where the volume of right-turning traffic
the approach fill to the structure. To achieve this
is very high. The principal operating difference
condition, the ramps have to be fairly long so
between the Single Point and the Simple
that queues backing up onto the freeway are
Diamond is that, in the former case, the right
less likely than on the Narrow Diamond. The
turns take place outside each other and in the
crossing road ramp terminals are also at ground
latter they are "hooking" movements.
level, which is a safer alternative than having
capacity of the Single Point Interchange is thus
the intersections on a high fill. Finally, because
higher than that of the Simple Diamond. It does,
the ramp terminals are remote from the struc-
however, require a three-phase signal plan and
ture, intersection sight distance is usually not a
also presents pedestrians with wide unprotected
problem.
crossings. 7-19 Chapter 7: Interchanges
The
Geometric Design Guide
ramps together at a point over the freeway. This
but is not without its advantages. The crossing
The various configurations of the Diamond
Three configurations of Par-Clo Interchange are
Interchange are illustrated in Figure 7.9
possible: the Par-Clo A, the Par-Clo B and the Par-Clo AB.
Par-Clo interchanges
As in the case of the Trumpet
Interchange, the letters have the significance of
Geometric Design Guide
the loops being in advance of or beyond the Par-Clo interchanges derive their name as a
structure. The Par-Clo AB configurationhas the
contraction of PARtial CLOverleaf, mainly
loop in advance of the structure for the one
because of their appearance, but also because
direction of travel and beyond the structure for
they were frequently a first stage development
the other. In all cases, the loops are on oppo-
of a Cloverleaf Interchange. In practice, they
site sides of the freeway. Both the Par-Clo A
could perhaps be considered rather as a distort-
and the Par-Clo B have alternative configura-
ed form of Simple Diamond Interchange.
tions: the A2 and A4 and the B2 and B4. These
Figure 7.9: Diamond interchanges
7-20 Chapter 7: Interchanges
configurations refer to two quadrants only being
from the intersections on the crossing road and
occupied or alternatively to all four quadrants
the only conflict is between right-turning vehi-
having ramps. The four-quadrant layout does
cles exiting from the freeway and through traffic
not enjoy much, if any, usage in South Africa.
on the crossing road. This makes two-phase signal control possible.
The various layouts are illustrated in Figures 7.10, 7.11 and 7.12.
The Par-Clo AB is particularly useful in the situation where there are property or environmental
Internationally, the Par-Clo A4 is generally
restrictions in two adjacent quadrants on the
regarded as being the preferred option for an
same side of the crossing road.
interchange between a freeway and a heavily
include a road running alongside a river or the
trafficked arterial. In the first instance, the loops
situation, frequently found in South Africa, of a
serve vehicles entering the freeway whereas, in
transportation corridor containing parallel road
the case of the Par-Clo B, the high-speed vehi-
and rail links in close proximity to each other.
Examples
loop. This tends to surprise many drivers and
The Rotary Interchange illustrated in Figure
loops carrying exiting traffic have higher acci-
7.12 has the benefit of eliminating intersections
dent rates than the alternative layout. Secondly,
on the crossing road, replacing them by short
the left turn from the crossing road is remote
weaving sections. Traffic exiting from the free-
Figure 7.10: Par-Clo A interchanges 7-21 Chapter 7: Interchanges
Geometric Design Guide
cles exiting the freeway are confronted by the
way may experience difficulty in adjusting speed
Kingdom as systems interchanges. In this con-
and merging with traffic on the rotary.
figuration, a two-level structure is employed. One freeway is located at ground level and the
Rotaries have also been used in the United
other freeway on the upper level of the structure
Geometric Design Guide
Figure 7.11: Par-Clo B interchanges
Figure 7.12: Par-Clo AB interchanges and rotaries 7-22 Chapter 7: Interchanges
with the rotary sandwiched between them. This
Where the need for the interchange derives
is the so-called "Island in the Sky" concept. The
purely from topographic restraints, i.e. where
Rotary is an interchange form that is unknown in
traffic
South Africa so that its application would com-
Interchange, illustrated in Figure 7.13, would be
promise consistency of design and thus be con-
adequate. This layout, also known as a Quarter
trary to drivers' expectations.
Link, provides a two-lane-two-way connection
volumes
are
low,
a
Jug
Handle
between the intersecting roads located in what-
7.7.4
Interchanges on non-freeway roads
ever quadrant entails the minimum construction and property acquisition cost.
Interchanges where a non-freeway is the major route are a rarity in South Africa. This applica-
Drivers would not expect to find an interchange
tion would arise where traffic flows are so heavy
on a two-lane two-way road and, in terms of
that a signalised intersection cannot provide suf-
driver expectancy, it may therefore be advisable
ficient capacity. In this case, the crossing road
to introduce a short section of dual carriageway
terminals would be provided on the road with
at the site of the interchange
the lower traffic volume. As a general rule, a simple and relatively low standard Simple
7.8
RAMP DESIGN
7.8.1
General
Diamond or a Par-Clo Interchange should suffice. An intersection with a particularly poor accident
A ramp is defined as a roadway, usually one-
history may also require upgrading to an inter-
way, connecting two grade-separated through
The accident history would provide
some indication of the required type of inter-
roads. It comprises an entrance terminal, a mid-
change.
section and an exit terminal.
Figure 7.13: Jug Handle interchange 7-23 Chapter 7: Interchanges
Geometric Design Guide
change.
between them;
The general configuration of a ramp is deter-
•
mined prior to the interchange type being selected.
The directional ramp also serving the right turn, with a curve only slightly in
The specifics of its configuration,
O
being the horizontal and vertical alignment and
excess of 90 degrees and free-flowing
cross-section, are influenced by a number of
terminals at either end, and
•
considerations such as traffic volume and com-
The collector-distributor road intended to remove the weaving manoeuvre from
position, the geometric and operational charac-
the freeway.
teristics of the roads which it connects, the local topography, traffic control devices and driver
The express-collector system discussed later is
expectations.
a transfer roadway and is not an interchange ramp.
