A Guide to the Geometric Design of Rural Roads ISBN: 0 85588 655 2 AP-G1/03
AUSTROADS ROAD DESIGN SERIES AUSTROADS
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Rural Road Design A Guide to the Geometric Design of Rural Roads
Rural Road Design
Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
Rural Road Design
Rural Road Design: A Guide to the Geometric Design of Rural Roads © Austroads Inc 2003 NAASRA Guides: First published 1955 Second Edition 1961 Third Edition 1967 Reprinted 1967 Reprinted 1968 Fourth Edition 1970 Fifth Edition 1973 Sixth Edition 1980
Austroads Guides Seventh Edition 1989 Reprinted 1991 Reprinted 1993 Reprinted 1997 Reprinted 1999 Eighth Edition 2003
This work is copyright. Apart from any use permitted under the Copyright Act 1968, no part may be reproduced by any process without the written permission of Austroads. National Library of Australia Cataloguing-in-publication data: Rural Road Design: A Guide to the Geometric Design of Rural Roads ISBN 0 85588 606 4 Austroads Project No. T&E.D.C.019 Austroads Publication No. AP-G1/03 Standards Australia and Standards New Zealand Handbook No. HB152:2002 Project Manager John Cunningham, VicRoads
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Prepared by Arup Group
Published by Austroads Incorporated Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 E-Mail:
[email protected] Website www.austroads.com.au Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of the information herein. Readers should rely on their own skill and judgement to apply information to particular issues. Design
ii
Kirk Palmer Design, Sydney
RURAL ROAD DESIGN
Rural Road Design
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A Guide to the Geometric Design of Rural Roads
SYDNEY 2002
RURAL ROAD DESIGN
iii
A U S T R O A D S I N C O R P O R AT E D
Austroads is the association of Australian and New Zealand road transport and traffic authorities whose purpose is to contribute to the achievement of improved Australian and New Zealand transport related outcomes by:
● undertaking performance assessment and development of
● developing and promoting best practice for the safe and
Within this ambit, Austroads aims to provide strategic direction for the integrated development, management and operation of the Australian and New Zealand road system — through the promotion of national uniformity and harmony, elimination of unnecessary duplication, and the identification and application of world best practice.
effective management and use of the road system ● providing professional support and advice to member organisations and national and international bodies ● acting as a common vehicle for national and international action ● fulfilling the role of the Australian Transport Council’s Road Modal Group
Australian and New Zealand standards ● developing and managing the National Strategic Research
Program for roads and their use.
AUSTROADS MEMBERSHIP
Austroads membership comprises the six State and two Territory road transport and traffic authorities and the Commonwealth Department of Transport and Regional Services in Australia, the Australian Local Government Association and Transit New Zealand. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations:
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● ● ● ● ●
Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Main Roads Queensland Main Roads Western Australia Transport South Australia
● Department of Infrastructure, Energy and Resources
Tasmania ● Department of Infrastructure, Planning and Environment
Northern Territory ● Department of Urban Services Australian Capital Territory ● Commonwealth Department of Transport and
Regional Services ● Australian Local Government Association ● Transit New Zealand
The success of Austroads is derived from the synergies of interest and participation of member organisations and others in the road industry.
HANDBOOK ENDORSEMENT
In December 1993 Austroads and Standards Australia signed a Memorandum of Understanding regarding the development of Standards and related documents primarily for the development and management of the Australian road system. Standards Australia's support for this handbook reflects the cooperative arrangement between the two organisations to ensure there is a coordinated approach in this area.
HB 152:2002
iv
RURAL ROAD DESIGN
In August 1995 Austroads, Transit New Zealand and Standards New Zealand signed an agreement regarding the development of Standards and related documents for endorsement of the appropriate Austroads publications as SNZ handbooks. Standards New Zealand and Transit New Zealand's support for this handbook reflects the cooperative arrangement with Austroads to ensure that there is a coordinated approach in this area.
FO R E W O R D This guide represents the combined experience and international best practices of Austroads member agencies and industry experts in the area of geometric design of rural roads. The Guide has been prepared as the common design tool for Australia and New Zealand. For a more detailed explanation of specific matters, which may vary from place to place, designers should check with the relevant road authority. It has been the aim of the Consultant and the Reference group to validate all tables, figures and graphs included in the Guide. The validation took the form of developed formulae, laboratory test results, field observations or references. In some cases the designer has been provided with a range of desirable and absolute values. A design can be produced which may take into account the design topography, the safety of the occupants and the design parameters. Care should be taken to ensure the combined use of absolute values does not create an inappropriate design. Each circumstance should be individually evaluated based on local conditions by experienced personnel. This document does not cover the geometric design of unsealed roads. The designer is directed to the ARRB document “Unsealed Roads Manual – Guidelines to Good Practice, 1993”. The document referred to will provide the practical and basic aspects for the maintenance design and construction of unsealed roads.
AUSTROADS REFERENCE GROUP
The Austroads Reference group for the guide:
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Members Project Manager Technical Editor
John Cunningham, Manager VicRoads Design, Victoria Dennis Maxwell, VicRoads, Victoria
Michael Brauer/Peter Ellis John Byrden Dennis Davis Geoff Clarke Tony Gill Geoff Glynn Rob Grove Arthur Hall Fritz Nabholtz Graeme Nichols Richard Saunders
Roads and Traffic Authority, New South Wales VicRoads, Victoria Transit New Zealand Commonwealth Department of Transport and Regional Services Department of Urban Services, Australian Capital Territory Municipal Association of Victoria Main Roads, Western Australia Department of Main Roads, Queensland Department of Infrastructure, Planning and Environment, Northern Territory Department of Infrastructure, Energy and Resources, Tasmania Department of Transport South Australia
Project Research and Writer
ARUP Group
RURAL ROAD DESIGN
v
vi
RURAL ROAD DESIGN
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P R E FA C E
This is the eighth edition of the Geometric Design of Rural Roads. The guide was last revised in 1989.
This revision of Rural Road Design: Guide to the Geometric Design of Rural Roads follows the 2002 release of Urban Road Design: Guide to the Geometric Design of Major Urban Roads.
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(Text to be added/revised)
RURAL ROAD DESIGN
vii
CONTENTS
TOPIC
PAGE NO.
FOREWORD
DESIGN INPUTS
13
v
7.
SPEED, USED FOR GEOMETRIC DESIGN
13
PREFACE
vii
7.1
Introduction
13
GLOSSARY OF TERMS
xii
7.2
Explanation of Terminology
13
7.2.1
Vehicle Speed on Roads
13
7.2.2
Operating Speed
14
7.2.3
Operating Speed of Trucks
14
7.2.4
Section Operating Speed
14
7.2.5
Design Value
14
PART 1
INTRODUCTION
1
1.
A BALANCED APPROACH
1
1.1
General
1
1.2
Design Standards
1
1.3
7.3
Estimating Operating Speeds on Rural Roads
14
Speed Concept
1
7.3.1
General
14
1.3.1
General
1
7.3.1.1
Driver Behaviour
14
1.3.2
High Speed Roads
2
7.3.1.2
Road Characteristics
14
1.3.3
Intermediate Speed Roads
2
7.3.1.3
Vehicle Characteristics
14
1.3.4
Low Speed Roads
3
7.3.2
Operating Speed Estimation Model
14
1.3.5
85th Percentile Speed
3
7.3.3
Acceleration On Straights Graph
17
2.
ROAD FUNCTIONAL CLASSES
3
7.3.4
Deceleration On Curves Graph
17
3.
DESIGN APPROACH
4
7.3.5
Section Operating Speeds
17
3.1
General
4
7.3.5.1
3.2
The Driver’s View
4
Length Of Road to be included in The Study
17
3.3
Co-ordination of Horizontal and Vertical Alignment 4
7.3.5.2
Identification of Sections
19
3.3.1
General
4
7.3.6
Estimating Speed on a Section of Road
21
3.3.2
Curvilinear Design
5
7.3.6.2
Step 2 – Estimate Speed at Point C
21
3.3.3
Combined Horizontal and Vertical Alignment
7.3.6.3
Step 3 – Estimate Speed at Point D
21
5
7.3.6.4
Step 4 – Estimate Speed at Point E
21
7.3.6.5
Step 5 – Estimate of Speed at Point F
21
7.3.6.6
Step 6 – Estimate of Speed at Point G
21
7.3.6.7
Step 7 – Estimate of Speed at Point H and I
21
PART 2
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PART 3
FUNDAMENTAL DESIGN CONSIDERATIONS
8
4.
TRAFFIC VOLUME & TRAFFIC COMPOSITION
8
5.
DESIGN VEHICLE
8
7.3.7
Effects Of Grades
21
Effect of Cross-Section
23
6.
ENVIRONMENTAL CONSIDERATIONS
9
7.3.8
6.1
Traffic Related Intrusion
9
7.3.9
Effect of Pavement Condition
23
7.3.10
Use of Operating Speed in the Design of Rural Roads
23
6.2
6.3
6.1.1
Visuals
9
6.1.2
Noise
9
6.1.3
Vibration
11
6.1.4
Air Pollution
11
6.1.5
Erosion
11
6.1.6
Environmentally Sensitive Areas
11
6.1.7
Clearing
12
Environmental Related Intrusion
12
6.2.1
Snow and Ice
12
6.2.2
Floods
12
6.2.3
High Winds
12
6.2.4
Animals and Birds
12
References
viii
RURAL ROAD DESIGN
12
7.4
Operating Speed of Trucks
24
7.5
Use Of Truck Operating Speeds
24
8.
SIGHT DISTANCES
24
8.1
General
24
8.2
Sight Distance Parameters
25
8.3
8.2.1
Object Height
25
8.2.2
Driver Eye Height
25
8.2.3
Driver Reaction Time
26
8.2.4
Ageing of Drivers
26
Stopping Sight Distance (SSD)
26
8.3.1
Derivation
26
8.3.2
Longitudinal Friction Factor
27
8.3.3 8.3.4 8.4
8.5
27
9.11.1
Truck to Road Object Stopping Sight Distance
Benching for Visibility on Horizontal Curves
51
27
9.11.2
Other Restrictions to Visibility
51
9.11 Sight Distance on Horizontal Curves
51
Overtaking Sight Distance
30
8.4.1
General
30
9.12.1
Introduction
52
8.4.2
Overtaking Model
30
9.12.2
Theoretical Considerations
52
8.4.3
Determination of Overtaking Provision
30
9.12.3
Advantages of Curvilinear Alignment
8.4.4
Determination of Percentage of Road Providing Overtaking
31
Manoeuvre Sight Distance
33
8.5.1
33
Derivation
8.6
Headlight Sight Distance
33
8.7
Horizontal Curve Perception Distance
34
PART 4
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Car to Road Object Stopping Sight Distance
GEOMETRIC DESIGN GUIDELINES
9.12 Curvilinear Alignment Design in Flat Terrain
HORIZONTAL ALIGNMENT
35
9.1
General
35
9.2
Movement on a Circular Path
35
9.3
Horizontal Curves
35
9.3.1
Types of Horizontal Curves
35
9.3.1.1
Reverse Curves
35
9.3.1.2
Compound Curves
35
9.3.1.3
Broken Back Curves
35
9.3.1.4
Transition Curves
35
52
9.13 Bridge Considerations
53
10.
54
VERTICAL ALIGNMENT
10.1 Introduction
54
10.2 Grades
35
9.
52
54
10.2.1
General
54
10.2.2
Vehicle Operation on Grades
54
10.2.3
Maximum Grades
55
10.2.4
Length of Steep Grades
55
10.2.5
Steep Grade Considerations
55
10.2.6
Minimum Grades
56
10.3 Vertical Curves
9.4
Side Friction Factor
36
9.5
Minimum Radii Values For Horizontal Curves
37
9.5.1
Minimum Radius Values
37
9.5.2
On Steep Down Grades
38
11.
56
10.3.1
General
10.3.2
Forms and Types of Curve
56 56
10.3.3
Crest Vertical Curves
56
10.3.3.1 Appearance
56
10.3.3.2 Sight Distance Criteria (Crest)
57
10.3.4
57
Sag Vertical Curves
10.3.4.1 Appearance and Comfort
57
10.3.4.2 Sight Distance Criteria (Sag)
58
10.3.5
Reverse/Compound/Broken Back Vertical Curves
58
CROSS SECTION
60
9.6
Horizontal Alignment Design Procedure
38
11.1 General
60
9.7
Superelevation
39
11.2 Traffic Lane Width
60
9.7.1
Maximum Values of Superelevation
42
11.3 Traveled Way
61
9.7.2
Minimum Values of Superelevation
42
11.3.1
Single Carriageways
61
9.7.3
Application of Superelevation
42
11.3.2
Divided Carriageways
62
9.7.4
Length of Superelevation Development
42
11.3.2.1 Independent Design of Carriageways
63
9.7.4.1
Rate of Rotation
43
11.3.2.2 Superelevation Issues
63
9.7.4.2
Relative Grade
43
9.7.4.3
Design Superelevation Development Lengths
11.3.2.3 Transitions Between Divided and Undivided Carriageways
63
44
11.4 Pavement Crossfall and its Considerations
63
9.7.5
Positioning Of Superelevation Runoff
44
11.5 Shoulder
65
9.7.5.1
Without Transitions
44
11.5.1
Function
65
9.7.5.2
With Transitions
46
11.5.2
Width
65
9.7.6
Superelevation on Bridges
48
11.5.3
Shoulder Sealing
66
11.5.4
Crossfalls
9.8
Curves With Adverse Crossfall
48
9.9
Minimum Horizontal Curve Length
48
11.6 Verge
67
9.10 Pavement Widening on Horizontal Curves
48
11.7 Batters
67
RURAL ROAD DESIGN
67
ix
11.7.1
Benches
69
13.7.4.2 Escape Exits
89
11.7.2
Batter Rounding
69
13.7.4.3 Spacing
89
11.8 Medians
69
13.7.4.4 Summary of Design Considerations
90
11.9 Roadside Drains
72
13.7.5
90
11.9.1
Table Drains
11.9.2
Catch Drains
72
13.8.1
Starting and Termination Points
90
11.9.3
Median Drains
72
13.8.2
Tapers
91
11.10 Noise Barriers
72
13.8.3
Cross Section
91
11.11 Right of Way
72
13.8.3.1 Pavement Width
11.12 Widths of Bridges
72
13.8.3.2 Shoulder Width
91
13.8.3.3 Crossfall
91
PART 5
OTHER DESIGN CONSIDERATIONS
13.8 Geometry of Auxiliary Lanes
75
90
91
13.8.3.4 Lane Configurations
91
13.8.4
92
Line marking and Signing
75
13.8.4.1 Signs
92
12.1 Financial Level
75
13.8.4.2 Linemarking
92
12.2 Safety
75
14.
VEHICLE STOPPING AREAS
92
12.3 Energy
75
14.1 General
92
12.4 Stage Construction
75
14.2 Service Facilities
92
12.
PRINCIPAL FACTORS
75
14.2.1
75
14.2.1.1 Major Rest Areas
93
13.2 Types of Auxiliary Lanes
75
14.2.1.2 Basic Rest Areas
93
13.3 Speed Change Lanes
76
14.2.1.3 Other Areas
94
13.
AUXILIARY LANES
13.1 General
Rest Areas
92
13.3.1
Acceleration Lanes
76
14.2.2
Location of Vehicle Stopping Areas
95
13.3.2
Deceleration Lanes
76
14.2.3
Heavy Vehicle Considerations
95
13.4 Overtaking Lanes/Climbing Lanes
76
15.
COMMUNITY CONSULTATION
96
76
16.
DRAINAGE
96
13.4.1.1 Overtaking Demand
76
16.1 General
13.4.1.2 Overtaking Opportunities
76
16.2 Flood Estimation
96
13.4.1.3 Warrants
79
16.3 Rational Method
97
13.4.1.4 Length
79
16.4 Design Considerations
98
13.4.1.5 Location
80
16.5 Water Quality
99
13.4.1.6 Spacing
80
17.
13.4.1
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72
Brake Check and Brake Rest Areas
Overtaking Lanes
13.4.1.7 Improvement Strategy For Overtaking Lanes
81
13.4.2
81
Climbing Lanes
96
ROADSIDE SAFETY
100
17.1 Safety Objectives
100
17.2 On-Road Safety
100
17.2.1
Intersections
100
Mid Block
101
13.4.2.1 General
81
17.2.2
13.4.2.2 Warrants
81
17.3 Recovery Area
101
13.4.2.3 Length
83
17.3.1
Clear Zone
101
13.5 Slow Vehicle Turnouts
83
17.3.2
Existing Hazards Within a Clear Zone
102
13.5.1
Partial Climbing Lanes
83
17.4 Safety Barriers
105
13.5.2
Passing Bays
83
17.5 Landscaping
108
13.6 Descending Lanes
85
17.6 Lighting
108
13.7 Runaway Vehicle Facilities
85
17.7 Pedestrians and Cyclists
108
13.7.1
General
85
17.8 Temporary Works During Construction
108
13.7.2
Types of Escape Ramps
86
17.9 Road Safety Auditing
108
13.7.2.1 Sand Pile
86
18.
109
13.7.2.2 Descending Grade
86
18.1 Horizontal Alignment
109
13.7.2.3 Horizontal Grade
86
18.2 Vertical Alignment
109
13.7.2.4 Ascending Grade
86
18.2.1
Road Grading
109
13.7.3
Location of Runaway Vehicle Facilities
86
18.2.2
Cross Section
112
13.7.4
Arrester Beds and Escape Exits
86
13.7.4.1 Arrester Beds
x
RURAL ROAD DESIGN
87
19.
RAILWAY LEVEL CROSSINGS
COMPUTER SOFTWARE FOR ROAD DESIGN
112
REFERENCES
113
APPENDICES
117
Appendix A – Characteristics of the Euler Spiral (Clothoid)
117
Appendix B – Vertical Curve Formulae
119
Appendix C – Derivation of Sight Distance Requirements at Railway Level Crossings 121 121
2. Case 1: Sight Distance Required for Give Way Control
121
3. Case 1(i): Decelerate and Safely Stop at the Stop or Holding Line
122
4. Case 1(ii): Proceed and Clear the Crossingwith an Adequate Safety Margin
122
5. Case 2: Sight Distance Required for Stop Sign Control
123
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1. General
RURAL ROAD DESIGN
xi
G LO S S A R Y O F T E R M S A
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B
xii
AADT
Annual Average Daily Traffic is calculated by counting the number of vehicles passing a roadside observation point in a year and dividing this number by 365.
Abutment
An end support of a bridge or similar structure.
Acceleration Lane
An auxiliary lane used to allow vehicles to increase speed without interfering with the main traffic stream. They are often used on the departure side of intersections.
Access
The driveway by which vehicles and/or pedestrians enter and/or leave property adjacent to a road.
Adverse Crossfall
A slope on a curved pavement that generates forces detracting from the ability of a vehicle to maintain a circular path.
Alignment
The geometric form of the centreline (or other reference line) of a carriageway in both the horizontal and vertical directions.
Alignment Co-ordination (coordinated alignment)
A road design technique in which various rules are applied to ensure that the combination of horizontal and vertical alignment is both safe and aesthetically pleasing.
Aquaplaning
Full dynamic aquaplaning occurs when a tyre is completely separated from the road surface by a film of water.