A variety of ramp configurations can be used. These include:
•
Geometric Design Guide
•
•
7.8.2
The outer connector, which serves the left turn and has free-flowing terminals
Guideline values for ramp design speeds are
at either end;
given in Table 7.2. Strictly speaking, the design
The diamond ramp, serving both the left-
speed of a ramp could vary across its length
and right-turns with a free-flowing termi-
from that of the freeway to that of the at-grade
nal on the freeway and a stop-condition
intersection, with the design speed at any point
terminal on the crossing-road;
along the ramp matching the operating speed of
The Par-Clo ramp, which serves the
the vehicles accelerating to or decelerating from
right turn and has a free-flowing terminal
the design speed of the freeway. The design
on the freeway and a stop-condition ter-
speeds given in the table apply to the controlling
minal on the crossing road, with a 180
O
curve on the mid-section of the ramp. The ramp
loop between them;
•
Design speed
design speed is shown as a design domain
The loop ramp, serving the right turn
because of the wide variety of site conditions,
and which has free-flowing terminals at
terminal types and ramp shapes.
O
both ends and a 270 degree loop 7-24 Chapter 7: Interchanges
In the case of a directional ramp between free-
the substantial difference between the freeway
ways, vehicles must be able to operate safely at
design speed and that of the loop ramp, it is
the higher end of the ranges of speeds shown in
advisable not to have a loop on an exit ramp if
Table 7.2.
this can be avoided. Safety problems on ramp curves are most likely
Factors demanding a reduction in design speed
for vehicles travelling faster than the design
include site limitations, ramp configurations and
speed. This problem is more critical on curves
economic factors. Provided the reduction is not
with lower design speeds because drivers are
excessive, drivers are prepared to reduce speed
more likely to exceed these design speeds than
in negotiating a ramp so that the lower design
the higher ranges. Trucks can capsize when
speed is not in conflict with driver expectations.
travelling at speeds only marginally higher than the design speed. To minimise the possibility of
For directional ramps and outer connectors,
trucks exceeding the design speed it is suggest-
higher values in the speed domain are appropri-
ed that the lower limits of design speeds shown
ate and, in general, ramp designs should be
in Table 7.2 should not be used for ramps carry-
based on the upper limit of the domain. The
ing a substantial amount of truck traffic.
constraints of the site, traffic mix and form of interchange, however, may force a lower design
If site specific constraints preclude the use of
speed.
design speeds that more-or-less match anticipated operating speeds, the designer should
A Par-Clo or a loop ramp cannot be designed to
seek to incorporate effective speed controls,
a high design speed. A ramp design speed of
such as advisory speed signing, special pave-
70 km/h, being the low end of the domain for a
ment treatments, long deceleration lanes and
freeway design speed of 130 km/h, would
the use of express-collector systems in the
require a radius of between 150 metres and 200
design.
metres, depending on the rate of superelevation
Sight distance on ramps
area, that the space required to accommodate a loop with this radius would be available. Even if
It is necessary for the driver to be able to see
the space were available, the additional travel
the road markings defining the start of the taper
distance imposed by the greater radius would
on exit ramps and the end of the entrance taper.
nullify the advantages of the higher travel
At the crossing road ramp terminal, lanes are
speed. The added length of roadway also adds
often specifically allocated to the turning move-
the penalty of higher construction cost. This
ments with these lanes being developed in
penalty also applies to the ramp outside the loop
advance of the terminal. The driver has to posi-
because it has to be longer to contain the larger
tion the vehicle in the lane appropriate to the
loop. In general, loop ramps are designed for
desired turn. It is therefore desirable that deci-
speeds of between 40 and 50 km/h. Because of
sion sight distance be provided on the 7-25
Chapter 7: Interchanges
Geometric Design Guide
7.8.3
selected. It is not likely, particularly in an urban
approaches to the ramp as well as across its
only commence at the nose.
The distance
length.
required to achieve the appropriate superelevation thus determines the earliest possible location of the first curve on the ramp.
Appropriate values of decision sight distance are given in Table 3.7.
7.8.4
Ramps are relatively short and the radii of
Horizontal alignment
curves on ramps often approach the minimum for the selected design speed. Furthermore, if
Minimum radii of horizontal curvature on ramps
there is more than one curve on a ramp, the dis-
are as shown in Table 4.1 for various values of
tance between the successive curves will be
emax. In general, the higher values of emax are
short. Under these restrictive conditions, transi-
used in freeway design and the selected value
tion curves should be considered.
should also be applied to the ramps.
Ramps are seldom, if ever, cambered and
Achieving the step down of radii from higher to
superelevation typically involves rotation around
lower design speeds on a loop may require the
one of the lane edges. Drivers tend to position
application of compound curves. In general, the
their vehicles relative to the inside edge of any
ratio between successive radii should be 1,5 : 1
curve being traversed, i.e. they steer towards
and, as a further refinement, they could be con-
the inside of the curve rather than away from the
nected by transition curves. The length of each
outside. For aesthetic reasons, the inside edge
arc is selected to allow for deceleration to the
should thus present a smoothly flowing three-
speed appropriate to the next radius at the entry
dimensional alignment with the outside edge ris-
to that arc. In the case of stepping up through
ing and falling to provide the superelevation.
successive radii, the same ratio applies but the
Where a ramp has an S- or reverse curve align-
design speed for the radius selected should
ment, it follows that first one edge and then the
match the desired speed at the far end of each
other will be the centre of rotation, with the
arc.
changeover taking place at the point of zero crossfall.
Geometric Design Guide
It is recommended that the designer develops a speed profile for the loop and bases the selection of radii and arc lengths on this speed profile.
In view of the restricted distances within which
As a rough rule of thumb, the length of each arc
superelevation has to be developed, the
should be approximately a third of its radius.
crossover crown line is a useful device towards rapid development. A crossover crown is a line
If a crossover crown line, discussed below, is
at which an instantaneous change of crossfall
not used, the crossfall on the exit or entrance
takes place and which runs diagonally across
taper between the Yellow Line Break Point and
the lane. The crossover crown could, for exam-
the nose is controlled by that on the through
ple, be located along the yellow line defining the
lanes.
edge of the left lane of the freeway, thus
Superelevation development can thus 7-26
Chapter 7: Interchanges
enabling initiation of superelevation for the first
having as little as 120 to 360 metres between
curve on the ramp earlier than would otherwise
the nose and the crossing road. The effect of
be the case. The crossover crown should, how-
the midsection gradient, while possibly helpful,
ever, be used with caution as it may pose a
is thus restricted. However, a steep gradient (8
problem to the driver, particularly to the driver of
per cent) in conjunction with a high value of
a vehicle with a high load. This is because the
superelevation (10 per cent) would have a
vehicle will sway as it traverses the crossover
resultant of 12,8 per cent at an angle of 53 to
crown and, in extreme cases, may prove difficult
the centreline of the ramp. This would not con-
to control.
The algebraic difference in slope
tribute to drivers' sense of safety. In addition,
across the crossover crown should thus not
the drivers of slow-moving trucks would have to
exceed four to five per cent.
steer outwards to a marked extent to maintain
O
their path within the limits of the ramp width.
7.8.5
Vertical alignment
This could create some difficulty for them. It is suggested that designers seek a combination of
Gradients
superelevation and gradient such that the gradient of the resultant is less than ten per cent.
The profile of a ramp typically comprises a mid-
Table 7.3 provides an indication of the gradients
section with an appreciable gradient coupled
of the resultants of combinations of supereleva-
with terminals where the gradient is controlled
tion and longitudinal gradient.
over the freeway, the positive gradient on the
The combinations of gradient and supereleva-
off-ramps will assist a rapid but comfortable
tion shown shaded in Table 7.3 should be avoid-
deceleration and the negative gradient on the
ed.
on-ramp will support acceleration to freeway speeds. In theory, thus, the higher the value of
Curves
gradient, the better. Values of gradient up to eight per cent can be considered but, for prefer-
As suggested above, decision sight distance
ence, gradients should not exceed six per cent.
should be available at critical points on ramps. The K-values of crest curves required to meet
Diamond ramps are usually fairly short, possibly
this requirement are given in Table 7.4. 7-27
Chapter 7: Interchanges
Geometric Design Guide
by the adjacent road. If the crossing road is
These values of K are based on the required
On sag curves, achieving decision sight dis-
sight distance being contained within the length
tance is not a problem so that the K-values
of the vertical curve. It is, however, unlikely that,
shown in Table 4.14 can be applied. If the inter-
within the confined length of a ramp, it would be
change area is illuminated, as would typically be
possible to accommodate the length of vertical
the case in an urban area, the K-values for com-
curve required for this condition to materialise.
fort can be used. Unilluminated, i.e. rural, inter-
The designer should therefore have recourse to
changes would require the application of K-val-
Equation 4.21 to calculate the K-value for the
ues appropriate to headlight sight distance.
condition of the curve being shorter than the
7.8.6 Cross-section
required sight distance. This equation is repeated below for convenience.
Horizontal radii on ramps are typically short and 4.21
hence are usually in need of curve widening.
Geometric Design Guide
The width of the ramps normally adopted is thus where K S
= =
Distance required for a
4 metres.
Because of the inconvenience of
1 % change of gradient (m)
changing the lane width of comparatively short
Stopping sight distance
sections, this width is applied across the entire
for selected design
length of the ramp.
speed (m) h1
=
Driver eye height (m)
In the case of a loop ramp, the radius could be
h2
=
Object height (m)
as low as fifty metres. At this radius, a semi
A
=
Algebraic difference in
trailer would require a lane width of 5,07 metres.
gradient between the
It is necessary for the designer to consider the
approaching and
type of vehicle selected for design purposes and
departing grades (%)
to check whether the four metre nominal width is 7-28
Chapter 7: Interchanges
adequate. If semi trailers are infrequent users
signalised or stop control at-grade intersection,
of the ramp, encroachment on the shoulders
the change in speed across the ramp is sub-
could be considered.
stantial.
Provision should thus be made for
acceleration and deceleration to take place The calculation of required lane width is
clear of the freeway so as to minimise interfer-
described in detail in Section 4.2.6. As sug-
ence with the through traffic and reduce the
gested above, regardless of the width required,
potential for crashes. The auxiliary lanes pro-
it should be applied across the entire length of
vided to accommodate this are referred to as
the ramp.
speed-change lanes or acceleration or deceleration lanes.
Ramp shoulders typically have a width of the
These terms describe the area
adjacent to the travelled way of the freeway,
order of 2 metres, with this width applying both
including that portion of the ramp taper where a
to the inner and to the outer shoulder. In con-
merging vehicle is still clear of the through lane,
junction with the nominal lane width of 4 metres,
and do not imply a definite lane of uniform width.
the total roadway width is thus 8 metres. This width would allow comfortably for a truck to pass
The speed change lane should have sufficient
a broken-down truck. In addition, it would pro-
length to allow the necessary adjustment in
vide drivers with some sense of security in the
speed to be made in a comfortable manner and,
cases where the ramp is on a high fill.
in the case of an acceleration lane, there should
7.8.7
Terminals
also be sufficient length for the driver to find and manoeuvre into a gap in the through traffic
The crossing road ramp terminals may be free-
stream before reaching the end of the accelera-
flowing, in which case their design is as dis-
tion lane. The length of the speed change lane
cussed below. Crossing road ramp terminals
is based on:
that are at-grade intersections should be
•
The design speed on the through lane,
designed according to the recommendations
i.e. the speed at which vehicles enter or
contained in Chapter 6. It is worth noting that,
exit from the through lanes;
•
from an operational point of view, what appears
est radius curve on the ramp, and
ing road ramp terminals on a Diamond
•
Interchange, is, in fact, two three-legged inter-
The tempo of acceleration or deceleration applied on the speed change lane.