Arrester Bed
An arrester bed is a safe and efficient facility used to deliberately decelerate and stop vehicles by transferring their kinetic energy through the displacement of aggregate in a gravel bed.
Arterial Road
A road that predominantly carries through traffic from one region to another, forming the principal avenue of communication for traffic movements.
Auxiliary Lane
The portion of the carriageway adjoining the through traffic lanes for speed change, or for other purposes supplementary to the through traffic movement.
Average Recurrence Interval (ARI)
The Average Recurrence interval (ARI) is the average interval of time during which an event will be equalled or exceeded once. It should be based on a lengthy period of records of the event. Statistically it is the inverse of the Average Exceedence Probability. The term replaces recurrence interval.
Batter
The uniform side slope of walls, banks, cuttings or embankments, expressed as a ratio of 1 vertical on x horizontal as distinct from grade.
Batter rounding
Curvature that is applied to improve the stability and appearance of the road at the intersection of the extension of the road crossfall and/or existing surface (hinge point), with the batter slope of an embankment or cutting.
Barrier
An obstruction placed to prevent vehicle access to a particular area.
Barrier Kerb
A kerb with a profile and height sufficient to prevent or discourage vehicles moving off the carriageway.
Bench
A ledge constructed in a batter or natural slope for the purpose of providing adequate horizontal sight distance, greater security against batter slippage or to assist with batter drainage.
Border
The area between the carriageway and the property line. It allows provision for services, footpaths, cycle path, shared paths, street trees and street furniture. Additional width will be required for bus bays or where major transmission services are to be provided in the verge. It includes the shoulder if provided.
Braking Distance
The distance required for the braking system of a vehicle to bring the vehicle to a stop from the operating speed.
Broken Back Curve
Two horizontal curves in the same direction separated by a short straight (a special case of the compound curve).
RURAL ROAD DESIGN
C
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D
Bunching
Grouping of vehicles travelling in the same direction with restricted speed caused by the slow moving head of the bunch and limited overtaking opportunities.
Bus Bay
An auxiliary lane of limited length at a bus stop or terminus usually indented into the shoulder or verge.
Carriageway
That portion of a road or bridge devoted particularly to the use of vehicles, inclusive of the shoulders and auxiliary lanes.
Catch drain
A surface channel constructed along the high side of a road or embankment, outside the batter to intercept surface water.
Catchment Area
The area that will contribute to the discharge of a stream after rainfall at the point under consideration.
Channelised Intersection
An intersection provided with channelised islands.
Centreline
The basic line that defines the axis or alignment of the centre of a road or other works.
Clear Zone
An area adjacent to the traffic lane that should be kept free from features potentially hazardous to errant vehicles.
Clearance
The space between a stationary and/or moving object.
Climbing Lane
A special case of an overtaking lane located on a rising grade.
Coefficient of Run-off
The ratio of the amount of water that runs off a catchment area to the amount that falls on the catchment.
Compound Curve
A curve consisting of two of more arcs of different radii curving in the same direction and having a common tangent point or being joined by a transition curve.
Crossfall
The slope, measured at right angles to the alignment, of the surface of any part of a carriageway.
Cross Section
The transverse elements of the longitudinal elements.
Crown
The highest point on the cross section of a carriageway with two-way crossfall.
Curvilinear Alignment
The alignment is a continuous curve with constant, gradual and smooth changes of direction.
Cycle Lane
A paved area adjacent to and flush with the traffic lane pavement, for the movement of cyclists. A lane designated for the exclusive use of cyclists.
Deceleration Lane
An auxiliary lane provided to allow vehicles to decrease speed.
Deck
The bridge floor directly carrying traffic loads.
Design Life
The period during which the quality of a structure (eg riding quality of a pavement) is expected to remain acceptable.
Design Speed
A speed fixed for the design and correlation of those geometric features of a carriageway that influence vehicle operation. Design speed should not be less than the operating speed.
Design Traffic
The predicted cumulative traffic at the design year, expressed in terms of vehicles.
Design Vehicle
A hypothetical road vehicle whose mass, dimensions and operating characteristics are used to determine geometric requirements.
Design Year
The predicted year in which the design traffic would be reached.
Discharge
The volumetric rate of water flow.
Divided Road (divided carriageway) A road with a separate carriageway for each direction of travel created by placing some physical obstruction, such as a median or barrier, between the opposing traffic directions.
RURAL ROAD DESIGN
xiii
G LO S S A R Y O F T E R M S ( co n t ’ d )
E F
G
H
I
Drainage
The natural or artificial means for the interception and removal of surface or subsurface water.
Ease
Section of rounding.
Footpath
A public way reserved for the movement of pedestrians and manually propelled vehicles. A separate facility for pedestrians remote from the road carriageway. It may also be the paved part of the “footpath” used by pedestrians.
Footway
Pedestrian facility on a bridge.
Formation
The surface of the finished earthworks, excluding cut or fill batters.
Frangible
Term is used to describe roadside furniture designed to collapse on impact. The severity of potential injuries to the occupants of an impacting vehicle is reduced, compared to those that could occur if the furniture was unyielding.
Freeway
A divided highway for through traffic with no access for traffic between interchanges and with grade separation at some interchanges.
Grade
The rate of longitudinal rise (or fall) of a carriageway with respect to the horizontal, expressed as a percentage.
Grade Separation
The separation of road, rail or other traffic so that crossing movements, which would otherwise conflict, are at different elevations.
Hinge Point
The point in the cross-section of a road at which the extended batter line would intersect the extended verge line.
Horizontal Alignment
The bringing together of the straights and curves in the plan view of a carriageway.
Horizontal Curve
A curve in the plan view of a carriageway.
Intensity of Rainfall
The rainfall in a unit of time.
Interchange
A grade separation of two or more roads with one or more interconnecting carriageways.
Intermediate Sight Distance
The ISD is equal to 2 x stopping distance for the operating speed.
Intersection
A place at which two or more roads meet.
Intersection Angle
1. The angle between two intersecting roads.
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2. The angles between the centrelines of two intersecting carriageways.
J, K
L
xiv
Intersection (at-grade)
An intersection where carriageways cross at a common level.
Intersection Leg
Any one of the carriageways radiating from and forming part of an intersection.
K Value
The length required for a 1% change of grade on a parabolic vertical curve.
Kerb
A raised border of rigid material formed at the edge of a carriageway.
Kerb and Channel
The kerb and channel combine to form an open drain to capture and discharge run off.
Kerb Clearances
A distance by which the kerb should be set back in order to maintain the maximum capacity of the traffic lane.
Lane (Traffic)
A portion of the carriageway allocated for the use of a single line of vehicles.
Lane Separator
A separator provided between lanes carrying traffic in the same direction to discourage or prevent lane changing, or to separate a portion of a speed change lane from through lanes.
Lateral Friction
The force which, when generated between the tyre and the road surface, assists a vehicle to maintain a circular path.
Level of Service (LOS)
A qualitative measure describing operational conditions within a traffic stream and their perception by motorists and passengers.
RURAL ROAD DESIGN
Limiting Curve Speed Standard
The curve speed at which f just equals f max, Vs.
Line of Sight
The direct line of uninterrupted view between a driver and an object of specified height above the carriageway in the lane of travel.
Longitudinal Friction Factor
The friction between vehicle tyres and the road pavement under locked wheel braking conditions, measured in the longitudinal direction.
Longitudinal Section
A vertical section, usually with an exaggerated vertical scale, showing the existing and design levels along a road design line, or another specified line.
Median
A strip of road, not normally intended for use by traffic, which separates carriageways for traffic in opposite directions.
Median Island
A short length of median serving a localised purpose in an otherwise undivided road.
Median Lane
The traffic lane nearest the median.
Median Opening
A gap in a median provided for crossing and turning traffic.
Minimum Turning Path
The path of a designated point on a vehicle making its sharpest turn.
Minimum Turning Radius
The radius of the minimum turning path of the outside of the outer front tyre of a vehicle.
Motorway
A divided highway for through traffic with no access for traffic between interchanges and with grade separation at some interchanges.
Multiple Combination Vehicles
The full range of truck, prime mover and semi trailers and road trains.
N
Normal Cross Section
The cross section of the carriageway where it is not affected by superelevation or widening.
O
Off-tracking
The radial offset between the path traced by the centre of the front axle and the centre of the effective rear axle.
One-way Road
A road or street on which all vehicular traffic travels in the same direction.
Operating Speed
The 85th percentile speed of cars at a time when traffic volumes are low and will allow a free choice of speed within the road alignment.
Overtaking
The manoeuvre in which a vehicle moves from a position behind to a position in front of another vehicle travelling in the same direction.
Overtaking Distance
The distance required for one vehicle to overtake another vehicle.
Overtaking Lane
An auxiliary lane provided to allow for slower vehicles to be overtaken. It is linemarked so that all traffic is initially directed into the left-hand lane, with the inner lane being used to overtake.
Overtaking Zone
A section of road on which at least 70 per cent of drivers will be prepared to carry out overtaking manoeuvres subject to availability of adequate gaps in the opposing direction.
Passing
The manoeuvre by which a vehicle moves from a position behind to in front of another vehicle, which is stationary or travelling at crawl speeds.
Passing Bay
A very short auxiliary lane (of the order of 100 m) that allows a slow vehicle to pull aside to allow a following vehicle to pass.
Pavement
That portion of a road designed for the support of, and to form the running surface for, vehicular traffic.
Perception Distance
The sight distance required accessing the curvature of horizontal curves on approach.
Property Line
The boundary between a road reserve and the adjacent land.
Rainfall Intensity
The rate of rainfall (mm/hr).
Rate of Rotation
The rate of rotation required achieving a suitable distance to uniformly rotate the
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M
P
Q, R
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G LO S S A R Y O F T E R M S ( co n t ’ d ) crossfall from normal to full superelevation. The usual value adopted is 0.025 rad/sec; 0.035 rad/sec is the maximum value. Reaction Distance
The distance travelled during the reaction time.
Reaction Time
The time between the driver’s reception of stimulus and taking appropriate action.
Re-alignment
An alteration to the control line of a road that may affect only its vertical alignment but, more usually, alters its horizontal alignment. A method of widening a road reservation. A section of road alignment consisting of two curves turning in opposite directions and having a common tangent point or being joined by a short length of tangent.
Residual Median
The remnant area of the median adjacent to right turn lanes.
Road Furniture
A general term covering all signs, streetlights and protective devices for the control, guidance and safety of traffic, and the convenience of road users.
Roadside Safety Barrier
A device erected parallel to the road to retain vehicles that are out of control.
Road (way)
A route trafficable by motor vehicles; in law, the public right-of-way between boundaries of adjoining property.
Roundabout
An intersection where all traffic travels in one direction around a central island.
Run-off
That part of the rainfall on a catchment which flows as surface discharge past a specified point.
Sag Curve
A concave vertical curve in the longitudinal profile of a road.
Section Operating Speed
The 85th percentile speed of cars traversing a section of road alignment.
Semi-Mountable Kerb
A kerb designed so that it can be driven across in emergency or on special occasions without damage to the vehicle.
Shared Path
A paved area particularly designed (with appropriate dimensions, alignment and signing) for the movement of cyclists and pedestrians.
Shoulder
The portion of formed carriageway that is adjacent to the traffic lane and flush with the surface of the pavement.
Sideways Friction Coefficient
The ratio of the resistance to side ways motion of the tyre of a vehicle (on a specified pavement) and the normal force on that wheel due to the vehicle mass.
Sight Distance
Approach Sight Distance (ASD) The distance required for a driver to perceive marking or hazards on the road surface approaching an intersection and to stop. Car Stopping Distance (SSD) The distance required for a car driver to perceive a hazard, react and brake to a stop. For design purposes, wet weather conditions and locked wheel braking are assumed. Entering Sight Distance (ESD) The sight distance required for minor road drivers to enter a major road via a left or right turn, such that traffic on the road is unimpeded Manoeuvre Sight Distance The distance required for an alert car driver to perceive an object on the road and to take evasive action. Minimum Gap Sight Distance (MGSD) “The minimum sight distance based on the gap necessary to perform a particular movement.” Overtaking Sight Distance The sight distance required for a driver to initiate and safely complete an overtaking manoeuvre. Railway Crossing Sight Triangle
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S
Reverse Curve
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The clear area required for a truck driver to perceive a train approaching an uncontrolled railway crossing and to stop the truck Safe Intersection Sight Distance (SISD) The distance required for a driver in a major road to observe a vehicle entering from a side road, and to stop before colliding with it. Sight Distance Through Underpass The distance required for a truck driver to see beneath a bridge located across the main road, to perceive any hazard on the road ahead, and to stop. Stopping Sight Distance The sight distance required by an average driver (car or truck depending on design requirements), travelling at a given speed, to react and stop before striking an object on the road. Truck Stopping Sight Distance The distance required for a truck driver to perceive a hazard, react and brake to a stop. For design purposes, the braking of an unladen vehicle in wet weather conditions without locking the wheels is assumed. Sight Triangle
The area of land between two intersecting roadways over which vehicles on both roadways are visible to each driver.
Skid Resistance
The frictional relationship between a pavement surface and vehicle tyres during braking or cornering manoeuvres. Normally measured on wet surfaces, it varies with the speed and the value of ‘slip’ adopted.
Slope
1. The inclination of a surface with respect to the horizontal, expressed as rise or fall in a certain longitudinal distance.
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2. An inclined surface.
T
Speed
85th Percentile Speed The speed at which 85 percent of car drivers will travel slower and 15 percent will travel faster. Operating Speed of Trucks The 85th percentile speed of trucks measured at a time when traffic volumes are low. Section Operating Speed The value at which vehicle speeds on a series of curves tend to stabilise, are related to the range of radii on the curves.
Speed-change Lane
A subdivision of auxiliary lanes, which cover those lanes used primarily for the acceleration or deceleration of vehicles. It is usual to refer to the lane by its actual purpose (eg. deceleration lane).
Sub-arterial Road
Road connecting arterial roads to areas of development, and carrying traffic directly from one part of a region to another.
Superelevation
A slope on a curved pavement selected so as to enhance forces assisting a vehicle to maintain a circular path.
Superelevation Development
The length over which the crossfalls on a carriageway are gradually changed from normal crossfall to full superelevation crossfall.
Superelevation Runoff
That part of superelevation development that goes from flat crossfall to full superelevation crossfall (on the outside of the curve, when there are segments rotating either side of the axis of rotation).
Swept Path
The area bounded by lines traced by the extremities of the bodywork of a vehicle while turning.
Swept Width
The radial distance between the innermost and outermost turning paths of a vehicle.
Table drain
The side drain of a road adjacent to the shoulder, having its invert lower than the pavement base and being part of the formation.
Tangent Runout
The length of roadway required to accomplish the change in crossfall from a
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normal crown section to a flat crossfall at the same rate as the superelevation runoff.
U, V
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Terrain
Topography of the land. Level Terrain Is that condition where road sight distance, as governed by both horizontal and vertical restrictions, are generally long or could be made to be so without construction difficulty or major expense. Undulating Terrain Is that condition where road sight distance is occasionally governed by both horizontal and vertical restrictions with some construction difficulty and major expense but with only minor speed reduction. Rolling Terrain Is that condition where the natural slopes consistently rise above and fall below the road grade and where occasional steep slopes offer some restriction to normal horizontal and vertical roadway alignment. The steeper grades cause trucks to reduce speed below those of passenger cars. Mountainous Terrain Is that condition where longitudinal and transverse changes in the elevation of the ground with respect to the road are abrupt and where benching and side hill excavation are frequently required to obtain acceptable horizontal and vertical alignment. Mountainous terrain causes some trucks to operate at crawl speeds.
Time of Concentration
The shortest time necessary for all points on a catchment area to contribute simultaneously to run-off at a specified point.
Traffic
A generic term covering all vehicles, people, and animals using a road.
Traffic Control Signal
A device that, by means of changing coloured lights, regulates the movement of traffic.
Traffic Island
A defined area, usually at an intersection, from which vehicular traffic is excluded. It is used to control vehicular movements and as a pedestrian refuge.
Transition
Transition length for increasing or decreasing the number of lanes.
Traffic Lane
A portion of the carriageway allocated for the use of a single line of vehicles.
Traffic Sign
A sign to regulate traffic and warn or guide drivers.
Transition Curve
A curve of varying radius to model the path of a vehicle entering or leaving a horizontal circular curve.
Transition Length for alignment
The distance within which the alignment is changed in approach from straight to a horizontal curve of constant radius.
Transition Length for crossfall
The distance required rotating the pavement crossfall from normal to that appropriate to the curve. Also called superelevation development length.
Transition Length for widening
The distance over which the pavement width is changed from normal to that appropriate to the curve.
Travelled way
That portion of a carriageway ordinarily assigned to moving traffic, and exclusive of shoulders and parking lanes.
Turning Lane
An auxiliary lane reserved for turning traffic.
Typical Cross Section
A cross section of a carriageway showing typical dimensional details, furniture locations and features of the pavement construction.
Verge
That portion of the formation not covered by the carriageway or footpath.
Vertical Alignment
The longitudinal profile along the design line of a road.
Vertical Curve
A curve (generally parabolic) in the longitudinal profile of a carriageway to provide for a change of grade at a specified vertical acceleration.
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1
PA R T
1.
I N T R O D U CT I O N to which a road project is built, results (due to the slightly increased cost) in the deferment of other projects to enable the higher cost project to be funded. Improved provision for future traffic results in greater deficiencies on the balance of the road system with respect to present traffic. The more constrained the financial situation, the more these tradeoffs become evident.
A BALANCED APPROACH
1.1 General Roads will continue to be an important part of our transport system for the foreseeable future by providing for the safe and operationally efficient movement of people and goods. A balanced approach towards road planning and design can improve road safety and public amenity, and reduce the effect of noise, vibration, pollution and visual intrusion on the areas through which a road passes. The objectives of new and existing road networks should be carefully considered to achieve the desired balance and must take into account the available resources to achieve them. In every situation designers will be faced with competing demands from different sections of the community as they endeavour to design safe, operationally efficient roads. The various chapters in this publication provide a guide to practitioners on the standards that can be achieved within social, environmental, economic and other constraints using best local and overseas practice.
1.2 Design Standards
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Geometric road design standards are used as an aid to achieving consistent and operationally effective road designs. Rapid expansion and improvement to road networks precipitated the need for standards to: ●
maintain a degree of uniformity, particularly across administrative boundaries;
●
enable satisfactory designs to be produced, even where there was not a high degree of expertise; and
●
ensure that road funds were not miss-spent, through inappropriate designs, or through inadequate provision for future traffic growth or for current operations.
Prior to the 6th edition of this guide, many of the standards adopted in Australia were based heavily on those used in the USA and other developed countries. However, with the 6th edition, standards that were more appropriate for Australia were promoted. There were two aspects to these new standards:
There are three distinct stages in the development of a country’s road system. The importance of geometric standards depends very much on the stage reached. ●
Stage 1 – Basic Network. The establishment of a basic network so that transport links exist where they are required. The roads must be trafficable. Geometric standards are relatively unimportant except as they affect matters like drainage and gradient;
●
Stage 2 – Increasing Capacity. Improving the road’s ability to carry increasing volumes of traffic. This includes structural strength, but geometric standards assume greater importance; and
●
Stage 3 – Quality of Service. Building operational safety, efficiency and convenience into the network, as embodied in a concept of ‘quality of service’. Alignment standards become important, and cross section standards need to be more generous to accommodate significant volumes of high-speed traffic.