Ideally, the crossing
road ramp terminal should be channelised to
In Chapter 3, reference was made to a deceler-
reduce the possibility of wrong-way driving.
ation rate of 3 m/s2 as the basis for the calculation of stopping sight distance. Research has
Vehicles entering or exiting from a freeway
shown that deceleration rates applied to off-
should be able to do so at approximately the
ramps are a function of the freeway design
operating speed of the freeway. Given the fact
speed and the ramp control speed. As both
that the crossing road terminal is invariably a
speeds increase, so does the deceleration rate, 7-29
Chapter 7: Interchanges
Geometric Design Guide
tion, i.e. the design speed of the small-
to be a four-legged intersection, e.g. the cross-
sections back-to-back.
The control speed of the ramp midsec-
which varies between 1,0 m/s2 and 2,0 m/s2.
length of the entering vehicle is not relevant in
For convenience, the deceleration rate used to
determination of the taper length.
develop Table 7.5 has been set at 2,0 m/s . 2
The lengths of the deceleration and acceleration lanes shown in Tables 7.5 and 7.6 apply to gra-
Deceleration should only commence once the
dients of between - 3 per cent and + 3 per cent.
exiting vehicle is clear of the through lane.
Acceleration lanes will have to be longer on
Assuming that a vehicle with a width of 2,5
upgrades and may be made shorter on down-
metres is correctly positioned relative to the
grades, with the reverse applying to decelera-
ramp edge line to be centrally located within an
tion lanes.
ultimate ramp width of 4 metres, its tail end will clear the edge of the through lane at a distance,
The actual entrance or exit points between the
L, from the Yellow Line Break Point where with
L
=
3,2 / T
freeway and the ramp can take the form of a
T
=
Taper rate (as listed in
taper or a parallel lane. The parallel lane has
Table 7.7)
the problem of forcing a reverse curve path, possibly followed after the nose by a further curve to the left, whereas the taper involves only
The maximum legal length of any vehicle on
a single change of direction. As such, drivers
South African roads is 22 metres and this should
prefer the taper. In cases where an extended
be added to the distance, L, to establish the dis-
acceleration
tance from the Yellow Line Break Point, at which
or
deceleration
distance
is
required, for example where the crossing road
the deceleration can commence so that
passes under the freeway resulting in the onramp being on an upgrade and the off-ramp on
LT
=
3,2/T + 22
a downgrade, a straight taper would result in an
Geometric Design Guide
inordinately long ramp. The parallel lane allows Values of LT, are listed in Table 7.5. From this
the speed change to take place in an auxiliary
table it follows that, in the case of a diamond
lane immediately adjacent to the through lanes
ramp without curves, the distance from the
thus reducing the spatial demands of the inter-
Yellow Line Break Point to the crossing road
change.
ramp terminal should be not less than 437
Furthermore, in the case of the on-ramp, the
metres.
parallel configuration provides an extended distance to find a gap in the adjacent traffic stream.
The acceleration rate can, according to
Typical configurations of on- and off-ramp termi-
American literature, be taken as 0,7 m/s2. The
nals are illustrated in Figures 7.14 to 7.17.
length of the acceleration lane is thus as shown Taper
in Table 7.6. An important feature of the acceleration lane is the gap acceptance length, which should be a minimum of 100 metres to 150
Two different criteria apply to the selection of the
metres, depending on the nose width.
taper rate, depending on whether the ramp is an
The 7-30
Chapter 7: Interchanges
rates corresponding to the condition of the wheel path having an offset of 150 millimetres
In the first case, the vehicle is merely required to
inside the Yellow Line Break Point.
achieve a change of direction from the freeway to the ramp. The taper rate should thus be suf-
These taper rates are higher than the rate of 1 :
ficiently flat to ensure that the vehicle path can
15 currently applied on South African freeways.
be accommodated within the lane width.
In
This rate derives from the application of an
Table 7.7, the radii of curvature corresponding
"operating speed" previously assumed to be 85
to a superelevation of 2,0 per cent for the vari-
per cent of the design speed and the further
ous design speeds are listed as are the taper
assumption that the wheel path could be
7-31 Chapter 7: Interchanges
Geometric Design Guide
exit from or entrance to the freeway.
allowed to pass over the Yellow Line Break Point.
that, in the case of existing interchanges, it is
In practice, a 1: 15 rate requires that drivers
not always necessary to incur the expenditure of
accept a higher level of side friction in negotiat-
upgrading to a flatter taper. Trucks do, however,
ing the change of direction. Passenger cars can
sometimes roll over at the start of the ramp taper.
negotiate tapers of this magnitude with ease so
Geometric Design Guide
Figure 7.14: Single lane exit
Figure 7.15: Two-lane exit 7-32 Chapter 7: Interchanges
Figure 7:17: Two-lane entrance 7-33 Chapter 7: Interchanges
Geometric Design Guide
Figure 7.16: One lane entrance
Figure 7.18: Major fork If there is a high percentage of truck traffic and
rate does not have to be as flat as suggested
if the incidence of roll-overs is unacceptably
above. As in the case of the climbing lane, a
high, it may be necessary to consider upgrading
taper length of 100 metres would be adequate.