The development of the Australian and New Zealand road network is a mixture of increasing the network capacity and providing for an improved quality of service. Parts of the more remote areas still have road development problems associated with the establishment of a basic network. Many of the imported geometric standards that were used prior to the 6th edition related to the quality of service that a road provides. Problems arose through their inappropriate application in areas where a basic network was still being developed. The main problem now for geometric standards is that there are many areas where the road system exhibits all three stages of development. In these areas motorists are more likely to be influenced by the geometry of the ‘Stage 2’ roads. Hence, they are likely to be more demanding of the standard of geometry on ‘Stage 1’ roads.
1.3 Speed Concept 1.3.1
●
●
Technical – relating to safety and efficiency of traffic operations and particularly to alignment design. Experience has shown that rigid adherence to the earlier standards did not always ensure a safe, operationally efficient road; and Costs of desirable road construction projects almost always exceed the total of funds that can be made available. In this situation, each upward increment in design standards
General
When assessing the major roles that a road should fulfil and the standard of this provision, engineering judgement will be required. Identified problems or concerns need to be carefully considered and a range of alternative solutions examined before deciding upon a particular course of action. Judgement of what is considered “acceptable” for the road in question will involve a balance between such issues as traffic capacity, the environment, speed, safety and road user comfort. It is
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1
important to determine which of the various demands should be given priority, taking into account function and operating conditions of the road and its relationship with other roads in the adjacent network. Use of the traditional design speed concept as a criterion for alignment consistency on rural roads was introduced in the USA in the 1930s in response to increasing numbers of accidents at horizontal curves. This concept was developed as a mechanism for designing rural road alignments permitting the majority of drivers to operate uniformly at their desired speed. However, as identified by researchers in various countries, the concept has not always produced safe and consistent alignments. Various speed studies in Australia, New Zealand and overseas have shown that on roads designed for speeds less than 100 km/h the 85th percentile driver exceeds the design speed by up to 20 km/h. The revised design procedure in Guide to the Geometric Design of Rural Roads (NAASRA, 1989) incorporated considerations of operating speeds to improve alignment consistency. The guide had four basic speed parameters: ● ● ● ●
1.3.2
High Speed Roads
These are roads with design speeds in excess of 100 km/h. On these high-speed roads operating speeds are not constrained by the geometry of the road but by a number of other factors, which include: ● ● ●
The degree of risk the drivers are prepared to accept; Speed limits and the level of policing of these limits; and Vehicle performance.
Roads with design speeds of 110 km/h and 130 km/h are likely to have similar operating speeds. McLean (Ref. 71) noted that drivers generally wish to travel at around 100 km/h to 110 km/h. On roads designed for lower speeds, drivers tend to “overdrive” the road. Conversely on roads designed for higher speeds, drivers adopt an operating speed of 100 km/h to 110 km/h. 1.3.2 High Speed Roads
desired speed; speed environment; design speed; and limiting curve speed standard.
There was some uncertainty in the application of the NAASRA (1989) design parameters because: ● ● ● ●
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●
different interpretations were given to the term speed environment; designers were reluctant accept the predicted speeds on some low radii; no clear instructions were available on the use of the design curves; results obtained by different designers were not consistent; and very long lengths of relatively straight road were required for vehicles to reach the speed environment.
In spite of these problems, the basic procedure provided appropriate outcomes. However, in order to make the procedures more transparent, there is a need for a more specific method for determining speeds on straight and horizontal curves. From observations of driver behavior in hilly terrain, it was noted that drivers initially reduce speed over the first few curves until they reach a speed that is the highest at which the driver feels comfortable. The driver then tends to maintain this speed unless confronted with a curve with a radius significantly below the general range of radii on the section of road. Conversely, the driver will not increase speed unless a straight (or near straight) is available and is >200 meters. On shorter straights drivers tend to maintain the speed attained on the preceding section of curves. This speed is called “section operating speed”. The research findings and accumulated design experience suggest that there are effectively three ranges of speed standard for roads, and that different design philosophies should be employed for each range. All have the fundamental objective of providing a road which accords with driver expectations.
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1.3.3
Intermediate Speed Roads
These are roads designed with minimum operating speeds of 80 km/h to 100 km/h. Operating speeds on these roads are generally constrained by the geometry. Drivers will, however, accelerate whenever the opportunity arises, such as on any straight or large radius curve. Curve radii on these roads are generally in excess of 160 m.
1.3.3 Intermediate Speed Roads
1.3.4 Low Speed Road
On roads with speed limits less than 100 km/h, the operating speed of vehicles will be determined by the geometric constraints of the road on the imposed speed limits and the corresponding operating speeds refer Section 7.2 and Figure 7.1.
1.3.5
85th Percentile Speed
The term “eighty fifth percentile speed” indicates that 85 percent of car drivers will travel at or below this speed and 15 percent will travel faster. In effect, this means that designs based on the 85th percentile speed will cater for the majority of drivers. For design purposes, the 15% of drivers who exceed this speed are considered to be aware of the increased risk they are taking and are expected to maintain a higher level of alertness, effectively reducing their reaction times.
2.
R O A D F U N CT I O N A L C L A S S E S
Roads fall into a hierarchy of functional classes ranging from major arterial to local access. Austroads has defined a system of functional classification for rural roads (see Table 2.1).
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Functional classes are not always clear-cut since almost all roads have some degree of local importance.
1.3.4
Low Speed Roads
These are roads having many curves with radii less than 150 m. Operating speeds on the curves vary from 50 km/h to 70 km/h. These roads are only used when difficult terrain and costs preclude the adoption of higher speeds. The alignments provided in these circumstances could be expected to produce a high degree of driver alertness, so those lower standards are both expected and acceptable. The most pragmatic approach to the design of individual elements in such constrained situations is to provide the best that appears practicable, and to check that it is within the absolute minimum standards for the predicted 85th percentile speed. Innovative, non-standard treatments will often be required when these standards cannot be met.
Rural roads of higher functional class generally cater for a higher (though normally still modest) proportion of longer length journeys, and it may be appropriate to select higher design standards for such roads so that the quality of service is more appropriate to the longer trip’s duration. However designers must be aware of placing too much importance on functional class alone where traffic volumes are low. Further discussion on functional classification of roads is given in Ref. 22.
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Table 2.1 Austroads Functional Rural Road Classification
The road, therefore, must be considered at all stages of design as a three-dimensional structure that should be safe, functional and economical but also aesthetically pleasing.
ARTERIAL ROADS 3.2 The Driver’s View Class 1 Those roads, which form the principal avenue for communications between major regions, including direct connections between capital cities.
Class 2 Those roads, not being Class 1, whose main function is to form the principal avenue of communication for movements between: ● A capital city and adjoining states and their capital cities; or ● A capital city and key towns; or ● Key towns.
Class 3
The driver sees a foreshortened and, thus, distorted view of the road, and unfavourable combinations of horizontal and vertical curves can result in apparent discontinuities in the alignment, even though the horizontal and vertical designs each comply separately with the provisions of their individual design requirements. Such combinations can mask from the driver a change in horizontal alignment or even a sag curve deep enough to conceal a significant hazard (the hidden dip problem). Only the consideration of the road as a three dimensional entity can reveal such deficiencies, and good design practice requires the elimination of all avoidable hazards even though some additional expense may be incurred. The removal of hazards is not, however, the only benefit, as the improved safety and performance potential is invariably accompanied by significantly enhanced amenity.
Those roads, not being Class 1 or 2, whose main function is to form an avenue of communication for movements: ● Between important centres and the Class 1 and Class 2 roads and/or key towns; or ● Between important centres; or ● Of an arterial nature within a town in a rural area.
Not only is the driver’s view constantly changing, but the duration of his view of successive elements of the road is also varying. Features situated in long, low sag curves remain in view for a considerable length of time whereas other features at or near an abrupt crest or on a tight curve are in view only fleetingly. It follows then that important features such as intersections are most favourably located on long sag curves.
LOCAL ROADS
Class 5
Visual cues to the driver from peripheral areas must be given adequate attention. While the designer views the whole road layout at once, and is aware of all changes in alignment, the driver sees much less at any one time. The driver’s inherently restricted view can be further limited at night, or in other times of poor visibility. The designer must, therefore, provide the driver with as many clues as possible as to what lies ahead, but must make sure that the roadside conditions do not convey messages which are ambiguous or misleading.
Those roads, which provide almost exclusively for one activity or function, which cannot be assigned to Classes 1 to 4.
3.3 Co-ordination of Horizontal and Vertical Alignment
Class 4 Those roads, not being Class 1, 2 or 3, whose main function is to provide access to abutting property (including property within a town in a rural area).
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3.3.1
3.
DESIGN APPROACH
3.1 General The subsequent sections are concentrated primarily on the physical attributes of good road design to satisfy the requirements of safety and performance. Whilst these needs are of prime importance, some compromise may be necessary in the need for convenience of access, amenity and economy. Considerations of amenity are those which concern the effect that a road and its traffic has upon the environmental and aesthetic senses of users and of those others who are affected by its construction and operation. The pleasing coordination of alignment and grading, the fitting of the road to the natural contours of the land surface, and the preservation or enhancement of the natural vegetation is all involved.
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General
It has been shown that the operation of a road is influenced partly by the nature of the terrain and partly by the horizontal alignment. It follows, therefore, that if the indications of these two factors are similar, the road will provide the best level of consistency in driver expectancy and thus safety. Further, a road having both horizontal and vertical curvature carefully designed to conform to the terrain will result in the desirable aesthetic quality of being in harmony with the landform. Perfect harmony of course is not always possible, and the designer must consider what matters are beyond his control and make full allowance for their influence on driver behaviour. From Section 7 it will be clear that, while it is possible to build a road with a high operating speed in adverse terrain, it is unlikely that there will ever be sufficient curvature in flat or gently rolling terrain to produce a lowspeed environment. Operating speeds will be high in the latter cases because of the terrain. The grading needs to
ensure adequate sight distances to potential hazards on the road and, where such sections merge into more constrained alignment sections, such transition must be accomplished gradually rather than suddenly. In flat open terrain, long straight road sections are common, but generally there is advantage in avoiding excessive lengths of straight road. A gentle curvilinear design, as discussed in Section 9, always helps to keep the operating conditions ‘under control’ and at the same time, affords scope for far more sympathetic fitting of the road to terrain. The increased flexibility of this approach enables more pleasing designs to be produced at no extra cost; economies in earthworks can often be achieved by fitting the road more closely to the terrain. In addition, safety is enhanced by making the driver more aware of his speed, by allowing him to make better assessments of the distances and speeds of other vehicles, by reducing headlight or sun glare in appropriate circumstances and by reducing boredom and fatigue. Even in flat country curvilinear designs can be used. Radii must be very large, so that all of the benefits of a curving alignment are achieved. Estimation of speed of oncoming vehicles is not significantly improved over a straight alignment when radius exceeds about 5,000m to 10,000m. It is the opinion of experienced designers, however, that sufficient benefits do still remain to make the exercise worthwhile.
3.3.2
Curvilinear Design
Curvilinear design is most readily applicable to divided roads with their less stringent sight distance requirements but the principles are just as relevant to single carriageway roads provided care is taken to ensure adequate overtaking opportunities are available.
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Very large radius curves can provide overtaking opportunities and, as mentioned above, retain at least some of the benefits of curvilinear alignment. If the topography is such that ‘natural’ curvature precludes the provision of overtaking sight distance, then the provision of overtaking zones may produce an economical as well as an aesthetic solution. Figure 3.1 illustrates basic examples of the method and benefits of proper fitting of the road to the terrain and of proper coordination of horizontal and vertical elements. In addition, there are some examples of poor design form, with indications of appropriate remedial measures. These latter examples are typical of the results likely if the designer does not consider the vertical and horizontal views simultaneously; particularly if a ‘minimum’ vertical standard is superimposed on a relatively unrestricted horizontal regime.
visualising the schemes in these dimensions using whatever aids are available.
3.3.3
Combined Horizontal and Vertical Alignment
The most pleasing three-dimensional result is achieved if the horizontal and vertical curvature is kept in phase, as this relates most closely to naturally occurring forms. Where possible, the vertical curves should be contained within the horizontal curves. This enhances the appearance in sag curves by reducing the three-dimensional rate of change of direction, and improves the safety of crest curves by indicating the direction of curvature before the road disappears over the crest. Thus, the best appearance requires the scale of the vertical and horizontal movements to be comparable: a small movement in one direction should not be combined with a large movement in the other. Drainage structures in sag curves that are combined with horizontal curves require careful design if a disjointed or kinked appearance is to be avoided. Culverts should introduce little aesthetic difficulty if they are contained within embankments and are made sufficiently long to accommodate full road formation widths. Bridges built on combined horizontal and vertical curvature can present considerable aesthetic problems, especially if reduced formation widths are used. Particular care should be devoted to the design of the bridge kerbs and railings, as well as to the location and transitioning of approach guard fences. In general, the more generous the curvature, the more pleasing and safer will be the result. Horizontal curves combined with crests have less influence on the appearance of a road than those combined with sags. Nevertheless, the effect on safety can be much greater, as the crest can obscure the direction and severity of the horizontal curve. Minimum radius horizontal curves, therefore, should not be combined with crest vertical curves.
3.3.3 Combined Horizontal and Vertical Alignment
The diagrams are not intended to be comprehensive, but serve merely to demonstrate the general concepts that should (or should not) be followed. In all cases, recognition of the deficiency is sufficient to indicate the appropriate remedy, and the recognition of the deficiency is dependent only on the designer taking a three-dimensional, rather than a twodimensional view of the problem. Specific rules are not appropriate to good design, as each particular project has its own peculiar problems and constraints. However, some benefit can be obtained from a consideration of what combinations of horizontal and vertical elements are most likely to produce satisfactory results, and
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Figure 3.1(a): Coordination of Vertical and Horizontal Alignments
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Figure 3.1(b): Coordination of Vertical and Horizontal Alignments
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2
PA R T
4.
F U N D A M E N TA L D E S I G N C O N S I D E R AT I O N S
TRAFFIC VOLUME & TRAFFIC COMPOSITION
Guide to Traffic Engineering Practice Part 2 (ref 15) provides details of highway capacity analysis. The Highway Capacity Manual Transportation Research Board, HCM 2000, provides a collection of state-of-the-art techniques for estimating the capacity and determining the level of service for transportation facilities, including intersections and roadways as well as facilities for transit, bicycles and pedestrians (Ref 93). Whilst a summary of key principles and issues is provided here, these references should be consulted for more detailed consideration of capacity issues. Level of Service (LOS) is defined as a qualitative measure describing operational conditions within a traffic stream as perceived by drivers and/or passengers. A level of service definition generally describes these conditions in terms of factors such as speed and travel time, freedom to manoeuvre, traffic interruptions, comfort and convenience and safety.
Designers need to consider future traffic demands for a road section to determine the required cross sectional configuration. A design period of 20 years is to be considered in determining capacity requirements. Consideration should be given to the staged construction or widening of roads over this period. Design requirements for rural roads are typically assessed by reference to forecasts of AADT. Design hour volumes may be derived by consideration of the flow pattern across hours of the year. A 30th highest hourly volume is often adopted as a design volume. In areas of high peak demands, such as recreational routes, special consideration may be required. Research (Ref. 57) has suggested an alternative specification of the design volume according to the percentage of traffic for which a selected level of service is to be exceeded (eg. provide LOS D or better for 85% of all traffic). In addition to capacity considerations traffic volume and composition is a key input to the structural design of pavement, culverts and bridges. Truck volumes are a critical input.
5. Level of Service A provides the best traffic conditions with no restrictions on desired travel speed or overtaking. Level of Service B to D describes progressively worse traffic conditions. Level of Service E occurs when traffic conditions are at or close to capacity, and there is virtually no freedom to select desired speeds or to manoeuvre within the traffic stream. Flow is unstable and minor disturbances within the traffic stream will cause breakdown of flow.
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The service flow rate is defined as the maximum hourly rate at which vehicles can reasonably be expected to traverse a uniform section of a lane or roadway during a given time period under the prevailing traffic and control conditions while maintaining a designated level of service. The service flow rate for LOS E therefore is taken as the capacity of a lane or roadway. Capacity of rural road sections is influenced by the following key characteristics: ● ● ● ● ● ● ●
Traffic volume; Road configuration – such as two lane two way, multi-lane divided or undivided; Operating speed; Terrain; Lane and shoulder width; Heavy vehicle (trucks and buses) proportions; and Grades.
In the case of two lane two way roads the following additional factors are important: ● ●
Directional distribution of traffic flow; and Overtaking opportunities - sight distance, overtaking lanes, climbing lanes or slow vehicle turnout lanes.
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RURAL ROAD DESIGN
DESIGN VEHICLE
The physical and operating characteristics of vehicles using major rural roads are controls in geometric design. The design vehicle is a hypothetical vehicle whose dimensions and operating characteristics are used to establish lane width, intersection layout and road geometry. This chapter discusses the design vehicle for mid-block sections. Historically, three general classes of vehicles have been selected for design purposes, namely: ● ● ●
Design prime mover and semi-trailer (19.0 m); Design single unit truck/bus (12.5 m); and Design car (5.0 m).
These three vehicle types are the basic design vehicles for most road and traffic design situations. The 19.0m prime mover and semi-trailer is to be used as the design vehicle for cross section elements and the car as the design vehicle for horizontal and vertical geometry. The geometric design should be checked for the largest design vehicle expected to use the road, as outlined below. The dimensions of the design vehicles are provided in Design Vehicles and Turning Path Templates (Ref. 36). Additional considerations for motorcycles are outlined in Guide to Traffic Engineering Practice, Part 15–Motorcycle Safety (Ref. 28). A functional layout based on the characteristics of a design vehicle should represent an economical level of design that caters safely and comfortably for at least 85% of vehicles
operating in accordance with normal traffic regulations. Larger vehicles (33 metre B-triple and 30 metre super B-double) and those operating under restricted access conditions may also be catered for, but this will usually involve encroachment into other traffic lanes. This may cause some inconvenience to other road users, but may be acceptable where there is a low frequency of occurrence together with the effect of special conditions associated with the permit. Where the route is designated for the use of special vehicles that fall outside the three general classes (other freight efficient vehicles, over-length buses, type 1 or 2 road trains), or where regular use of the route by these vehicles could reasonably be expected (access to industrial areas, bus routes), the design should satisfy the needs of such vehicles. The operation of these vehicles should not be compromised by having to encroach into other traffic lanes. The geometric design should be checked for B-doubles and special vehicles where the need is demonstrated and at the areas where problems are most likely to occur. Most arterial rural roads are likely to have some B-double operation even if they are not specific B-double routes. Table 5.1 describes the provisions that need to be made for trucks. These can also be used for special vehicles. Design guidelines for the various geometric issues in the table are discussed in subsequent sections.
Visual
The visual intrusion of a road project can have a dramatic effect on abutting individual residents and communities. The visual amenity of a project can be greatly enhanced by the design of creative and functional landscaping. The expense of visual landscaping can be shared by the other functions that the landscaping will aid, such as soil erosion control, replacement vegetation and amenity.