to the tapers suggested in Table 7.7. Parallel entrances and exits have a major In the case of the entrance ramp, the driver of
advantage over straight tapers in respect of the
the merging vehicle should be afforded suffi-
selection of curves on the ramps. In the case of
cient opportunity to locate a gap in the opposing
a design speed of 120 km/h, a straight taper
traffic and position the vehicle correctly to merge
provides a distance of just short of 300 metres between the Yellow Line Break Point and the
into this gap at the speed of the through traffic.
nose. A portion of this distance should ideally
Taper rates flatter than those proposed in Table
be traversed at the design speed of the freeway
Geometric Design Guide
7.7 should thus be applied to on-ramps. A taper
with deceleration commencing only after the
of 1:50 provides a travel time of about 13 sec-
vehicle is clear of the outside freeway lane.
onds between the nose and the point at which
About 220 metres is available for deceleration
the wheel paths of merging and through vehi-
and this suggests that a vehicle exiting at 120
cles would intersect. This has been found in
km/h is likely still to be travelling at a speed of
practice to provide sufficient opportunity to pre-
about 95 km/h when passing the nose. The first curve on the ramp could, allowing for superele-
pare to merge into gaps in the through flow
vation development to 10 per cent, be located
because drivers typically begin to locate usable
100 metres beyond the nose so that, at the start
gaps in the freeway flow prior to passing the
of the curve, the vehicle speed would be in the
nose.
range of 80 to 90 km/h. The first curve should thus have a radius of not less than 250 metres.
Parallel
In the case of the parallel ramp, the radius could obviously be significantly shorter, depending on
The parallel lane configuration is also initiated
the length of the deceleration lane preceding the
by means of a taper but, in this case, the taper
nose. 7-34
Chapter 7: Interchanges
7.9
COLLECTOR - DISTRIBUTOR
7.10.1 Ramp metering
ROADS Ramp metering consists of traffic signals Collector-distributor roads are typically applied
installed on entrance ramps in advance of the
to the situation where weaving manoeuvres
entrance terminal to control the number of vehi-
would be disruptive if allowed to occur on the
cles entering the freeway. The traffic signals
freeway. Their most common application, there-
may be pretimed or traffic-actuated to release
fore, is at Cloverleaf interchanges. The exit and
the entering vehicles individually or in platoons.
entrance tapers are identical to those applied to
It is applied to restrict the number of vehicles
any other ramps. The major difference between
that are allowed to enter a freeway in order to
Cloverleaf interchange C-D roads and other
ensure an acceptable level of service on the
ramps is that they involve two exits and two
freeway or to ensure that the capacity of the
entrances in quick succession. The two exits
freeway is not exceeded. The need for ramp
are, firstly, from the freeway and, secondly, the
metering may arise owing to factors such as:
split between vehicles turning to the left and
•
Recurring congestion because traffic
those intending to turn to the right. The two
demand exceeds the provision of road
entrances are, firstly, the merge between the
infrastructure in an area;
•
two turning movements towards the freeway
Sporadic congestion on isolated sec-
and, secondly, the merge with the freeway
tions of a freeway because of short term
through traffic.
traffic loads from special events, normally of a recreational nature;
•
The distance between the successive exits
As part of an incident management sys-
should be based on signing requirements so as
tem to assist in situations where a acci-
to afford drivers adequate time to establish
dent downstream of the entrance ramp
whether they have to turn to the right or to the
causes a temporary drop in the capacity
left to reach their destination. Nine seconds is
of the freeway; and
•
generally considered adequate for this purpose
Optimising traffic flow on freeways
design speed of the freeway as they pass the
Ramp metering also supports local transporta-
nose of the first exit, the distance between the
tion management objectives such as:
•
noses should desirably be based on this speed.
Priority treatments with higher levels of service for High Occupancy Vehicles; and
The distance between successive entrances is
•
based on the length required for the accelera-
Redistribution of access demand to other on-ramps.
tion lane length quoted in Table 7.6. It is important to realise that ramp metering
7.10
OTHER INTERCHANGE DESIGN
should be considered a last resort rather than as
FEATURES
a first option in securing an adequate level of service on the freeway. Prior to its implementa-
7-35 Chapter 7: Interchanges
Geometric Design Guide
and seeing that vehicles may be travelling at the
tion, all alternate means of improving the capac-
An express-collector system could, for example,
ity of the freeway or its operating characteristics
be started upstream of one interchange and run
or reducing the traffic demand on the freeway
through it and the following, possibly closely
should be explored. The application of ramp
spaced, interchange, terminating downstream
metering should be preceded by an engineering
of the second. The terminals at either end of the
analysis of the physical and traffic conditions on
express-collector system would have the same
the freeway facilities likely to be affected. These facilities include the ramps, the ramp terminals
standards as applied to conventional on- and
and the local streets likely to be affected by
off-ramps. The interchange ramps are connect-
metering as well as the freeway section
ed to the express-collector system and not
involved.
directly to the freeway mainline lanes.
The stopline should be placed sufficiently in
Traffic volumes and speeds on the express-col-
advance of the point at which ramp traffic will
lector roads are typically much lower than those
enter the freeway to allow vehicles to accelerate
found on the mainline lanes, allowing for lower
to approximately the operating speed of the
standards being applied to the ramp geometry
freeway, as would normally be required for the
of the intervening interchanges.
design of ramps. It will also be necessary to ensure that the ramp has sufficient storage to
The minimum configuration for an express-col-
accommodate the vehicles queuing upstream of
lector system is to have a two-lane C-D road on
the traffic signal.
either side of a freeway with two lanes in each direction. The usual configuration has more than two mainline lanes in each direction
The above requirement will almost certainly lead to a need for reconstruction of any ramp that is to be metered. The length of on-ramps is typi-
A similar configuration is known as a dual-divid-
cally determined by the distance required to
ed freeway, sometimes referred to as a dualdual freeway. In this case, the C-D roads are
enable a vehicle to accelerate to freeway
taken over a considerable distance and may
speeds. Without reconstruction, this could result
have more than two lanes. Furthermore, con-
Geometric Design Guide
in the ramp metering actually being installed at
nections are provided at intervals between the
the crossing-road ramp terminal.
outer and the core lanes along the length of the freeway. These, unfortunately, create the effect
7.10.2 Express-collector systems
of right-side entrances and exits to and from the outer lanes and are thus contrary to drivers'
Express-collector systems are used where traf-
expectations. The geometry of the situation is
fic volumes dictate a freeway width greater than
otherwise similar to that of the express-collector
four lanes in each direction. The purpose of the
system.
express-collector is to eliminate weaving on the mainline lanes by limiting the number of entrance and exit points while satisfying the demand for access to the freeway system.