6.1.2
Noise
The potential for noise disturbance to individuals and communities resulting from traffic use of road networks is high. Concern regarding the adverse effects of noise in the environment has resulted in strict noise regulations being developed and enforced by relevant authorities. Factors affecting noise levels that should be considered by designers include: ●
Number, speed, type and condition of vehicles;
●
Road surface type, condition and gradient;
●
Distance of the noise sensitive land use from the road (particularly intersections);
●
Shielding (natural/built) between the road and noise sensitive area;
●
Type of terrain (reflective/absorptive) between the road and noise sensitive area; and
6.1 Traffic Related Intrusion
●
Meteorological conditions (prevailing winds).
The various impacts of roads in the rural environment are of growing concern to individuals and communities. It is important to fully consider the impact of these issues in any road design. Reduction of adverse environmental impact should be one of the main objectives of any road project.
Methods available to the road designer to reduce the impact of noise from traffic include:
Guide to Traffic Engineering Practice, Part 5 – Intersections at Grade (Ref. 18) provides detailed guidance on intersection design.
6.
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6.1.1
ENVIRONMENTAL CONSIDERATIONS
New rural roads should not only be constructed to link major rural centres, but also to bypass areas sensitive to traffic impacts. Good design should aim to ensure that sensitive environments are not disturbed. The careful design of rural roads can incorporate the means to ameliorate the environmental intrusion of road infrastructure and associated traffic. In particular, consideration should be given to visual amenity through the use of landscaping and creativity with structures and noise barriers. At the design stage, measures to address safety and access issues for all road users will reduce the impact of road projects. Traffic related intrusions perceived by people include: ● ● ● ● ● ● ● ● ●
Visual; Noise; Vibration; Air pollution; Erosion; Risk of accidents and intimidation (Chapter 17); Deterioration of water quality (Chapter 16); Adverse effect on environmentally sensitive areas; and Clearing.
●
Where possible, locating the route away from noise sensitive areas;
●
Using pavement surfaces that have been developed for reduced tyre/surface noise (eg. open graded friction course asphalt);
●
Using geometric design features that encourage the smoother flow of traffic, such as flatter grades and the elimination of at-grade intersections;
●
Locating the road in a cutting or a tunnel where the effects of noise are constrained except at the ends. Cuttings, tunnels and retaining walls could be fitted with noise absorptive cladding; and
●
Providing shielding with landscape features such as earth mounds with appropriate plantings, or with noise attenuation barriers. These barriers may be an architectural feature or designed to blend into the surroundings. Transparent barriers can be used to maintain views.
The required height, location and material type of barriers should be based on acoustic modelling. Cross-sectional detail to provide for noise barriers is shown on Figure 11.7.
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Table 5.1: Provision for Trucks LOCATION
PROVISION FOR TRUCKS
Intersections
Provide for the swept paths of trucks. Refer to Design Vehicles and Turning Path Templates. (Ref. 36). Roadside obstructions shall be located 600 mm clear of the swept path that is travelled when the vehicle’s wheels are in the tray of the kerb and channel. Provide truck stopping sight distance shown on Table 8.3(b) (lateral sight distance restrictions are often critical, particularly at intersections in hilly terrain or near bridge piers). Provide truck stopping sight distance (refer to Table 8.3(b)) for intersections on or near crest vertical curves. Provide truck stopping sight distance (refer to Table 8.3(b)) to allow large/special vehicles to turn safely into each road. Vehicle stability should be considered for turning movements by providing radii appropriate for the turning speeds and providing a uniform rate of change for crossfall.
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Provide stopping sight distance to railway crossings, speed change areas and merge areas such as lane drops. Horizontal curves
As far as possible, avoid locating features that are likely to require large/special vehicles to break on curves, such as intersections where the major road is on a low radius curve. Note that the extra braking distances required on horizontal curves are not compensated by higher driver eye height.
Reverse curves
Provide a straight 0.6V metre long or transition curves between reverse curves to allow for the spiral tracking of trucks. Where deceleration is required on the approaches to a lower radius curve, sufficient distance must be provided to enable drivers to react and decelerate.
Compound curves
If deceleration is likely to be required, allow sufficient distance for drivers to react and decelerate. However, the use of compound curves is not desirable.
Transition curves
Provide transition curves wherever possible. However, any transition should involve a shift of >0.25m.
Grades
Provide sufficient signs to warn drivers of steep downhill grades. Provide adequate sight distance on approaches to curves on steep downhill grades.
Sag vertical curves
Provide stopping sight distance and adequate clearance beneath overpasses.
Superelevation
Avoid adverse superelevation where practicable. The length of superelevation development should be adequate to ensure safe vehicle rotation. Check that superelevation has been increased on downgrades.
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RURAL ROAD DESIGN
6.1.3
Vibration
6.1.5 Controlling Erosion
Vibration from traffic on rural roads is very unlikely to be significant and action to ameliorate the intrusion will not usually be necessary. However, where vibration is an issue, the airborne sound pressure issue can be mitigated through noise attenuation or window design.
6.1.4
Air Pollution
Motor vehicles have an adverse effect on air quality. This results from the discharge into the air of reactive and nonreactive pollutants. The amount of vehicle emissions is dependent on traffic volume, composition of traffic, traffic flow characteristics and road geometry. The impact of the adverse effect of the emissions is dependent on topography, meteorological and atmospheric conditions and the distance of the receptor from the road. On rural roads this intrusion has minimal effect and need not be considered further.
6.1.5
Erosion
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The construction of rural roads can rapidly disturb the environment, leaving extensive scars on the landscape. The cooperation of road engineers, soil conservationists and all personnel involved is essential to reduce the impact of road construction on the environment. The large areas cleared by earthmoving equipment during road construction are a potential soil erosion hazard. Areas, that do not have a cover of grass to slow and reduce water runoff, are subject to excessive water flows and can result in severe loss of soil. Erosion on construction sites can affect adjacent properties and cause the sedimentation of private and public lands, streams, water storage dams, rivers, harbours and lakes. Sediment can destroy vegetation and the natural habitat of native fauna. Soil erosion and sediment can pose a serious threat to the safety, stability and durability of the road itself. These problems can be greatly reduced if adequate planning is undertaken during design and control measures are implemented for each stage of construction. An erosion management plan, which has been developed by all responsible agencies and authorities, shall be the corner stone of rural road projects. The best results will be achieved when an erosion management plan, developed by agreement between the responsible agencies, is in place and a suitably qualified person is engaged to manage and control its implementation.
●
Using earth banks to divert water from disturbed areas;
●
Lining drains to prevent scouring and gollying;
●
Establishing vegetation using suitable plant species; and
●
Implementing an appropriate maintenance program.
post-construction
The advantages of a properly managed erosion management plan are: ●
Greatly reduced erosion repair costs;
●
Marked decrease in down-time following wet weather, resulting in substantial financial benefits;
●
Significant improvements in catchment protection and a more acceptable environment adjacent to the site;
●
Increased safety.
Specific control measures may include: ●
Training construction personnel to understand and implement the control measures;
●
Developing culvert and drainage works prior to major construction;
●
Minimising disturbance of natural vegetation cover, particularly adjacent to drainage lines;
The cost of erosion and sediment control is likely to be a fraction of the total project costs, but the aesthetic and general benefits of implementing control measures are far greater.
6.1.6
●
Stockpiling topsoil for later respreading to assist the revegetation of areas disturbed during construction;
A proposed rural road may highlight other environmental issues either within or close to the road reserve, such as: ● ●
●
Building sedimentation traps;
Environmentally Sensitive Areas
●
Native flora and fauna; Cultural heritage (indigenous and non-indigenous); and Water quality.
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11
The construction, use and maintenance of the road must be sensitive to these issues. For example, it is important to retain significant areas of remnant native vegetation, including grasses, in and adjacent to the road reserve. Road design, construction works and maintenance activity should all aim to reduce impact on native flora and fauna habitat.
6.2.3 High Wind
Identifying and managing any potential impact on sites of historical or archaeological interest should involve a qualified archaeologist and representatives of relevant local Aboriginal Land Councils and heritage bodies. If required, a program for archaeological monitoring should be developed in consultation with the road authority to determine the most appropriate construction methods to avoid or reduce disturbance to the site. Designers should also consider the influence of social issues when planning and designing rural roads (see Section 15 Community Consultation). Runoff from the road surface contains pollutants, which can be detrimental to the receiving waters. When AADT is greater than 30,000, the amount of resultant pollutants is very high and the runoff from rural roads should be considered for treatment over the full length of the project. When AADT is less than 30,000, lengths of a project traversing sensitive receiving environments should be considered for treatment to improve the runoff water quality (Ref Section 16.5).
6.1.7
Clearing
The clearing of all forms of vegetation should be kept to a minimum within the works area. Cleared areas rob soil of the natural protection from erosion, which vegetation provides. Close attention is to be given to determining the extent of clearing when preparing the erosion protection strategy plan for a project.
6.2 Environmental Related Intrusion 6.2.1
Snow and Ice
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Snow and ice can pose a traffic hazard that may require maintenance action and signage to accommodate the safe passage of vehicles.
6.2.2
Floods
Many areas are inundated with flood waters that over-top the rural road formation. Special signage and possible route relocations may result from these incidents.
6.2.3
High Winds
High winds that blow adjacent to the road alignment in exposed locations need to be considered in the design stage. The winds can cause concern to all vehicles and special signage and wind socks are used to bring the attention of the driver to the intrusion.
6.2.4
Animals and Birds
Animal and bird intrusion can be in the form of farm (fenced) or station (unfenced) animals or natural animals and birds. All these require signage and in some cases road cattle grids or
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RURAL ROAD DESIGN
special crossings or fences to limit intrusion. Cattle underpasses or overpasses can be installed to allow for the safe movement of stock. In the case of natural animals the special crossings and fences may be installed to provide a safe crossing for migratory reasons.
6.3
References
Guidelines prepared by Austroads (Ref. 31) establish a range of procedures to evaluate environmental impacts and summarise the legislation and operation of Australian Federal and State procedures for use when assessing major road projects. The impacts that need to be addressed to meet the objectives of ecologically sustainable development strategy are described in another Austroads publication (Ref. 35). This strategy is a key document to assist road planners and designers. Further consideration of these issues is set out in Ref. 32 and 39 and various environmental protection policies or guidelines prepared by local environmental authorities.
3
PA R T
7.
DESIGN INPUTS
S P E E D , U S E D FO R G E O M E T R I C DESIGN
7.1 Introduction Among the principal parameters used in road design are “stopping distance”, “sight distance”, “curve radius”, “lane width” and “superelevation”. As these parameters are related directly to the speed of traffic on the road, one of the first requirements in design is to establish the appropriate speed or speeds to use for design. Historically, a single “design speed” was used as the basic parameter for each road. Although roads designed in this way had consistent minimum design standards, problems arose because vehicle operating speeds differed from the design speed and, in some cases, and the speed difference was sufficient to create a hazard. The most common location where problems occur is at the end of straights where vehicle operating speeds often exceed the design speed of the curve. To overcome these problems, designers are now required to obtain more rigorous estimates of 85th percentile vehicle operating speeds on each element of the road and then to ensure that the design speed of every element is either equal to, or greater than, the 85th percentile operating speed on that element.
actual vehicle operation in the field enables the designers to visualise themselves in the position of a driver negotiating the road. Use of this procedure can help designers to identify features, which could influence the operating speed; it is also a useful technique for identifying other problems associated with the design. In addition to simulating vehicle behaviour on curves, the estimation model has the following built-in safety factors: ●
The model identifies the use of lateral friction factors which exceed specified values; and
●
The model identifies the development of excessive speed inconsistencies along the alignment. The model restricts speed differences between design elements to less than 10km/h and in most cases the difference is significantly less than this.
7.2 Explanation of Terminology 7.2.1
Vehicle Speed on Roads
Vehicle speed range is as follows: High speed:
Intermediate: 80km/h to 99km/h Low speed:
The decision to use the 85th percentile operating speed was based on:
79km/h or less
Driver operating speeds are not constrained by the geometry of the road but by a number of other factors, which include:
The need to overcome the problems associated with the use of a single design speed as mentioned above;
●
The degree of risk the drivers are prepared to accept;
●
Recent design practice in Europe and the USA;
●
Speed limits and the level of policing of these limits; and
●
The premise that drivers of the fastest vehicles, generally travel in a more alert state than the average driver and therefore a reduction in reaction time can be assumed which will compensate, to some extent, for the difference between the operating speed of these vehicles and the design speed of the road; and
●
Vehicle performance.
●
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100km/h or greater
●
Figure 7.1: Comparison between Observed 85th Percentile Speeds and pre 1980 Curve Speed Standard
Practical constraints. It is not possible for practical reasons (mainly economic) to design for the 100th percentile vehicle.
Operating speeds can either be measured or estimated. Wherever possible, operating speeds should be measured both for cars and for trucks. As this is not possible with new road proposals, operating speeds have to be obtained by other means including measurements of speed on similar roads, and estimates using the method described in Sections 7.3 and 7.4. The estimation procedure in Sections 7.3 to 7.4 was developed to simulate or model the actual behaviour of vehicles on the road. This correlation between the mathematical model and
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Figure 7.1 indicates that on roads designed for lower speeds, drivers tend to overdrive the road. Conversely on roads designed for higher speeds, drivers adopt an operating speed of 100 km/h to 110 km/h. In some cases, where a speed limit is 110km/h, operating speeds may be higher such as on long downhill grades.
7.2.2
separately, such as on the approaches to intersections. At intersections, the stopping distance on each approach should be based on the operating speed for that approach. Operating speeds can be affected by the frequency of intersections.
7.3 Estimating Operating Speeds on Rural Roads
Operating Speed 7.3.1
The term “Operating Speed” in this guide is the 85th percentile speed of cars at a time when traffic volumes are low, that is when drivers are free to choose the speed at which they travel. In effect, this means that designs based on the 85th percentile speed will cater for the majority of drivers. For design purposes, the 15% of drivers who exceed this speed are considered to be aware of the increased risk they are taking and are expected to maintain a higher level of alertness, effectively reducing their reaction times. On straight flat rural roads with low traffic volumes, the 85th percentile Operating Speed of cars is generally close to 110km/h. On higher standard roads with a posted speed of 110km/h, the Operating Speed may be marginally higher.
The following procedure will enable designers to consider the behaviour of a typical 85th percentile driver. There are three basic elements: the driver, the road and the vehicle.
7.3.1.1 Driver Behaviour Consider first a typical driver approaching a straight section of road, which is followed by a series of curves at the end of the straight.
Vehicle speeds on a series of curves and short straights tend to stabilise at a value related to the range of curve radii. This speed is called the “Section Operating Speed”.
The driver's initial response will depend on the speed at this time and the length of straight. If the straight is too short, the driver is likely to continue at the same speed. On longer straights, the driver will accelerate until terminal speed is reached, which is related to the length of straight and the initial speed. They will then continue at this speed to within approximately 75m of the curve. The driver then decelerates to a speed, which is considered safe for the curve ahead. Truck drivers will generally decelerate to the appropriate speed for the curve because of the dangers associated with braking trucks on curves. Car drivers are likely to enter at a speed that is high for the curve as indicated by some further deceleration, which commonly occurs within the first 80m of the curve. Speeds remain at this level until the driver has a clear view of the curve or straight ahead. If it is a straight, the driver will accelerate out of the curve; if another relatively low radius curve follows, the driver is likely to reduce speed further. This loss of speed continues until the vehicle reaches a speed at which he feels comfortable. This is the section operating speed for the series of curves. This speed is then maintained until the end of the section.
7.2.5
7.3.1.2 Road Characteristics
A procedure for estimating vehicle speeds of cars in rural areas is provided in Section 7.3.
7.2.3
Operating Speed of Trucks
The term “Operating Speed of Trucks” is the 85th percentile speed of trucks at a time when traffic volumes are low. Operating speeds of trucks are required for checking design details such as stopping distances for trucks at intersections.
7.2.4
Section Operating Speed
Design Value
Operating Speed is the value adopted for the design of each element of the road. Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
General
On roads designed for high-speed travel, speeds remain relatively constant permitting the use of a single design value for the road. Note that although operating speeds are relatively constant, they can differ significantly from the design value as indicated on Figure 7.1. On roads with operating speeds less than 100 km/h, operating speeds vary along the length of the road depending on the road geometry and, to some extent on other factors such as speed limits and policing. For the design of rural roads, most weight is given to the effects of the geometry of the road as speed limits and the level of policing can change. On these roads operating speed needs to be determined for each element of the road. For design purposes on two-way carriageways, operating speeds are either measured or estimated for each element of the road and for each direction of travel. In many cases the higher of the two values will be adopted as the design value of the curve. There will be some circumstances where each direction has to be considered
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RURAL ROAD DESIGN
The effect of grading, cross section and surface conditions all impact on the operating speed. There is insufficient investigation to accurately understand their impact but it is important to be aware of their characteristics. This is explained further in Sections 7.3.7 to 7.3.9.
7.3.1.3 Vehicle Characteristics Two design vehicles are considered: cars and the truck (design semi trailer 19.0m). Speeds are determined first for cars. Truck speeds are then obtained using Table 7.2.
7.3.2
Operating Speed Estimation Model
The model used to estimate Operating Speeds is based on a large number of observations of the behaviour of traffic. The Operating Speed of vehicles is estimated by establishing the approach speed of the vehicle for the direction of traffic flow being considered. The approach speed is then applied to the first curve and an operating speed is read. This speed then becomes the approach speed for the subsequent curves and separating straights. The Operating Speed estimating graphs are:
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Figure 7.2: Acceleration on Straights (Hilly to Mountainous Terrain)
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Figure 7.3: Deceleration on Curves
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Operating Speed (in sequence)
● ●
Acceleration on Straights (Figure 7-2), Deceleration on Curves (Figure 7-3).
7.3.3
Car Acceleration On Straights Graph
The car Acceleration on Straights graph Figure 7.2 allows the designer to estimate the speed at which a vehicle can accelerate over a given length. Large radius curves may be considered as straights, as depicted on Figure 7.3, where the Operating Speed from 50 to 120km/h is no longer influenced by a further increase in the radius. The change in speed, read from Figure 7.2 assumes that the terrain is constant and the maximum visible length of straight is 1000 metres.
7.3.4
Car Deceleration On Curves Graph
The car Deceleration on Curves graph Figure 7.3, allows the designer to estimate the speed to which a vehicle decelerates to, when entering a given curve radius line, or matches the Section Operating Speed. The intersect with the higher speed value is the element Departure Speed. Figure 7.3 allows the designer to then consider the given curve radius against the Desirable Minimum Radius and check that it does not approach the Absolute Minimum Radius for the Approach Speed. The example on Figure 7.3 shows an Approach Speed of 100km/h intersecting with a given radius of 320m, resulting in a Departure Speed of 93km/h. The curve radius intersection is about the Desirable Minimum Radius limit. The Departure Speed is more than the Section Operating Speed in this case. It is necessary to redesign the alignment in those circumstances where the intersection of the Approach Speed and the Section Operating Speed (or radius) encroaches more than half way toward the Absolute Minimum Radius line. An acceptable solution would be for the intersect to be midway or better between the Desirable and Absolute Minimum radius lines. In the case of existing roads, adequate signage needs to be provided to inform drivers of the restricted alignment.
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Information required to use this graph includes: ●
The approach speed to the curve. This is likely to be either: - The speed on the preceding curve; or - The speed at the end of the preceding straight;
●
The length of the curve or straight;
●
The section operating speed being considered;
●
Radii.