7-36 Chapter 7: Interchanges
TABLE OF CONTENTS 8
ROADSIDE SAFETY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1.1 General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1 8.1.2 Safety objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 8.1.3 The "Forgiving Roadside" approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4 8.1.4 Design Focus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4 8.1.5 Roadside safety analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5 8.1.6 Road safety audits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6
8.2
ROADSIDE HAZARDS AND CLEAR ZONE CONCEPT . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.2.2 Elements of the clear zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8 8.2.3 Factors influencing the clear zone design domain . . . . . . . . . . . . . . . . . . . . . . . . . 8-9 8.2.4 Determining width of clear zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10 8.2.5 Best practices in respect of roadside vegetation . . . . . . . . . . . . . . . . . . . . . . . . . 8-10
8.3
SIGN AND OTHER SUPPORTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 8.3.1 Basis for Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 8.3.2 Breakaway supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13 8.3.3 Design and Location Criteria for Sign Supports . . . . . . . . . . . . . . . . . . . . . . . . . . 8-15 8.3.4 Design approach for lighting supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16
8.4
TRAFFIC SAFETY BARRIERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16 8.4.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16 8.4.2 Determining Need for Safety Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-17 8.4.3 Longitudinal roadside barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-18 8.4.4 Median barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-27
8.5
IMPACT ATTENUATION DEVICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30 8.5.1 Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-30 8.5.2 Design/selection of impact attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 8.5.3 Functional considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 8.5.4 Sand-filled plastic barrel impact attenuators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32
8.6
RUNAWAY VEHICLE FACILITIES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 8.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 8.6.2 Types of escape ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 8.6.3 Criteria for provision of escape ramps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-35 8.6.4 Location of runaway-vehicle facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36 8.6.5 Arrestor bed design features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-38
8.7
BRAKE CHECK AND BRAKE REST AREAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-39
LIST OF TABLES Table 8.1 Design elements that influence road safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 Table 8.2 Clear zone distances (metres) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11 Table 8.3: Roadside obstacles normally considered for shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-23 Table 8 4. Recommended minimum offset distances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-24 Table 8.5 Recommended maximum flare rates for barrier design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-25 Table 8.6 Recommended run-out lengths for barrier design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-26 Table 8.7 : Impact attenuator and end terminal application. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-31 Table 8.8 : Space requirements for plastic drum attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32 Table 8.9: Length of Arrestor Bed (Over Uniform Gravel Depth) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-38
LIST OF FIGURES Figure 8.1: Roadside safety analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7 Figure 8.2: Roadside recovery zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9 Figure 8.3: Adjustment for clear zones on curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-12 Figure 8.4: Breakaway supports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13 Figure 8.5: Classification of traffic barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-17 Figure 8.6: Classification of longitudinal barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-19 Figure 8.7: Warrants for use of roadside barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-22 Figure 8.8: Roadside barrier elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-23 Figure 8.9: Length of need for adjacent traffic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-26 Figure 8.10: Length of need for opposing traffic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-26 Figure 8.11: Space requirement for plastic drum attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-32 Figure 8.12: Typical arrangement of sand-filled barrel attenuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-33 Figure 8.13: Typical arrestor beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-34 Figure 8.14: Layout of arrestor bed adjacent to carriageway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-36 Figure 8.15: Layout of arrestor bed remote from carriageway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-37
Chapter 8 ROADSIDE SAFETY 8.1
INTRODUCTION
ing road safety and that, in the design of new roads or the upgrading of existing ones, particu-
8.1.1
General
lar attention should be given to safety as a prime design criterion.
Road crashes, to varying degrees, are caused A safe road should:
by defects attributable to the vehicle, the driver
•
or the road or by combination of these defects.
Warn and inform road users of changes in the approaching road environment;
In addition, a significant percentage of deaths
•
on roads in South Africa occur as a result of
Guide and control road users safely through the road environment;
•
pedestrians being on the road. The road, or more fully, the road environment, has been esti-
Provide a forgiving roadside environment;
•
mated to contribute to 28 per cent of all road
Provide a controlled release of information;
crashes in South Africa. Various studies have
•
indicated that up to 40 per cent or more of crash
Provide an aesthetically pleasing landscape;
reduction, which could reasonably be expected
•
on the road system, could accrue from the pro-
Maintain road user interest and concentration;
vision of safer roads. The cost of road crashes
• •
to society in South Africa exceeds the annual expenditure on roads, thus the expenditure of
Not surprise road users; Give consistent messages to road users; and
•
considerable sums of money can be justified in
Provide good visibility for all road users.
improved design appropriate and standards and
Numerous research projects have established
by catering for the presence of pedestrians on
relationships between crashes and geometric
roads. Crashes resulting from simply leaving
design elements (as well as operating speeds
the roadway regardless of the underlying cause,
and traffic volumes).
represent a substantial portion of the total road crash problem i.e. "run-off-the-road"
The various geometric design features of a
(ROR)
accidents account for 25 per cent of all road
road, shown in Table 8.1, affect safety by:
crashes in South Africa. They occur on both
•
Influencing the ability of the driver to
straight and curved sections of road and gener-
maintain vehicle control and identify
ally involve either rollover of the vehicle or colli-
hazards. Significant features include:
sion with fixed objects, such as trees, roadside structures etc.
Lane and shoulder width; Horizontal and vertical alignment;
It is thus obvious that the roadside environment and its design have a vital role to play in improv8-1
Chapter 8: Roadside Safety
Sight distance; Superelevation; and
Geometric Design Guide
reducing the crash rates on roads through
•
roadside design can mitigate the severity of a
Pavement surface and drainage.
Influencing the number and types of
crash. This interaction between road, driver and
opportunities that exist for conflict
vehicle characteristics unfortunately compli-
between vehicles. Significant features
cates attempts to estimate the accident reduc-
include:
•
o
Access control;
o
Intersection design;
o
Number of lanes;and
o
Medians.
tion potential of a particular safety improvement. The construction of a road is typically a trade-off between standards and the cost of providing
Affecting the consequences of an out-of-
them. High design standards might be expen-
control vehicle leaving the travel lanes.
sive to provide. However, the cost to society of
Significant features include:
road crashes and deaths often exceeds the total
o
Shoulder width and type;
annual expenditure on roads. Reducing initial
o
Edge drop;
o
Roadside conditions;
o
Side slopes; and
o
Traffic barriers.
construction (or capital) costs of road projects can result in increased life cycle costs if the cost of accidents, injuries and deaths is included in the economic calculations. It is the design engineer's responsibility to inform the client of the
In addition to geometric features, a variety of
consequences of inadequate expenditure on
other factors affect road safety, including other
safety.
elements of the overall road environment, such as:
• • • • • •
Geometric Design Guide
•
Pavement condition;
It is often extremely difficult, if not impossible, to
Weather;
correct safety defects at a later stage without
Lighting;
major reconstruction. For this reason, designing
Traffic flows;
for safety should occur at the outset, or be pro-
Traffic regulation;
vided for in stage construction drawings. Road
Presence of pedestrians; o
Intoxication and
o
Age;
safety audits on the design, carried out by an independent person or team should take place at various stages in the project, as provided for
Vehicle characteristics, such as: o
Size;
in Volume 4 of the South African Road Safety
o
Mass; and
Manual. Although this design guide focuses on
o
Braking capability.
road design features, the psychological aspects of driver behaviour are always present. An error
The effect of road design is somewhat obscured
in perception or judgement or a faulty action on
by the presence of these extraneous factors and
the part of the driver can easily lead to a crash.
most accidents result from a combination of factors interacting in ways that prevent a single fac-
Roads should be designed in such a manner
tor being identified as the cause of a crash.
that only one decision at a time is required from
However, even when a vehicle leaves the road
a driver, ensuring that he/she is never surprised
owing to driver error or mechanical failure, good
by an unexpected situation and that adequate 8-2
Chapter 8: Roadside Safety
8.1.2
time is provided to make the decision.
Safety objectives
Research has shown that the number of accidents increases as the number of decisions required
Broadly, there are three avenues of action pos-
by the driver increases.
sible for mitigating road crashes and their con-
cussed in more detail in chapter 3 of this docu-
sequences;
ment, as well as in Volume 1 of the South
•
By reducing the possibility that run-off will occur;
African Road Safety Manual.
•
Once the run-off does occur, by providing opportunities for the driver of the
Standardisation in road design features and traf-
vehicle to recover and return to the road
fic control devices plays an important role in
without incident, and
•
reducing the number of required decisions, as
If a crash does occur, by providing design elements to reduce the severity
the driver becomes aware of what to expect on
of that collision.
a certain type of road. However it should be noted that standardization alone does not necessarily ensure a safe facility, hence the require-
The first of these areas of endeavour is dealt
ment for a safety audit of the design.
with by incorporating features in the overall 8-3 Chapter 8: Roadside Safety
Geometric Design Guide
This matter is dis-
design of the road which will reduce the possi-
ophy
and
approaches.
Transportation
bility that run-off will occur. These features are
Research Circular 435 states that:-
influenced to a large extent by the selected "Basically a forgiving roadside is one free of
design speed.
obstacles which could cause serious injuries to occupants of an errant vehicle. To the extent
This chapter deals with the latter two of these
possible, a relatively flat, unobstructed roadside
three possibilities. It provides the designer with
recovery area is desirable, and when these con-
guidance on the design of roadside environment
ditions cannot be provided, hazardous features
that includes elements to allow for recovery on
in the recovery area should be made breakaway
the part of the driver, as well as features which
or shielded with an appropriate barrier".
are intended to reduce the severity of such accidents as do occur.
When a vehicle leaves the traffic lane, the path of the vehicle and any object in or near that path
More specifically, it is recommended that the fol-
become contributing factors to the degree of
lowing safety objectives be adopted when a
severity of the crash. By designing a forgiving
road is designed:
• • •
Separate potential conflict points and
roadside the severity of crashes can be
reduce potential conflict areas;
reduced. The concept of designing a forgiving
Control the relative speeds of the con-
roadside should not be regarded as a by-prod-
flicting vehicles;
uct of the application of safety criteria to each
Guide the driver through unusual sec-
design element, but as an integral part of the
tions;
•
total engineering for the road.
Ensure that the needs of pedestrians and cyclists (if relevant) are also consid-
The need for a forgiving roadside is paramount
ered;
Geometric Design Guide
•
Provide a roadside environment that for-
on the outside of horizontal curves with radii of
gives a driver's errant or inappropriate
less than 1 000 metres, where the possibility of
behaviour, by attention to details such
an errant vehicle running off the road is great-
as the safe placement of roadside furni-
est.
ture and by the location and selection of
that horizontal curves with radii in excess of
types of traffic barrier.