7.3.5
Section Operating Speeds
As previously stated, when drivers travel along a series of curves of similar radii, their speed will stabilise at a level at which the driver feels comfortable. This is the Section Operating Speed. The effects of grade, cross-section and pavement conditions, as explained further in Sections 7.3.7, 7.3.8 and 7.3.9 may influence Section Operating Speed.
7.3.5.1 Length Of Road to be included in The Study Section Operating Speeds can be obtained directly from Table 7.1. However, as a first step, it is necessary to segment the alignment into sections commencing approximately 1km to
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Figure 7.4: Road Study Length
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Figure 7.5: Single Curve Disparity
Figure 7.6: Road Length Sections
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1.5km before the start of the section for which speed estimates are required.
only includes radii up to 600m, radii beyond that range should be considered as a straight. Also refer Section 7.3.3 and Figure 7.3.
If, for example, speed estimates were required for the curves between C and I in Figure 7.4, the speed study would extend from A to I (Assuming a one way road in the direction from A to I). If the diagram represented a two-way road, the study would include the section from A to J.
Further research is required to establish a minimum length of straight that may be considered as a section. In the meantime, it is suggested that 200m should be adopted as the minimum length of straight that may be considered as a section. Straights, shorter than 200m have no effect on vehicle operating speed.
The extensions are necessary because the first speed estimate at the start of the extensions, at points A and J, are not particularly accurate. Accuracy then increases with distance depending on the alignment. The choice of 1.5km is considered conservative.
It is also considered that:
7.3.5.2
Identification of Sections
●
Individual curves separated by straights longer than 200m are treated as individual elements.
●
Curves inconsistent in radius to the preceding curves where acceleration is likely are treated as individual elements.
A series of similarly sized curves, separated by small straights, or spirals that can be grouped together function as a single element and drivers will travel along this portion of road at the Section Operating Speed.
Acceleration occurs whenever speed has been reduced below the Section Operating Speed or the section speed. For example, the stable speeds on sections 1 and 2 of Figure 7.6 could be 70km/h and 80km/h respectively. Speed can thus be expected to increase on the first few curves of section 2 until stability is reached at 80km/h. The rate of increase can be 1 km/h for every 30m with limited sight distance (Figure 7.2) to 1km/h for every 5m with unlimited sight distance (flat to undulating terrain).
Spiral lengths should be divided in two, with the length of the two halves being included in the adjoining elements. Table 7.1
Section Operating Speeds for single curve sections and curve groups are listed in Table 7.1.
In some circumstances, the radius of a single curve cannot be grouped with curves to create a section because of the disparity between the radii. In this instance, the single curve has to be treated as a section as shown on Figure 7.5.
Table 7.1: Section Operating Speeds
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Range of Radii In Section (m)
Single Curve Section Radius (m)
Section Operating Speed (km/h)
Range of Radii In Section (m)
Single Curve Section Radius (m)
Section Operating Speed (km/h)
45-65
55
50
180-285
235
84
50-70
60
52
200-310
260
86
55-75
65
54
225-335
280
89
60-85
70
56
245-360
305
91
70-90
80
58
270-390
330
93
75-100
85
60
295-415
355
96
80-105
95
62
320-445
385
98
85-115
100
64
350-475
410
100
90-125
110
66
370-500
440
103
100-140
120
68
400-530
465
105
105-150
130
71
425-560
490
106
110-170
140
73
450-585
520
107
120-190
160
75
480-610
545
108
130-215
175
77
500-640
570
109
145-240
190
79
530+
600
110
160-260
210
82
RURAL ROAD DESIGN
19
In the following example the series of curves are joined by short straights and transition curves unless otherwise stated.
to form a section, it must be treated as a Single Curve Section Radius in Table 7.1.
Example Calculations
The curves between C and F range in radii between 270m and 320m. This range fits within the section in Table 7.1, which has a Section Operating Speed of 93km/h.
Identify individual sections for the alignment shown in Figure 7.7. Between A and B the curve radii range is from 230m to 320m. This range fits within the “Range of Radii in Section” column in Table 7.1, suggesting that 89km/h should be adopted as the Section Operating Speed. The next section is the straight between points B and C. Consideration must then be given to the curves between points C and I where radii range between 165m and 320m. As this range will not fit within any listed in Table 7.1, the curves must be grouped into two or more sections. The problem curve is clearly the one with a radius of 165m. As this curve cannot be grouped with any of the adjacent curves Figure 7.7: Road Length Detail
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Figure 7.8: Section Identification
Figure 7.9: Section Operating Speed
20
RURAL ROAD DESIGN
Section FG is an isolated 165m radius curve section. Interpolated from column 2 in Table 7.1, this curve has a Section Operating Speed of 76km/h. The two curves between G and I both have radii of 300m. From Table 7.1, the section operating speed of this section is 91km/h. In this case the Section Operating Speed can be obtained from the single curve column or alternatively by picking a range of radii which is spread evenly on each side of the 300m radius. Both methods give the same result. The sections identified above are shown diagrammatically on Fig. 7.8.
Section operating speeds within these sections are shown in Fig. 7.9.
7.3.6.6
7.3.6
On Figure 7.12, follow the approach speed line (93km/h) down to the intercept with the radius (165m) or the Section Operating Speed (76km/h) whichever comes first. The radius intersect is first and the departure speed is 81 km/h.
Estimating Speed on a Section of Road
An estimate of Section Operating Speed is required between C and I in the direction from C to I on Figure 7.7. For the purpose of this exercise the pavement condition and cross section remain constant. The undulating terrain is also constant. There has been no allowance for steep grades. See Section 7.3.7 to 7.3.9 for further clarification. As vehicle speed at every site depends on the road geometry on the approaches, it is necessary in all investigations to consider the alignment for 1km and 1.5km on each approach. As this is a one-way road it is only necessary to consider the approach between points A and C.
7.3.6.1
Step 1 – Estimate Speed on Section A – B.
From Table 7.1 for radii 230m to 320m the section operating speed is 89km/h and the speed at point B may be taken as 89km/h.
7.3.6.2
Step 2 – Estimate Speed at Point C
Step 6 – Estimate of Speed at Point G
The intersection of the approach speed 93km/h and the radius 165 m is at the Absolute Minimum Radius line. This is an unacceptable solution. The radius needs to be increased to relocate the intercept of the Approach Speed (93km/h) and the radius to at least midway between the Desirable and Absolute Minimum Radii for the Approach Speed (93km/h). If this cannot be achieved, warning curve signs need to be provided to inform drivers of the restricted alignment. A radius of 220m intersects midway and results in a Departure Speed of 85km/h. In absolute situations where it is unavoidable to increase such a radius careful attention needs to be given to clearing runoff areas, sight distance lines, lighting and sufficient advanced warning signs, in an attempt to minimise the potential for accidents.
that is near the end of the straight. From Figure 7.2, the speed at the end of the straight is 100km/h (assuming an initial speed of 89km/h on straight 600m long).
It is desirable to redesign the alignment in those circumstances where the intersection of the approach speed and the Section Operating Speed (or radius) is to the left of the absolute minimum radius line.
7.3.6.3
7.3.6.7
Step 3 – Estimate Speed at Point D (departure speed on first curve)
On Figure 7.10, follow the 100km/h curve approach speed line down until in intercepts either with the radius of 320m or with the section operating speed – determined earlier as 93km/h (whichever comes first). In this case the departure speed for this curve is 93km/h.
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Note the location of the intercept with the radius line. The fact that this is close to the Desirable Minimum indicates that the curve radius is using desirable lateral friction. The curve is acceptable.
7.3.6.4
Step 4 – Estimate Speed at Point E
Step 7 – Estimate of Speed at Point H and I
The 85th percentile vehicle, having significantly reduced speed on curve F G Figure 7.9 will accelerate on subsequent elements. Most drivers will attempt to achieve the section operating speed again, provided the driver can see some benefit. The driver will not accelerate over a short length only to decelerate around another tight curve. Acceleration will apply on both straights and curves provided the driver doesn’t exceed the element operating speed. The acceleration on straights graph can be used to estimate the increase in speed. If the distance between points G and I was 310m with approach speed 81km/h, from Figure 7.2, the approximate speed of a car at point I would be 88km/h.
(departure speed on second curve)
7.3.7 On Figure 7.11, follow the approach speed line (which is now 93km/h) to the intercept with the radius or the Section Operating Speed (whichever comes first). In this case the Section Operating Speed is 93km/h. The departure speed at Point E is equal to the Section Operating Speed (93km/h). Note also the location of the intercept between the radius (270 m) and the curve speed. In this case it is close to the desirable minimum radius line. This indicates that the radius used is desirable.
7.3.6.5
Step 5 – Estimate of Speed at Point F
As for Step 4, Figure 7.11 can be used again to demonstrate that the Section Operating Speed (93km/h) again prevails.
Effects Of Grades
Insufficient information is available to provide firm guidelines on the effect of grades. However, designers are expected to consider the grading and make adjustments to speed estimates, refer Table 10.1. The following assumptions can be made. These corrections for grade must be made for each element of the road as the speed estimate is made. ●
The operating speed of cars may be reduced on up hill grades longer than 200m.
●
The operating speed of laden trucks will be significantly reduced on long up hill grades.
RURAL ROAD DESIGN
21
Figure 7.10: Speed at Point ‘D’ (see Figure 7.3)
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Figure 7.11: Speed at Point ‘E’ & Point ‘F’ (see Figure 7.3)
22
RURAL ROAD DESIGN
Figure 7.12: Speed at Point ‘G’ (see Figure 7.3)
●
Cars will generally travel at the operating speed on steep down hill grades, however, some increase could be expected toward the end of the down hill grade.
hierarchy of the road and either equal to or greater than the predicted 85th percentile operating speed for the road with consideration given to both cars and trucks.
●
Trucks may be required to significantly reduce their speed prior to steep down hill grades.
If the road being designed is a high-speed road with operating speeds of 110km/h, then a single operating speed can be adopted and the road designed using this speed to select the design standards used.
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Corrections for grade should be considered for each element of the road. This is particularly necessary when there is a significant change in topography.
7.3.8
Effect of Cross-Section
Speed estimates in preceding sections are appropriate for typical road cross-sections, such as those with traffic lanes wider than 3m. On roads with lanes narrower than 3m, the speed estimates can be reduced by up to 3km/h.
7.3.9
On other roads the operating speeds will vary along the length of the road. The basic steps to be followed in the design of this type of road are listed below: ●
Prepare a draft alignment and grading in the normal manner taking into account desirable minimum curve radii, road hierarchy and terrain. A design feature is the use of relatively large radii at the end of straights where high speeds can be expected;
●
Using the draft alignment, estimate the operating speeds in each direction of travel using the procedure outlined in Sections 7.3 to 7.5. The location of intersection points on the deceleration on curves graph will indicate whether the design is appropriate or not. If any of the intersection points between the curve radius and operating speed lie on the left of the desirable minimum radius line, then some adjustments will be required, either to the design to reduce the approach speed to the curve, or to increase the radius of the curve – usually the latter;
●
Modify the alignment;
Effect of Pavement Condition
Average pavement conditions were assumed for the speed estimates in the preceding sections. On roads with poor or broken surfaces, speeds can be reduced by 5km/h to 10km/h.
7.3.10
Use of Operating Speed in the Design of Rural Roads
The normal design procedure is to prepare a preliminary alignment and grading with standards that are as high as possible within realistic constraints. The minimum standards used must be appropriate for the terrain, consistent with the
RURAL ROAD DESIGN
23
Check the operating speeds on the modified alignment. (Repeat if necessary until all intercept points on the speed on curve graph are either on, or to the right of the desirable minimum radius line).
●
If a very short length smaller radius curve exists, the driver usually transitions the vehicle path to a larger radius than the curve centre line. A short length curve can therefore be defined as a curve where the radius of the transitioned driver path is considerably greater than the radius of the centre line of the roadway. Any short length curves can usually be found by visual inspection of the alignment. The radius of the transitioned driver path can be obtained by assuming a 2m wide vehicle approaching and departing the curve in the centre of the lane and transitioning to just touch the centre line of the roadway or the edge line midway around the curve. By using this method larger radius curves can be used in the analysis of short absolute minimum radius curves.
●
The lower operating speed for trucks is an average condition with truck speeds varying more than car speeds due to grades, poorer acceleration etc.
●
When checking braking and stopping sight distance provision for trucks, it is acceptable to use the lower truck operating speed for a corresponding car operating speed. This is because an acceptable level of safety is provided through the assumptions of: - Wet conditions - Unladen state - No antilock braking system
●
Check that the maximum difference in speeds between design elements does not exceed 10km/h.
Table 7.2 Truck/Car Speed Relationships
●
Compare the operating speeds for each direction of traffic on each element of the roadway (other than those at intersections) and adopt the higher of the two speeds as the design speed for each element. Where intersections are involved, both operating speeds have to be used as speeds on each approach can differ and the appropriate speed has to be used for sight distance checks on each approach;
●
●
●
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●
●
●
Otherwise, truck speeds in Table 7.2 may be used.
Additional factors that support the truck speeds in table 7.2 are:
No further reduction in operating speed due to wet conditions.
Car Speed (km/h)
110
100
Truck Speed 110* 100* (km/h)
90
80
70
60
50
80
70
60
52
43
Note: *On high-speed rural roads truck speeds equal car operating speeds.
Check sight distances on all curves noting where benching is likely to be required. It is often impractical in steep country to meet the sight distance requirements. In these circumstances consideration should be given to alternative treatments such as the use of sealed shoulders of sufficient width to enable one vehicle to manoeuvre around a stationary vehicle in the lane ahead.
7.5
Using the checklist in Table 5.1, check the alignment for potential problem sites for trucks. If any problem areas are identified, then it is necessary to estimate the 85th percentile truck operating speeds for each site. Truck sight distances can then be checked. If the site proves to be a problem for trucks, the design should be reviewed and, if necessary, amended;
Further research is required to determine the speed of trucks on individual geometric elements and the maximum allowable decrease in speeds between successive geometric elements.
Prepare superelevation diagrams based on the critical speeds obtained for each element; and Prepare detail design plans for the project.
7.4
Operating Speed of Trucks
As with cars, truck speeds should be measured wherever possible. Where it is not practical to measure the speed of trucks, speed has to be estimated. The following rules should be used as a guide: ●
On high-speed roads, truck speeds can be taken to be the same as that of cars.
●
Provided sufficient length of acceleration is available, truck speeds will closely match car speeds on flat terrain.
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RURAL ROAD DESIGN
Use Of Truck Operating Speeds
Although the basic design vehicle for road alignments is still the car, designers are now required to check all designs to ensure that they are safe for trucks. Specific locations where providing for trucks is likely to be required are listed in Table 5.1.
8.
S I G H T D I S TA N C E S
8.1 General The principal aim in road design is to ensure that the driver is able to see any possible road hazards in sufficient time to take action to avoid mishap. To provide a calculable parameter that can be related to the geometry of the road, the concept of Sight Distance is used. This concept is based on a number of somewhat stylised assumptions of particular hazards and corresponding driver behaviour. The hazard is assumed to be an object of sufficient size to cause a driver to take evasive action, intruding into the driver’s field of view. Specific values are assumed for the driver’s reaction time (though in practice there would be a distribution of values) and the dimensions determining the geometry of the sight line. Normally, selection of extreme values for every parameter is not appropriate, as the probability of all factors occurring
together is extremely low, and the resultant designs would become impractical. The assumed parameter values lead to sight distances that produce a satisfactory design. Greater distances allow for less probable hazard situations and thus produce greater margins of safety. Subject to their effect on overtaking and economics they may be an advantage. Where possible the sight distance provided should be greater than the values used in these guidelines. Adequate sight distance is essential for safe and efficient traffic operation. The designer should consider the length of vertical curves, the radius of horizontal curves and the terrain on the inside of horizontal curves in providing adequate sight distance. This section does not consider sight distance at intersections. For required sight distances at intersections, including roundabouts, refer to Guide to Traffic Engineering Practice, Part 5 and 6 (Ref. 18 and 19).
head lights or taillights would be necessary. Larger objects would be visible sooner and provide longer stopping distances. To perceive a very small hazard, such as a surface defect, a zero object height would be necessary. However, at the required stopping sight distances for high speeds, small pavement variations and small objects (especially at night) may not be visible to most drivers. Thus, most drivers travelling at high speeds would have difficulty in stopping before such a small obstruction. The length of vertical curve required at crests increases significantly as the object height approaches zero. The general figure adopted which produces satisfactory design is 200mm. Lower object heights, even zero, can be used at intersections, where it is necessary to see road markings, and at locations such as causeways, floodways and cuttings, kerb and channel noses, where there is a high probability of water, rocks or other debris being on the road (Ref. 51).
8.2 Sight Distance Parameters When determining sight distance, assumptions must be made about the following elements:
For geometric design of rural roads the object heights shown in Table 8.1 are to be used. Table 8.1: Object Heights
● ● ●
Object height; Driver eye height; and Driver reaction time.
Sight distance is measured between the driver eyes and an object or pavement marking on the road ahead, as shown on Figure 8.1. An object in view may not always be perceived. There is evidence that when a driver is travelling on sharp curves or when the vehicle is rapidly accelerating or decelerating and the driver is subject to unusual forces, his ability to perceive an object is reduced. Fatigue and drugs add to the time of perception and may increase an individual’s reaction time.
8.2.1
Object height
Situation
0.0m (Pavement)
• Intersection design • Sight to line-marking configuration
0.2m (Object)
• Mid-block crest curve design • Horizontal curve line of sight
0.6m • Impact on vertical clearance (Car taillights) • Sight to vehicles at end of (Car traffic indicator) intersection queues • Sight over roadside safety barrier installations
Object Height
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8.2.2 The object height to be used in the calculation of stopping sight distance is a compromise between the length of sight distance and the cost of construction. Stopping is generally in response to another vehicle or large hazard in the roadway. To recognise a vehicle as a hazard at night, a line of sight to its
Driver Eye Height
Driver eye height is a combination of the height of driver stature and driver seat height. A number of studies (eg. Ref. 4, 41, 50 and 64) have investigated car driver eye height trends and found that they have progressively reduced over time,
Figure 8.1: Sight Distance
RURAL ROAD DESIGN
25
consistent with the changing vehicle fleet. Historically and internationally, car driver eye heights used range between 1.15 m and 1.00 m. Based upon recent research and consideration of the characteristics of the vehicle fleet and the ageing of drivers, a car driver eye height of 1.05 m is to be used for the geometric design of rural roads. For general geometric design a truck driver eye height of 2.4m is to be used. The 2.4m value for sag curves is particularly important for checking the effect of overhead structures on sight distance. The reduction of car driver eye height will have implications on geometric design elements (such as length of vertical curves) used in other road design publications, which should be considered by the designer when this guide is used in conjunction with previously published guides.
For truck drivers, the 2.5 second time actually consists of a 2.0 second initial reaction time (which is a reflection of the fact that truck drivers are professional drivers and in traffic, are usually able to see over vehicles in front) plus a 0.5 second inherent delay in the operation of the air brake system that is used on heavy vehicles (see Ref 55). Braking tests by Mack Trucks Australia support this time delay, being in the range of 0.47 seconds to 0.6 seconds.
8.2.4
As people age, they experience decreasing physical and mental capabilities and become more susceptible to injury and shock. Human functions subject to deterioration due to ageing include: ● ● ●
8.2.3
Driver Reaction Time
Reaction time is the time for a driver to perceive and react to a particular stimulus and take appropriate action. This time depends on the complexity of the decision or task involved. Research studies have shown that an average reaction time of 2.5 seconds is typical although the variance of the distribution of reaction times is very high (Ref. 6, 54, 68 and 94). Values of up to 7 seconds have been recorded at one extreme, and at the other extreme, 1.0 second has been measured with forced stops (Ref. 6). One reason for the large variability is that reaction time depends on a driver’s level of alertness at the time. Similarly, anticipation or pre-signalling of an event, the absence of uncertainty on multiple choices, and the familiarity with the task can each lower reaction time.
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Given the above, it has been reported that most drivers can react simply to a clear stimulus in less than 2.5 seconds in an urgent situation. This represents an upper (possibly the 85th percentile) value for normal drivers and is close to the mean for degraded drivers (Ref. 94). Consequently, the reaction time of 2.5 seconds is a commonly adopted value, although a number of European countries specify a value of 2.0 seconds. A recent study investigating road safety and design for older drivers (Ref. 53) recommended a minimum reaction time of 2.5 seconds at intersections. For mid-block sections a desirable minimum reaction time of 2.5 seconds and an absolute minimum of 2.0 seconds is to be used. The aging of drivers (refer to Section 8.2.4) emphasizes the importance of these values. A driver reaction time of 2.5 seconds is to be used in this Guide for the geometric design of rural roads. However, in mid-block situations where there is an expectation for increased driver alertness, such as locations with additional signs or line marking, or where it may not be practicable to design for a 2.5 second reaction time, such as low speed alignments in difficult terrain, a minimum reaction time of 2.0 seconds may be considered. It is noted that the driver reaction time will have implications on geometric design elements (such as sight distance) used in other road design publications, which should be considered by the designer when this Guide is used in conjunction with previously published guides.
26
RURAL ROAD DESIGN
Ageing of Drivers
●
Visual ability; Attention capacity; Reaction time; and Contrast sensitivity.
As a group, older drivers do not currently represent a major road safety problem in most Western societies when compared with other age groups. However, older drivers are involved in significantly more serious injury and casualty crashes per kilometre travelled. Furthermore, as the proportion of older people in Australia and New Zealand is expected to roughly double over the next 40 years, older drivers are likely to become a more significant problem in the years ahead (Ref. 53). Recent research (Ref. 53) indicates that a number of road design elements may be associated with older driver crashes in Australasia. In particular, it was concluded that improvements to intersection sight distances, provision for separate turn phases at traffic signals, more conspicuous traffic signal lanterns and more clearly defined vehicle paths have the potential to reduce crash and injury risk for older drivers. The research includes a detailed description of measures that should be implemented immediately in Australia to increase the safety of older road users.
8.3 Stopping Sight Distance (SSD) Stopping sight distance is the distance to enable a normally alert driver, travelling at the design speed on wet pavement, to perceive, react and brake to a stop before reaching a hazard on the road ahead. This distance is considered to be the minimum sight distance that should be available to a driver.
8.3.1
Derivation
Stopping sight distance has two components, namely the distance travelled during the driver’s perception-reaction time and distance travelled during braking. SSD
= d1 + d2
where (R V) d1 = reaction distance = T (m) 3.6 2 (V ) d2 = braking distance = (m) 254(F + 0.01g1) RT = reaction time (2.5 secs) V
= operating speed (km/h)
F
= longitudinal friction factor
g1
= longitudinal grade (%, + for upgrades and – for downgrades).
Table 8.2: Longitudinal Friction Factors Operating Speed (km/h) 50
60
70
80
90
100
110
120
130
Cars
0.52
0.48
0.45
0.43
0.41
0.39
0.37
0.35
0.35
Trucks
0.29
0.29
0.29
0.29
0.29
0.28
0.26
0.25
0.24*
*Extrapolated Values of RT (from Section 8.2.3) and F must be assumed in order to compute the SSD appropriate to the operating speed, Table 8.2.
derived from US research (Ref. 55) and were based on the behavior of an empty prime mover-trailer combination on a wet pavement.
8.3.2
8.3.3
Longitudinal Friction Factor
The longitudinal friction factor is a measure of the longitudinal friction between the vehicle tyres and the road surface. It depends on factors such as the speed of the vehicle, the tyre condition and pressure, the type of road surface and its condition, including whether it is wet or dry. Currently design values of the longitudinal friction factor for bituminous and concrete surfaces are shown in Table 8.2. The review of available literature indicates that the longitudinal friction factors for cars that are currently in use appear too high relative to the actual friction that can be confidently expected on wet surfaces. The friction factors appear to have been increased relative to those given in NAASRA, 1973 “Policy for geometric design of rural roads (Metric Units)” without direct vindication.
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McLean (Ref. 71) notes that the limiting values for longitudinal friction factor were based on producing stopping sight distance requirements leading to what was considered to be an appropriate balance between horizontal and crest vertical curve standards. The balance achieved appears to be generally consistent with international practice, although, relative to North America and earlier Australian (1973) practice, minimum sight distance requirements are a little low. Concerns have been raised in relation to the high values of longitudinal friction factor for trucks. However, little mention of truck longitudinal friction factors is given in current or past research literature. The adopted figures in Table 8.2 were
Car to Road Object Stopping Sight Distance
The concept of car stopping sight distance is illustrated in Figure 8.2. It is measured between the driver’s eye and a small object on the road. SSD values for cars are calculated using the adopted longitudinal friction factor values, are shown in Table 8.3(a).
8.3.4
Truck to Road Object Stopping Sight Distance
A comparison of international sight distance design practices (Ref. 56) noted that SSD only refers to cars. Truck stopping sight distance is not considered by most of the countries reviewed. A typical reason for this can be found in AASHTO (Ref. 1): “The derived minimum stopping sight distances directly reflect passenger car operation and might be questioned for use in design for truck operations. Trucks as a whole, especially the larger and heavier units, require longer stopping distances for a given speed than passenger vehicles do. However, there is one factor that tends to balance the additional braking lengths for trucks for given speeds with those for passenger cars. The truck operator is able to see the vertical features of the obstruction substantially farther because of the higher position of the seat in the vehicle. Separate stopping sight distances for trucks and passenger cars, therefore, are not used in highway design standards.”
Figure 8.2: Stopping Sight Distance
RURAL ROAD DESIGN
27
However, this is quite contrary to the findings of a review of references on truck performance characteristics (Ref. 48, 52 and 87), which suggest that the sight distance advantages provided by the higher driver eye level in trucks do not compensate for the inferior braking of trucks. Particularly at locations with lateral sight distance restrictions, the benefits of the higher eye level could be lost and provision of longer SSD or other remedial measures such as signing and higher friction surfaces would be needed.
●
The braking of articulated vehicles must be in the form of controlled braking without wheel locking in order to avoid jackknifing if wheels lock at different times. Without the aid of antilock braking systems, the friction coefficient used in controlled braking is usually less than that for locked wheel braking. The friction coefficient for cars in Table 8.2 involve locked wheel braking.
●
Truck tyres are designed primarily for wear resistance. Consequently, they tend to have lower wet friction coefficients than cars.
The reasons for the longer truck braking distances include: ●
Poor braking characteristics of empty trucks. Empty trucks have poor braking characteristics and this is reflected in comparatively high crash rates. The problem relates to the suspension and tyres, which are designed for maximum efficiency under load.
In situations where driver eye height provides no advantage, the only parameter that offsets the poorer braking performance of trucks is the assumed lower operating speed as per Table 7.2. Therefore, some further justification or basis of the truck operating speeds should be given. For example:
●
Uneven load between axles. When the load is not evenly distributed between axles, one axle can slip sideways and create instability in others (up to 15% of braking efficiencies can be lost).
●
The lower operating speed for trucks is an average condition with truck speeds varying more than car speeds due to grades, poorer acceleration, etc.
● ●
Inefficient brakes of articulated trucks. Fifty percent of trucks tested on the roads in the US could not meet the required braking standards. Many drivers immobilise their front brakes to reduce the possibility of jack-knifing.
When checking braking and stopping sight distance provision for trucks, it is acceptable to use the lower truck operating speed for a corresponding car operating speed. This is because an acceptable level of safety is provided through the assumptions of:
● ●
Effect of road curvature. Trucks require longer SSD on curves than on straights because some of the friction available at the road/tyre interface is used to hold the vehicle in a circular path.
Wet conditions; Unlade state; No antilock braking system; and There is no additional assumption of a reduction in operating speed due to wet conditions.
● ● ●
Table 8.3(a): Minimum Car Stopping Sight Distances (1.05m to 0.2m) Operating Speed (km/h) 50
60
70
80
90
100
110
120
130
0.52
0.48
0.45
0.43
0.41
0.39
0.37
0.35
0.35
2.5 Des. min.
54
71
91
114
140
170
205
245
280
(m, level grade) 2.0 Abs. min.
47
63
82
103
128
157
190
229
262
2%
-
-1
-2
-3
-4
-5
-7
-9
-11
4%
-1
-2
-4
-5
-7
-9
-13
-17
-21
6%
-2
-3
-5
-7
-10
-14
-18
-24
-31
8%
-3
-4
-7
-9
-13
-17
-23
-30
-38
-2%
-
1
2
3
4
6
7
10
14
-4%
2
3
4
6
8
12
16
21
27
-6%
3
4
7
10
13
18
25
34
44
-8%
4
6
9
13
19
26
36
48
62
Longitudinal Friction Factor
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SSD
Correction for Grade (m) Upgrade
Downgrade
Note: • Desirable minimum stopping sight distances are calculated for a reaction time of 2.5 seconds and absolute minimum stopping sight distances are calculated for a reaction time of 2.0 seconds. • Corrected stopping sight distances should be rounded conservatively to the nearest 5 metres.
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RURAL ROAD DESIGN
Figure 8.3: Truck Stopping Sight Distance
Table 8.3(b): Minimum Truck Stopping Sight Distances (2.4m to 0.2m) Operating Speed (km/h) 50
60
70
80
90
100
110
120
130
0.29
0.29
0.29
0.29
0.29
0.28
0.26
0.25
0.24*
2.5 Des. min.
69
91
116
143
173
210
259
310
367
(m, level grade) 2.0 Abs. min.
62
82
106
131
160
197
244
294
349
2%
-6
-9
-12
-16
-20
-24
-30
-35
-42
4%
-11
-16
-22
-28
-36
-44
-53
-64
-78
6%
-15
-22
-30
-39
-49
-60
-73
-87
-110
8%
-19
-27
-36
-47
-60
-74
-90
-107
-125
-2%
8
11
15
20
25
31
37
45
55
-4%
18
26
35
46
58
71
86
103
122
-6%
32
46
62
81
102
126
153
182
212
-8%
52
74
101
132
167
206
249
296
345
Longitudinal Friction Factor
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SSD
Correction for Grade (m) Upgrade
Downgrade
Note: • Desirable minimum stopping sight distances are calculated for a reaction time of 2.5 seconds and absolute minimum stopping sight distances are calculated for a reaction time of 2.0 seconds. • Corrected stopping sight distances should be rounded conservatively to the nearest 5 metres. * Extrapolated
RURAL ROAD DESIGN
29
To balance between the costs and benefits for making provision for trucks, rural roads are to be designed to cater for cars. Truck stopping sight distances should be used for checking purposes at locations that could be potentially hazardous for trucks (as summarised in Table 5.1).
8.4.2 Overtaking Sight Distance
At crest and sag points truck stopping sight distance is measured as shown on Figure 8.3. The designer should consider measures such as additional signs and line marking to improve safety if stopping sight distance is found to be inadequate for trucks and it is not possible to improve the geometric design. However, it is emphasised that signage and line marking are not substitutes for achieving standard design practices. SSD values for trucks have been calculated using the adopted longitudinal friction factor values are shown in Table 8.3(b).
8.4 Overtaking Sight Distance 8.4.1
General 8.4.2
Overtaking sight distance is the distance required for the driver of a vehicle to safely overtake a slower moving vehicle without interfering with the speed of an oncoming vehicle. It is measured between the driver’s eyes of the overtaking and oncoming vehicles.
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Overtaking sight distance is considered only on two-lane twoway roads. On these roads, the overtaking of slower moving vehicles is only possible when there is a suitable gap in the oncoming traffic accompanied by sufficient sight distance and appropriate line marking. Sections with adequate overtaking sight distance should be provided as frequently as possible, as they are an essential safety measure by reducing driver frustration and risk taking. The desirable frequency is related to the operating speed, traffic volume and composition, terrain and construction cost. Overtaking demand increases rapidly as traffic volume increases, while overtaking capacity in the opposing lane decreases as volume increases. As a general rule, if overtaking sight distance cannot be economically provided at least once in each 5km of road or V/20 which is 3 to 5 minutes of driving time apart, (Ref. 95), consideration should be given to the construction of overtaking lanes (Refer Section 13.4.1). In practice, overtaking zones will usually be the fortuitous result of road alignment and cross section. Because of the large sight distances involved, it is often not practical to achieve overtaking zones through design alone (costly to provide). However, good design practice will include a check on the overtaking zones that are provided and may result in cases where an overtaking zone can be achieved through a practical refinement of the design. More commonly though, the proportion of road that provides overtaking is used in conjunction with traffic volumes to assess the level of service provided by a section of road and hence determine whether overtaking lanes are warranted. The Austroads parameters for determining the start and finish of overtaking zones dictate that there are few passing opportunities on New Zealand roads. The New Zealand practice to provide a desirable minimum overtaking sight distance for vertical curve design is to double safe stopping sight distance.
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RURAL ROAD DESIGN
Overtaking Model
The overtaking manoeuvre has a large number of variables: The judgement of the overtaking driver and the risks he is prepared to take; ●
●
The speed and size of vehicles to be overtaken;
●
The speed of the overtaking vehicle;
●
The speed of a potential on-coming vehicle; and
The evasive action or braking undertaken by the vehicle or the overtaken vehicle. ●
Since the 6th edition of this guide, overtaking has been assessed by means of a model that was derived from research into overtaking on Australian Rural Roads (Ref 95). There are two main considerations with the Overtaking Model: Refer Figure 8.4 ●
Establishment: A minimum sight distance that is adequate to encourage a given proportion of drivers to commence an overtaking manoeuvre. This is called the Establishment Sight Distance (ED) as it establishes a length of road as a potential overtaking zone.
●
Continuation: A critical sight distance, which if maintained for some length of road after the ED has become available, will enable an overtaking driver to either complete or abandon a manoeuvre already commenced with safety. This is called the Overtaking Continuation Sight Distance (OSD). After the establishment sight distance first becomes available, an overtaking zone is assumed to extend as long as this shorter distance remains available, subject to the constraint in the next paragraph.
8.4.3
Determination of Overtaking Provision
ARRB has carried out a major research project on overtaking on Australian rural roads. (Ref. 95). The values in this Guide
Figure 8.4: Overtaking Manoeuvre
(refer Tables 8.4 (a) & 8.4 (b)) are the distances for the 85th percentile overtaking manoeuvres, adopted from the research. These distances indicate the overtaking sight distances to be used in determining the overtaking zones on MCV (Multiple Combination Vehicle) routes.
8.4.4
Determination of Percentage of Road Providing Overtaking
Sections of road assumed to provide overtaking will: ●
Commence at a point where ED is available; and
●
Terminate where OSD ceases to be available, or alternatively at a distance equal to Operating Speed divided by 20 (km) from the last location where ED was available if this is less than the length over which OSD has been maintained. As long as the OSD remains available, any overtaking manoeuvre commenced can be successfully completed. However if the ED does not occur again at intervals, insufficient drivers will be encouraged to commence overtaking, and capacity (at high volumes) or quality of service (at low volumes) will suffer. The distance equal to Operating Speed divided by 20 should be treated as an approximate rather than a precise figure. It corresponds to about 3 to 5 minutes travel time.
Briefly: ●
Establishment Sight Distance is derived from the size of the time gap accepted by a potential overtaking driver and is derived by the time taken to complete phases 1,2,3 and 4 of the total manoeuvre (see Figure 8.4).
ED = GT85
(V + u) 3.6
where:
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GT85 u V
= 85th%ile critical time gap secs. = V/1.17 (speed of slow vehicle) = operating speed
●
Continuation Sight Distance is derived from the time taken to complete phases 2 and 3 of the manoeuvre (see Figure 8.4).
●
The oncoming vehicle is assumed to travel at the operating speed.
●
The overtaken vehicle is assumed to travel at a lesser speed, taken as the mean speed for its direction of travel.
●
The sight distances with the 1.05m driver eye height to 1.05m object height are used in this guide.
●
The distance travelled by oncoming traffic is represented in Figure 8.4 by phase 4.
In checking a length of road, the OSD will be found to be the critical parameter in allocating a ‘percent allowing overtaking’ to the road section. The OSD ensures that the road distance used by the overtaking vehicle would be visible at the ‘point of no return’, and an approaching vehicle would be visible if it is within the zone where it could affect the manoeuvre.
The Operating Speeds to be used in selection of the overtaking distances will be the Section Operating Speed over a length of road in both directions. A section of road must be used rather than an individual geometric element, as Operating Speed may vary. Also, since one element in the overtaking provision is the speed of the oncoming vehicle, and as Operating Speed may vary by direction of travel, the mean of both directions must be used. The proportion of road offering overtaking provision is the sum of such sections, divided by the overall length of the road section being considered. O.P.
=
∑ O.L’s x 100 TSL
where: O.P. ∑ O.L’s T.S.L.
= Proportion of road offering overtaking provision (%) = Sum of overtaking lengths in road section (m) = Total road section length (m)
The sight distances to be used in the analysis of overtaking are presented in Table 8.4. The time gaps from which they were derived are also shown.
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31
Table 8.4 (a): Overtaking Sight Distances for Determining Overtaking Zones on MCV Routes when MCV speeds are 10km/h less than the Operating Speed. Road Section Operating Speed (km/h)
Continuation Sight Distance (m)
Establishment Sight Distance (m)
Overtaken Vehicle speed (km/h)
Prime mover Semitrailer
B-Double
Type 1 Road Train
Type 2 Road Train
580
260
280
310
350
670
730
320
340
380
430
770
820
890
370
400
460
530
890
930
990
1,080
450
490
550
650
1,070
1,120
1,200
1,310
540
580
660
770
Overtaken Vehicle
Semitrailer B-Dble
Road Trains
Prime mover Semitrailer
B-Double
Type 1 Type 2 Road Train Road Train
70
50
50
490
510
540
80
59
59
610
630
90
67
67
740
100
76
76
110
84
84
Given a low eye height of 1.05m, most car drivers cannot adequately distinguish differences in sight distance for values greater than about 1000m. Therefore, listed sight distance values greater than 1000m can be assumed to be satisfied whenever the actual sight distance exceeds 1000m. The listed sight distance values have been derived from the Troutbeck (1981) overtaking model. Sight distance values have been rounded to the nearest 10m. Given the inherent level of precision in the overtaking model, it would be incorrect to determine that an overtaking zone does not exist when the actual sight distance falls below a relevant listed value by about 10m.