However, this statement does not imply
1 000 metres are always safe since the phenomena of "risk adaptation", whereby drivers
8.1.3
The "Forgiving Roadside" approach
concentrate less and drive at higher speeds on sections of road they consider to be safe, should
The forgiving roadside concept, coined in the
be taken into account.
1960's, relates to the approach of making provision for errant vehicles leaving the roadway by
8.1.4
Design Focus
incorporating design elements that reduce the consequences of such departure. This concept
The focus of design measures outlined in this
is an integral part of modern road design philos-
chapter is primarily one of improving road safe8-4
Chapter 8: Roadside Safety
ty through roadside hazard management by the
ments and other aspects of the facility design,
design and provision of appropriate recovery
and between the road itself, the driver and the
and protection measures. The effectiveness of
vehicle. As a result, information touching on
road safety features depends greatly on five
road design issues necessarily is available from
aspects;
many sources. Designers should not rely on
•
Knowledge of the safety characteristics
this Guide as the sole source of information on
and limitations of roadside features by
roadside design issues, particularly when deal-
the designer (and the maintenance per-
ing with unusual or local conditions that depart
sonnel);
from generally accepted situational norms.
The correct choice of appropriate treat-
Particular attention should be paid to the South
ment;
Africa Road Safety Manual produced by the
• •
The correct installation of the roadside
South African Committee of Land Transport
safety features;
•
Officials (COLTO).
The maintenance of the roadside safety features and roadside environment; and
•
8.1.5
Regular monitoring of installations to
Roadside safety analysis
ensure they perform adequately. The design of the roadside environment is a complex problem.
Evaluation of alternative
There are two needs that are the key to effective
designs and choosing between them are difficult
attention to safety in the roadside design
tasks, which involve
process.
•
the occurrence of crashes;
The need for explicit evaluation of
•
design trade-offs with an impact on road safety.
severity; and
In the traditional design process, attention to
•
safety has usually been implicit, not explicit.
the real costs of the property damage, injuries, and fatalities which can result.
The common myth among designers is that if current "standards" are met, then the road is
Nonetheless, such analysis which provides an
safe. The reality is that road design "standards"
explicit framework for considering design trade-
are often no more than a limit : one should not
offs is a much more desirable approach to road-
provide less than the standard stipulates but,
side safety design than meeting arbitrary "stan-
within limits, to provide more is often better.
dards" whose underpinnings may or may not be
Furthermore, just meeting the standard does not
appropriate to a given situation. Figure 8.1 illus-
mean that an appropriate amount of safety has
trates an algorithm for conducting a roadside
been provided. 2.
the outcome of crashes in terms of
safety analysis.
The need to recognize that the design
of the roadside environment is a highly complex and probabilistic process. There are many lev-
The process is generally based on two funda-
els of interaction between different roadside
mental models.
•
design components, between roadside ele-
Predictive models that provide a way of
8-5 Chapter 8: Roadside Safety
Geometric Design Guide
1.
degrees of uncertainty with respect to
•
estimating collision frequencies and
cally investigates the roadside safety of a partic-
severities under a wide variety of condi-
ular project, has a proven potential to improve
tions; and
the safety of both proposed and existing facili-
Cost-effectiveness models that provide
ties.
a way of quantifying the life-cycle costs (and benefits) associated with any given
Road safety audits, especially during the design
set of safety measures.
stage, create the opportunity to eliminate, as far Predictive models have been developed and
as possible, road safety problems in the provi-
deployed by a number of agencies in North
sion of new road projects. They should be seen,
America.
however, as part of the broader scope of the phi-
Although
the
latest AASHTO
Roadside Design Guide probably represents the
losophy of roadside hazard management.
most current and widely accepted effort in this regard, designers should be aware that the state
A road safety audit is a formal examination of
of the art in this area is continually developing
any road project which interacts with road users,
and should be monitored regularly for new mod-
in which a qualified and independent examiner
els and techniques which may have application
reports on the projects accident potential and
to their design challenges.
safety performance. The audit may be conducted at the project's:
The techniques of cost-effectiveness analysis
• • • • •
are well established and are applied for a variety of purposes in transportation and highway design agencies.
A number of alternative
approaches are available but, most commonly,
Feasibility stage; Draft design stage; Detailed design stage; Pre-opening stage; and On existing roads.
the tools used by transportation agencies are built on life-cycle costing models and use pres-
The earlier a road is audited within the design
ent worth or annualised cost techniques as their
and development process, the better.
Geometric Design Guide
underlying analysis methodology.
All these
approaches are built on fundamental assump-
The subject is dealt with more fully in Chapter
tions regarding parameters such as discount
2.5 of this document, and particularly in Volume
rates and unit crash costs. In order to enforce
6 of the South African Road Safety Manual.
consistent and comparable results across the
8.2
road authority, these basic assumptions are
ROADSIDE HAZARDS AND CLEAR ZONE CONCEPT
usually set as a matter of policy and represent a "given" for designers to use in their analyses.
8.2.1 8.1.6
Overview
Road safety audits Research has shown that in 50 per cent of all
First developed in the United Kingdom, Australia
run-off-the-road accidents the vehicle leaves the
and New Zealand, this process, which specifi-
road in a skidding manner. Roadside hazards 8-6
Chapter 8: Roadside Safety
•
can significantly increase the severity of crashes and it is necessary to manage roadside haz-
signage;
•
ards in such a manner as to decrease the severity of these crashes.
Drainage structures such as culverts, drains, drop inlets;
Figure 8.1 illustrates this
• • • •
process. Common existing roadside hazards include:
•
Supports and poles (for lighting, utilities,
Trees;
Bridge abutments/piers; Side slopes such as embankments; Ends of traffic barriers, bridge railings; Incorrectly positioned traffic barriers, i.e.
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