Table 8.4 (b): Overtaking Sight Distances for Determining Overtaking Zones on MCV Routes when MCV speeds are equal to the Operating Speed.
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Road Section Operatin gSpeed (km/h)
Overtaken Vehicle speed (km/h)
Establishment Sight Distance (m)
Continuation Sight Distance (m)
Overtaken Vehicle
Semitrailer B-Dble
Road Trains
Prime mover Semitrailer
B-Double
Type 1 Road Train
Type 2 Road Train
Prime mover Semitrailer
B-Double
Type 1 Road Train
Type 2 Road Train
70
60
60
570
600
640
690
300
320
360
420
80
69
69
710
740
790
860
370
400
450
510
90
77
77
850
890
950
1,040
440
470
530
620
100
86
84
1,020
1,070
1,130
1,240
530
560
630
740
110
94
84
1,230
1,290
1,200
1,310
620
680
660
770
Given a low eye height of 1.05m, most car drivers cannot adequately distinguish differences in sight distance for values greater than about 1000m. Therefore, listed sight distance values greater than 1000m can be assumed to be satisfied whenever the actual sight distance exceeds 1000m. The listed sight distance values have been derived from the Troutbeck (1981) overtaking model. Sight distance values have been rounded to the nearest 10m. Given the inherent level of precision in the overtaking model, it would be incorrect to determine that an overtaking zone does not exist when the actual sight distance falls below a relevant listed value by 10m.
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RURAL ROAD DESIGN
8.5 Manoeuvre Sight Distance
Table 8.5: Evasive Action Distance Operating speed (km/h)
Designers shall make every effort to provide car stopping sight distance along traffic lanes on all roads. However, in some circumstances manoeuvre sight distance (MSD) may be used to avoid costly construction. MSD is generally only 6% less than SSD. MSD, therefore, is the absolute sight distance that must be provided. For example, on a two-lane two-way road, it may be much cheaper to provide full width paved shoulders on an existing substandard crest curve than to reconstruct with improved vertical geometry. Manoeuvre sight distance may be used on isolated vertical curves on a straight or sufficiently large radius horizontal curve where lowering of the grade line would mean expensive excavation into hard rock materials or major geological problems. Manoeuvre sight distance should not be used on a horizontal curve with a radius that requires close to the absolute maximum side friction. The designer must ensure that the pavement width is sufficient to enable drivers to manoeuvre around stationary or slow moving vehicles or an object on the road. Sealed shoulders with a desirable minimum width of 2.5m (or absolute minimum width of 1.5m) can provide a reasonable space for evasive action provided the combined seal width of lane plus sealed shoulder exceeds 5m. However if the area adjacent to the shoulder is clear of hazards and traffic volumes are low, an unsealed shoulder may be accepted.
Speed Range Slowed To (km/h)
50
15.0
30 - 35
60
25.0
35 - 40
70
35.0
40 - 50
80
50.0
40 - 50
90
70.0
40 - 60
100
95.0
35 - 60
110
125.0
35 - 60
120
155.0
25 - 60
130
190.0
25 - 60
Note: * Derived from Queensland “Road Planning and Design Guide”. Table 8.6: Manoeuvre Sight Distance Manoeuvre Manoeuvre Sight Time Distance (m) (sec)
Operating Speed (km/h)
Reaction Time (sec)
50
2.0
3.2
45
60
2.0
3.6
60
70
2.0
3.9
75
80
2.0
4.3
95
90
2.0
4.8
120
100
2.0
5.6
155
d3 = the distance travelled during the evasive action (m)
110
2.5
6.3
195
Evasive action distance is the distance a driver requires to undertake an evasive manoeuvre. The evasive manoeuvre consists of braking to comfortable speed followed by a swerving manoeuvre to avoid the object. The values given in Table 8.5 are based on empirical evidence gained in Australia.
120
2.5
7.0
235
130
2.5
8.0
275
8.5.1
Derivation
Manoeuvre sight distance, for a single vehicle to manoeuvre around on obstruction is the sum of two components: MSD = d1 + d3 where: d1
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Evasive Action Distance (m)*
(R V) = the distance travelled during the reaction time = T 3.6 (see section 8.3.1) (m)
●
Significant improvement is unlikely, as a fivefold light increase is necessary for a 15 km/h increase in speed, and a tenfold increase for a 50% reduction in object size;
●
In any case, the joint requirements of driving vision and minimising glare for oncoming traffic set limits to beam intensity.
The manoeuvre sight distance for a range of operating speeds is shown in Table 8.6.
8.6 Headlight Sight Distance The most common obstruction on a normal rural road is another vehicle that may or may not be stopped. Even if its lights are not operating, it will have retro-reflective material at strategic locations, situated higher than the ‘object cut off height’ used in the stopping sight distance calculations. As far as small, unilluminated objects are concerned, research has shown that: ●
Only larger, light-coloured objects can be perceived at speeds above 80 km/h at the stopping sight distances set out herein;
A general limit of 120m to 150m sight distance is all that can be safely assumed for visibility of an object on a bitumen roadway. This corresponds to a satisfactory stopping distance for 80 km/h to 90 km/h, and a manoeuvre time of about 5 seconds at 100 km/h. Beyond this, it is only large or lightcoloured objects that will be perceived in time for reasonable evasive action to be taken on unlit roads. The relatively small number of accidents involving objects on the roadway at night is probably due to the factor of safety implicit in the various assumptions in sight distance calculations.
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In addition to the problem of beam illumination, the question of the angle of the beam is relevant in sags. It is inappropriate for the beam to be aimed above the horizontal position because of glare to opposing drivers and a figure of 0.5o depression is an appropriate assumption. A headlight aiming angle of 0.5 degrees depressed will allow on effective 1 degree elevation of the beam to be used in design due to vertical spread. The length of sag curves to give stopping sight distance measured from a headlight height of 750 mm to zero is considerably more than that required to achieve reasonable riding comfort. In addition, increasing the length of sag curve to produce a theoretical sight distance may not give the desired result. If there is a horizontal curve in addition to the sag, the headlights shine tangentially to the horizontal curve and off the pavement (refer Figure 10.1). The only method of achieving full compatibility between theoretical sight distances by day and night is by roadway lighting. However, two matters act to redress the imbalance, one outside the control of designers and one at least partly in their domain. Firstly, the majority of hazards encountered comprise other vehicles, which are either illuminated or visible because of the requirement for retro-reflective fittings. Secondly, because retro-reflective materials respond too much lower light levels than the non-reflective objects, they are perceived well outside the direct headlight beam. Thus, the provision of retro-reflective road furniture (including items like flood gauge markers, which frequently occur in sags) is an important offset to the difficulties of night time driving.
8.7 Horizontal Curve Perception Distance
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A major characteristic of low speed roads and intermediate speed roads is the way drivers will speed up on longer straights and through larger radius horizontal curves then slow down where necessary for smaller radius curves. Since the 6th edition of this guide, such roads have been designed so that the geometric elements matched the operating speeds along the road. This means that when vehicles have to slow down for a horizontal curve, drivers must see a sufficient amount of the curve in order to perceive its curvature, react and slow down appropriately for the curve. As a result of not perceiving the curvature, drivers may not slow down appropriately for them. Therefore, these curves should only be used when the perceived curve operating speed is no more than 5 km/h less than the operating speed on the approach to the curve. Normally, sufficient sight distance for a horizontal curve is provided through the practice of not having a horizontal curve start over a crest. However, there are times where this cannot be avoided and the following criteria should be applied in order to check that sufficient visibility is provided for the curve. ●
A driver eye height of 1.05 m.
●
A zero object height because the driver needs to see the road surface in order to perceive the curvature. Road edge guide posts and cut batters can only be considered as supplementary aids.
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RURAL ROAD DESIGN
●
A driver needs to see sufficient length of the curve in order to judge its curvature. The driver must be able to see the minimum of : – 5 degrees of arc – About 80 metres of arc – The whole curve.
However, if the curve is transitioned, at least 80% of the transition length needs to be seen and desirably all of the transition. ●
The length of arch that needs to be perceived must be seen from a point that allows the driver to react then decelerate. See section 8.2.3 for the reaction time. The deceleration should only require comfortable braking. Therefore a maximum deceleration rate of 2.5m/s/s should be used. Typically, this means a distance of 25 m will accommodate a 10 km/h speed reduction from 90 km/h and 40 m will accommodate a 15 km/h speed reduction. Deceleration distances should be adjusted for the effect of grade.
●
The sight distance is the sum of the reaction distance, arc length for perception and deceleration distance. If the curve is transitioned, it is possible for the deceleration distance to coincide with up to the first half of the transition. If the curve is untransitioned, deceleration up to the curve tangent point can be assumed.
Provision of horizontal curve perception distance may require a larger crest than is required for stopping sight distance.
8.7 Horizontal Curve Perception Distance (sequential)
4
GEOMETRIC DESIGN GUIDELINES
PA R T
speed of successive geometric elements; and
9.
H O R I Z O N TA L A L I G N M E N T ●
Diminishing radii should be avoided on steep downgrades; and
●
Motorcyclists may experience instability of the motorcycle as a result of the abrupt changes in centripetal force required due to the change in radius.
9.1 General The horizontal alignment of a road is usually a series of straights (tangents) and circular curves that may or may not be connected by transition curves. The following section outlines various design criteria that are to be considered when adopting a horizontal alignment.
9.2 Movement on a Circular Path As a vehicle traverses a circular curve, it is subject to a centripetal force that must be sufficient to balance the inertial forces associated with the circular path. For a given radius and speed a set force is required to maintain the vehicle in this path. In road design, this is provided by side friction developed between tyre and pavement and by superelevation. For the design vehicle types and side friction coefficient and normal values of superelevation, side friction coefficient and curve radius the following formula is accepted: e+f
=
V2 127R
9.3 Horizontal Curves
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9.3.1.3 Broken Back Curves Broken back curves have a straight less than 0.60V long or a large radius curve between two relatively low radius unidirectional curves. Generally the following guidelines apply to broken back curves: ●
These curves are unsightly and should be avoided where possible; and
●
Where unavoidable, the length of straight should be no less than the design speed in metres.
...... (9.1)
where e = pavement superelevation (m/m or tangent of angle). This is taken as positive if the pavement falls toward the centre of the curve f = side frictional factor (see Section 9.4) V = speed of vehicle (km/h) R = curve radius (m).
9.3.1
Although inconclusive, some literature suggests that a small radius curve immediately following a large radius curve (both turning in the same direction) gives drivers inadequate perception of the small radius. This is reported to lead to a higher single vehicle accident rate. Generally, this geometry should be avoided.
Types of Horizontal Curves
9.3.1.1 Reverse Curves A reverse curve is a section of road alignment consisting of two curves turning in opposite directions and having a common tangent point at the end and start of transition curves or being joined by a short length of tangent. This tangent length is desirably 0.6V metres long. However, where deceleration is required on the approaches to a lower radius curve, sufficient distance must be provided to enable drivers to react and decelerate.
9.3.1.4 Transition Curves Transition curves are normally used to join straights and circular curves, although they may be omitted when largeradius curves are used. Transition curves: ●
Provide a natural path for vehicles moving from a straight to a circular curve and enable centripetal acceleration to increase gradually from zero at the start of the transition to their maximum value at the start of the circular curve. If a transition curve is not provided some drivers will occupy adjoining lanes when entering and leaving the curve;
9.3.1.1 Reverse Curves
9.3.1.2 Compound Curves Curves comprising two or more contiguous curves of different radii in the same direction are known as compound curves. Generally the following guidelines apply to compound curves: ●
Radii less than 1,000 m are undesirable;
●
Where radii less than 1,000 m are unavoidable, there should be no more than 10 km/h difference in the design
RURAL ROAD DESIGN
35
●
Allow for superelevation development and pavement widening; and
●
Improve the appearance of the curve ahead.
The need for transition curves was learned from the early days of railway building when problems were encountered with passenger comfort and track wear due to the sudden application of curvature with untransitioned curves. However, the fact that road vehicles are not rigidly confined to a specific path together with the characteristics of road vehicle steering mean that shorter transition lengths are more appropriate than those used for railways. This is why it is current road design practice to base transition lengths on superelevation runoff length (see Section 9.7.4) instead of a comfort criterion that was once used. The use of longer transitions than those based on superelevation runoff length should be avoided when curve operating speeds are such that drivers have to reduce speed for the curve. Drivers regulate their speed from the apparent curvature of the road ahead and in practice, there is some variation in curve entry speeds. In these circumstances, longer transitions may cause drivers to perceive a higher standard of curvature than there is, with consequent increased speed and friction demand on the circular section of the curve. Overseas studies have found that there have been higher accident rates on some curves with a combination of long transition (typically with more than twice the length based on superelevation development) and small to medium radius.
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For most curves the average driver can achieve a suitable transition path within the limits of normal lane width. However, with particular combinations of high speed, heavy vehicles and a large difference in curvature between successive geometric elements, the resultant vehicle transition path can result in a sideways movement within the lane and sometimes actual occupation of adjoining lanes. Trucks have more problems because of their wider wheelbase and heavier, less responsive steering. Trucks also require more width on curves because: ●
Rear axles of semi trailers track outwards when travelling around curves at speed;
●
At low speeds the trailers track inwards;
●
Truck trailers swing from side to side at speed; and
●
The effective width of trucks increases on curves (vehicle swept path considerations).
In the abovementioned circumstances, transition curves have been applied to obtain the following advantages: ●
●
A properly designed transition curve allows the vehicle’s centripetal acceleration to increase or decrease gradually as the vehicle enters or leaves a circular curve. This transition curve minimises encroachment on adjoining traffic lanes. The transition curve length provides a convenient desirable arrangement for superelevation runoff. The transition between the flat cross slope and the fully superelevated section on the curve can be effected along the length of the transition curve in a manner closely fitting the speedradius relation for the vehicle traversing it.
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RURAL ROAD DESIGN
●
Where superelevation runoff is affected without a transition curve, it has been common practice to match the superelevation runoff with the likely transition path the vehicles take when entering or leaving the circular curve.
●
A transition facilitates the change in width where the pavement section is to be widened around a circular curve. Use of transitions provides flexibility in the widening on sharp curves.
●
The appearance of rural roads is enhanced by the application of transitions.
Despite the advantages of using transition curves, there are also possible adverse effects associated with transitions. Some research studies undertaken indicate the following: ●
Transitions at the start of horizontal curves give the impression of magnifying the radius of the curve ahead. This encourages drivers to approach the curve too quickly;
●
Transitions hide the tangent-to-curve point making it difficult to identify the start of the curve. This results in drivers reducing speed on the approach to curves so that they can judge when to commence braking;
●
Transition curves at the start of circular curves are reported to lead to a higher single vehicle accident rate than circular curves without transitions, for the above reasons. However, other studies indicate that single vehicle accident rates on circular curves without transitions are similar to those for circular curves with transitions (Ref. 66); and
●
When drivers brake on curves, a combination of forces applies on the tyres, effectively reducing the maximum force that can be developed for braking or cornering. Articulated trucks also have problems with braking on curves because of the tendency of these vehicles to jackknife. On curves with transition approaches, braking occurs on the spiral. This could create a problem if the driver does not commence braking sufficiently early.
Sections of road where the operating speed is less than 60 km/h do not require transition curves. The most frequently used form of transition is the clothoid (or Euler) spiral where the curvature changes at a uniform rate along the curve. The clothoid is easier to set out in the field compared with other forms of transition curves (the Lemniscate and the cubic parabola). Basic properties of the clothoid transition are shown in Appendix A A transition may be omitted when the associated shift (see Appendix A) is less than 0.25m.
9.4 Side Friction Factor A vehicle travelling round a circular horizontal curve requires a radial force that tends to effect the change in direction and consequent centripetal acceleration. This force is provided by side friction between the tyres and the road surface. If there is insufficient force provided by side friction, the vehicle will tend to slide tangentially to the road alignment.
Side friction factor f is the friction force divided by the weight perpendicular to the pavement and is expressed as the following formula: f
=
V2 –e 127R
...... (9.2)
where V = operating speed, km/h R = radius of horizontal curve, m e = superelevation, m/m.
The values of side friction factor f for use in geometric design are shown in Table 9.1.
The upper limit of this factor is that at which the tyre is skidding or at the point of impending skid (Ref 1). The side friction factor at which side skidding is imminent depends on: ● ● ●
Vehicle operating speed; The type and condition of the roadway surface; and The type and condition of the tyres.
If the vehicle speed were less than the permissible operating speed V, the side friction factor being called upon would be less than the design maximum side friction factor fmax , and as the travel speed approaches V, then f will approach fmax . The speed at which f just equals fmax can be considered as a limiting (safe) speed Vs and if a vehicle is travelling in excess of Vs, then the side friction factor being called upon will exceed fmax . Vs, is called the Limiting Curve Speed Standard. The amount by which Vs exceeds V can be considered to indicate a lower bound for the margin of safety against the friction being demanded exceeding the friction that is available. That is, the quantity Vs – V can be considered a design margin of safety. The available friction can vary both spatially (from one curve to another, at the same time) and temporally (from one time to another time at the same curve). Temporal variations in the available side friction factor are often due to changes in weather and are inevitable, and the most practicable way to minimise the total variation is to minimise the spatial variations by providing a spatially uniform road surface.
It is important to note that the absolute maximum values for f given in Table 9.1 assume construction and maintenance techniques that will ensure an adequate factor of safety against skidding. The susceptibility of the wearing surface to polishing, the macro-texture of the surface and the amount of bitumen used, evident at wearing surface, are all important matters in the initial construction of a pavement contributing to skid resistance. Freedom from contamination by oil spillage or loose aggregate and resealing when surface texture becomes too smooth are important aspects in maintenance of skid resistance. Normally, a pavement, which is properly maintained, will retain adequate resistance to skidding under all but extreme conditions of driver behaviour or weather. The desirable maximum values should be used on intermediate and high-speed roads with uniform traffic flow, on which drivers are not tolerant of discomfort. These values should be adopted, if possible, to allow vehicles to maintain their lateral positions within a traffic lane and be able to comfortably change lanes if necessary. On low speed roads with non-uniform traffic flow, drivers are more tolerant of discomfort, thus permitting employment of absolute maximum amount of side friction for use in design of horizontal curves (Ref. 1) The f values given in Table 9.1, which apply only to sealed pavements, have been derived from observations of driver speed behaviour on rural road curves and revised by ARRB (Ref 42). A reduction of 0.04 is applied to all values when applied to unsealed pavements (Ref. 66).
9.5 Minimum Radii Values For Horizontal Curves
Table 9.1: Side Friction Factors
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Variation in the margin of safety arises from both variations in the available friction (friction supply) and the friction demanded (friction demand) by drivers. The geometric design will have little (if any) effect on the available friction, but it can influence the behaviour of drivers (and particularly their choice of speed) (Ref 82).
Operating Speed (km/h)
Des max.
f Abs max.
50
0.30
0.35
60
0.24
0.33
The minimum radius of a horizontal curve for a given operating speed can be determined from the formula (9.1). It can be rearranged as follows:
70
0.19
0.31
Rmin =
80
0.16
0.26
90
0.13
0.20
100
0.12
0.16
where Rmin = V = emax = fmax =
110
0.12
0.12
120
0.11
0.11
130
0.11
0.11
9.5.1 Minimum Radius Values
V2 127(emax + fmax ) minimum radius (m) operating speed (km/h) maximum superelevation (m/m) maximum coefficient of side frictional force developed between vehicle tyres and road pavements.
Using the values for fmax from Table 9.1, the approximate minimum radii for various vehicle speeds for typical maximum superelevations are as shown in Table 9.2.
RURAL ROAD DESIGN
37
Table 9.2 Minimum Radii of Horizontal Curves Based on Superelevation and Side Friction at Maximum Values Operating
Km/h Minimum Radius m (rounded up)
Speed
Flat Terrain e = 3%
Undulating Terrain e = 6%
Rolling Terrain e = 7%
Mountainous Terrain e = 10%
(km/h) Des min
Abs min
Des min
Abs min
Des min
Abs min
Des min
Abs min
50
60
52
56
40
53
47
49
44
60
105
79
95
73
91
71
83
66
70
175
113
154
104
148
102
133
94
80
265
173
229
157
219
153
194
140
90
315
219
335
245
319
236
277
213
100
525
415
437
358
414
342
-
-
110
635
635
529
529
501
501
-
-
120
810
810
667
667
-
-
-
-
130
950
950
782
782
-
-
-
-
9.5.2
On Steep Down Grades
On steep down grades, the minimum curve radius from Section 9.5.1 should be increased by 10% for each 1% increase in grade over 3%.
Step 4
RMIN on Grade = RMIN from Table 9.2 [1 + (G – 3)/10]
Prepare a trial grade line, taking into account vertical controls and drainage aspects. Co-ordinate horizontal and vertical alignments as in Section 11.
where G = grade (%) R = radius (m)
9.6 Horizontal Alignment Design Procedure Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
end of straights because of the high speeds that can be developed at these locations.
Step 1
On down grades, minimum curve radii should be increased by 10% for each 1% increase in grade over 3%. Refer Section 9.5.2. Step 5 Check that all radii are compatible with estimated vehicle operating speeds using the procedure described in Section 7.
Identify all major controls on the alignment and categorise them as mandatory or discretionary.
Step 6
Step 2
Adjust the alignment so that:
Decide upon an operating speed that is appropriate both for the class of road and for the terrain. Minimum radii for these operating speeds are then obtained from Table 9.2. Radii used are chosen to fit the terrain and desirably should exceed the minimum.
●
All mandatory controls are met;
●
Discretionary controls are met as far as possible;
●
Curve radii are consistent with operating speeds at all locations;
●
Other controlling criteria are satisfied with special consideration given to the location of intersections and points of access to ensure that minimum sight distances and critical crossfall controls are met; and
Step 3 Prepare a trial alignment using a series of straights and curves, using the radii determined in Step 2. On low and intermediate speed alignments, curves used should generally be consistent. Special care must be taken with curves at the
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RURAL ROAD DESIGN
●
Earthworks are minimised.
Where minimum standards cannot be achieved and compromises have to be made, the designer requires a broad understanding of basic theory and the assumptions made in the development of the guidelines.
9.7 Superelevation
pavement will dictate a different superelevation. This is acceptable if the resultant side friction is suitable for the curve design speed and consistent with that for any adjacent curves. With the “linear distribution method”, the superelevation (e1) for a curve of radius R, which is greater than Rmin is given by: e1 =
The superelevation to be adopted is chosen primarily on the basis of safety, but other factors are comfort and appearance. The superelevation applied to a road should take into account:
Note that fmax may be either the absolute maximum value or the desirable maximum value for the design speed V.
●
Operating Design Speed of the curve, which is taken as the speed at which the 85th percentile driver is expected to negotiate it;
The value of e1 is usually rounded upwards (eg. 4.0% but 4.1% becomes 5%) and the corresponding coefficient of side friction is calculated from:
●
Tendency of very slow moving vehicles to track towards the centre;
f1 =
●
Stability of high laden commercial vehicles;
●
Difference between inner and outer formation levels, especially in flat country; and
●
Length available to introduce the necessary superelevation.
However, it is noted “although the dynamics of vehicle movement show that the selection of superelevation is important for traffic safety, research findings suggest that it does not make much of a difference for drivers, who are primarily affected by the radius of curvature in choosing their speed” (Ref. 61). The proportion of centripetal acceleration as a result of the combination of superelevation and sideways friction needs to be controlled to provide a constant driving experience. There are a number of methods to determine the superelevation (and hence resultant side friction) for curves with a radius larger than the minimum radius for a given design speed. It must also be reiterated that the length of such curves should be checked to ensure that the length does not cause the operating speed to increase beyond the curve design speed when the design speed is less than 110 km/h. Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
V2emax 127R(emax + fmax )
The “linear method” distribution to be used in this Guide is for the superelevation and side friction to be varied linearly from 0 for R = infinity to emax for Rmin. This then results in the proportions of the required centripetal acceleration due to superelevation and side friction being the same for larger radii as they are at Rmin, considering the following practical considerations: ●
For construction expediency, superelevation values are normally rounded (upwards) to a multiple of 1% so that there is a corresponding adjustment of side friction.
●
The perceived benefits of uniformity are only possible on high-speed rural roads (where the design or operating speed exceeds 100 km/h), because section operating speeds vary on intermediate and low speed rural roads.
●
Other methods have been used in the past so that there are likely to be many cases where the reuse of existing
V2 – e1 rounded 127R
However, if specific controls cannot be met then actual e values may be used. With different possibilities for emax and fmax (absolute maximum vs. desirable maximum) different values of superelevation may be attributed to a given combination of radius and design speed. However, the subjective basis of the “linear distribution method” (and indeed most other methods) and the practice of rounding the superelevation value, allows a practical rationalisation to be made, refer Figures 9.1(a) & (b) and Figures 9.2(a) & (b). For rural roads, rationalisation of the parameters has been achieved by distributing the parameters. High speed rural roads use 6% as the maximum e that should be applied. Intermediate speed rural roads of 80 to 100km/h, use a maximum e of up to 7%. Low speed rural roads may use up to a maximum e of 10%. Superelevation of 10% should not be used where there are vehicles with high centres of gravity. In addition, the rationalisation of both desirable and absolute maximum f values has been used for superelevations of 6% to zero. For superelevations of 7%, 8% 9% and 10%, the maximum values of f as per Table 9.1 have been used. This rationalisation will provide high-speed rural roads with the best practice control, over the variation of centripetal acceleration. This gives the best overall consistency in the margin of safety, which is defined as the difference between the speed at which the maximum permissible design side friction would be called upon and the design speed (Ref. 82). In New Zealand the practice has traditionally been to reduce the side friction demand at radii less than the minimum for e any design speed using the factor e + f as a constant. This method is described in Transit New Zealand’s State Highway Geometric Design Manual and is the only method to be used in New Zealand.
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39
Figure 9.1(a): Relationship between Speed, Radius and Superelevation Based on Desirable Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
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Figure 9.1(b): Relationship between Speed, Radius and Superelevation Based on Desirable Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
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RURAL ROAD DESIGN
Figure 9.2(a): Relationship between Speed, Radius and Superelevation Based on Absolute Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
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Figure 9.2(b): Relationship between Speed, Radius and Superelevation Based on Absolute Maximum f for e > 6% and a Linear Distribution of f for e ≤ 6%
RURAL ROAD DESIGN
41
9.7.1
Maximum Values of Superelevation
Use of maximum superelevation will need to be applied in steep terrain or where there are constraints on increasing the radius of an individual curve in a group. The current design practice shows that superelevation exceeding 7% is rarely used. In mountainous terrain there is normally insufficient distance to fully develop steep (more than 7%) superelevation and in less rugged terrain the use of steep superelevations is questionable considering the potential adverse effect on high centre of gravity vehicles. Therefore, the absolute maximum superelevation should be 7% with 6% being the normal maximum superelevation for high-speed rural roads. The maximum superelevation (low speed 70
60
> 70
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Radius, R (m) 75
4.3
4.0
100
4.1
3.8
100 – 200
3.8
3.8
> 200 Notes: All lane widths have been calculated using 0.6m for the lateral clearance, C, and have been rounded up to the nearest 0.1 m Radii below absolute minimum radii for operating speed – not to be used. Refer Table 9.2 Lane widening is not required. A standard lane width of 3.5m is adequate. Where the operating speed is substantially < 60 km/h, lane widening should be calculated using the formula for Wc.
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Figure 9.5: Horizontal Stopping Sight Distance
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●
The swept path width of the design vehicle accommodates the swept path width of smaller vehicles plus provides room for steering variation (and driver skill variation) with the smaller vehicles; and
●
The now common use of full width or part width paved and sealed shoulders compensates for not having a steering allowance component for the design vehicle.
Table 9.10 shows the width of traffic lane, including widening for a range of circular curves and design vehicles. For lane widening with transitioned curves, it is normal practice to apply half of the curve widening to each side of the road. However, this means that the shift associated with the transition (shift = LP2/24R, where LP is the length of transition curve, and R is the radius of the circular arc) must be greater than the curve widening that is applied to the outer side of the curve so that the design vehicle will make use of the widening and for appearance. This will usually only be a problem when the curve widening has to suit a road train and a greater proportion of the total widening will have to be applied on the inside of the curve. The painted centreline will then be offset from the control line in order to provide equal lane widths. For untransitioned curves, it is normal practice to apply all the curve widening to the inside of the curve with the painted centreline then being offset from the control line in order to provide equal lane widths. This practice aids drivers in making their own transition. For more information refer to Section 11.2 Traffic Lane Width and to Guide to the Geometric Design of Major Urban Roads (Ref. 40). Refer Section 12.2, Traffic Lane Width.
9.11
Sight Distance on Horizontal Curves
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Horizontal curves with minimum radii shown in Table 9.2 do not necessarily meet the sight distance requirements described in Section 8. Where a lateral obstruction off the pavement such as a bridge pier, cut slope or natural growth restricts sight distance, the stopping sight distance appropriate to the design speed of the curve determines the minimum desirable radius of curvature.
9.11.1 Benching for Visibility on Horizontal Curve
Figure 9.5 shows the relationship between horizontal sight distance, curve radius and lateral clearance to the obstruction and is valid when the sight distance at the appropriate design speed is not greater than the length of curve. This relationship assumes that the driver’s eye and the sighted object are above the centre of the inside lane, 1.75m in from the outer edge of lane based on a standard 3.5 m lane width. When the design sight distance is greater than the length of curve, a graphical solution is appropriate. For alignments on lower speed roads, particularly in difficult terrain, it may not be feasible to achieve the 2.5 seconds reaction time stopping sight distances shown in Section 8. Increasing curve radius to improve the sight distance may increase the operating speed so that longer, and still unavailable, design stopping sight distances are required. In these situations, the designer should provide the maximum sight distance practicable, and ensure that it is not less than the stopping sight distance corresponding to a 2.0 second reaction time. Where sight benches in side cuttings are required on horizontal curves or a combination of horizontal and vertical curves, the horizontal and vertical limits of the benching are determined graphically or by modeling.
9.11.1
Benching for Visibility on Horizontal Curves
Benching is the widening of the inside of a cutting on a curve to obtain the specified sight distance. It usually takes the form of a flat table or bench over which a driver can see an approaching vehicle or an object on the road. In plan view, the envelope formed by the lines of sight fixes the benching. The driver and the object he is approaching are assumed to be in the centre of the inner lane and the sight distance is measured around the centre line of the lane, the path the vehicle would follow in braking. Benching adequate for inner lane traffic more than meets requirements for the outer lane. Where a horizontal and crest vertical curve overlap, the line of sight between approaching vehicles may not be over the top of the crest but to one side and may be partly off the formation. Cutting down the crest on the pavement will not increase visibility if the line of sight is clear of the pavement, and the bottom of the bench may be lower than the shoulder level. In these cases, as well as in the case of sharp horizontal curves, a better solution may be to use a larger radius curve so that the line of sight remains within the formation. However, this will tend to increase the operating speed, which in turn will increase the sight distance required.
9.11.2
Other Restrictions to Visibility
There are other minor constraints on sight distance that must be kept in mind by the designer: ●
In avenues of trees, visibility can be reduced at a sag owing to the line of sight being interrupted by the foliage. The same may happen where a bridge crosses a sag and the line of sight is obstructed;
●
Guard fencing, bridge handrails, median kerbs and similar obstructions can restrict the visibility available at horizontal and vertical curves;
RURAL ROAD DESIGN
51
●
There is a sizeable difference between the length of sight distance available to a driver depending on whether the curve ahead is to the left or the right.
9.12
Curvilinear Alignment Design in Flat Terrain
9.12.1
Introduction
The traditional approach to the design of road alignment in the flat terrain has been to use long tangents with relatively short curves between them. In some cases, the length of straight has become exceptionally long, resulting in monotonous driving conditions leading to fatigue and reduced concentration. The problems of the long tangent/short curve alignment have been recognised for some time. A general conclusion has been that the ideal alignment is a continuous curve with constant, gradual, and smooth changes of direction. This has led to the concept of curvilinear alignment which has been defined as consisting of long, flat circular curves, simple and compound, connected by fairly long spiral transitions, about two thirds of the alignment being on the circular arcs and one third on spirals. Inherent in this definition is the premise that the alignment is made up of a range of curves varying in radius from about 10,000 metres to a maximum of 30,000 metres. If the whole alignment can be made up of curves of the 10,000 metres to 30,000 meters radii, the need for spiral transitions is essentially removed.
9.12.2
Theoretical Considerations
The basis for using curvilinear alignment is found in the consideration of visual requirements and the effect of speed on perception and vision. As speed increases: ● ● ● ●
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●
Concentration increases; The point of concentration recedes; Peripheral vision diminishes; Foreground detail begins to fade; and Space perception becomes impaired.
Thus the higher the speed, the further ahead the driver focuses his vision and the more concentrated the angle of vision becomes. This restriction of vision (called “tunnel view” by some) may induce fatigue unless the point of concentration is made to move around laterally by means of a curvilinear layout of the road. Space perception is achieved with the help of memory, and by assessing relative changes in the size and position of objects. It is therefore necessary to have a lateral component to enable a driver to discern movement and its direction. This lateral component is provided on curves, the rate of such movement depending on the radius of the curve. The radius that should be adopted depends on several factors including the type of topography and the expected speed of travel, the desired radius depending on how far ahead the driver can see the road. At high speeds, a driver looks from 300 metres to 600 metres ahead and a curve should be at least this long to be visually significant when the driver is on it. It is desirable to design on the basis of at least 30 degrees of deflection angle as a minimum, which will result in the
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adoption of curve radii of from 3,000 to 30,000 metres depending on how far ahead the road can be seen. A further consideration is the requirement of overtaking sight distance. It is desirable that overtaking sight distance be provided if possible and in flat country this can easily be achieved. A 15,000 m radius curve allows overtaking sight distance for 120 km/hour to be achieved. The optimum radius range is about 16,000 to 18,000 metres. The larger the radius, however, the closer the alignment comes to a straight line and the less the advantages become and in this respect further consideration may need to be given to the desirable maximum length of curve in one direction. There is no point in using radii larger than 30,000 metres for this reason.
9.12.3
Advantages of Curvilinear Alignment
A road with curvilinear alignment is much more pleasant to drive on than one with long straight tangents since it unfolds itself smoothly with no unexpected checks. The driver is more able to judge the distance to an approaching vehicle, and to assess its rate of approach since the driver sees it to one side, the lateral component of its movement providing the necessary information for the driver assessment. Judgements on the safety of overtaking manoeuvres are easier to make under these circumstances. Because of the continuously curving alignment, the view ahead is constantly changing and it is also possible to direct the road towards interesting features of the countryside for short periods. This removes much of the monotony of the long straight alignment and can create a sense of anticipation in the driver for what is beyond. At night, curvilinear alignment removes much of the approaching headlight glare problem common to long straight roads in flat country. On long straights, headlights become visible from a very long distance away and can be annoying and distracting from a distance of over 3 kilometres. Where vehicles approach each other on curvilinear alignment, the glow of the approaching vehicle headlamps can be seen well before the lamps become visible, and the rate of approach of the vehicle can be assessed. In the daytime when driving in the direction of the sun curvilinear alignment removes much of the approaching glare problem caused by the sun’s rays common to long straight roads running in a westerly/easterly direction in flat country. Conditions for both day and night driving are therefore much more comfortable on a road with curvilinear alignment. On treeless plains, some of the effect of the curvilinear alignment is lost. It may be that in such circumstances, the smaller (optimum) radii would be more effective in that it will increase the driver’s perception of relative change. The principles of curvilinear alignment can be applied in a wide range of conditions using a wide range of curve radii together with spirals. Considerable improvements in the quality of our road system can be achieved at no extra cost by the application of these principles.
9.13 Bridge Consideration
9.13
Bridge Considerations
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Bridge carriageway width and width of road on the approaches to the bridge are based on providing a consistent level of service along a section of road. The following factors should be considered: ● ● ● ● ●
Road geometry; Traffic volumes and composition; Terrain; Climatic conditions; and Bridge location.
The traffic lane widths provided on the bridge should not be less than the widths provided on the approach roadway. On short bridges (20m long or less for most rural roads), it is normal practice to carry the full width of shoulders and pavement, including auxiliary lanes, across the bridge.
reduced in order to avoid widening on bridges. If possible, it may be preferable to relocate the auxiliary lane. The following principles are to be adopted for the alignment of elevated structures on major rural roads: ●
Avoid multiple and varying geometrics on the structure, including superelevation transitions, where possible;
●
Skew angle should not exceed 35 o;
●
Avoid curve radii below 500 m;
●
Avoid short end spans on bridges;
●
Provide a constant crossfall on bridges;
●
If curvature is unavoidable, the bridge should lie fully within the circular arc and the radius should be as large as possible with maximum 6% superelevation; and
●
The designer should seek advice from bridge engineers in relation to construction economies, provision for future duplication and the location of tangent points.
Where necessary, additional bridge width should be provided: ●
To carry a kerbed footway on the bridge and on the approaches; and
●
To achieve satisfactory sight distance and curve widening.
Further consideration of geometric requirements for bridges is set out in the Austroads Bridge Design Code (Ref. 29).
Auxiliary lane lengths and, in particular, tapers should not be
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53
10. 10.1
10.2
Grades
10.2.1
General
VERTICAL ALIGNMENT Introduction
Vertical alignment is the longitudinal profile along the centreline of a road. It is made up of a series of grades and vertical curves. The profile is determined by a consideration of the planning, access, topographic, geological, design controls earthworks and other economic aspects.
Generally, grades should be as flat as possible, consistent with economy and longitudinal drainage requirements (where kerbing is to be incorporated). Flat grades permit all vehicles to operate at the same speed. Steeper grades produce variation in speeds between vehicles with varying power to weight ratios both in the uphill and down hill direction. This speed variation: ●
Leads to higher relative speeds between vehicles producing the potential for higher rear end vehicle accident rates; and
The grades are generally expressed as a percentage of one vertical divided by the horizontal component.
●
The vertical curves are parabolic in shape and are expressed as a K Value. The K Value is the vertical curve constant, used to define the size of a parabola. It is the length (m) required for a 1% change of grade.
Results in increased queuing and overtaking requirements which gives rise to further safety problems, particularly at higher traffic volumes.
In addition, freight costs are increased due to the slower speed of heavy vehicles.
For design purposes the K value concept also has the advantage of easily determining the radius at the apex of a parabolic vertical curve: R = 100K. Within the range of grades used for road design there is little variation between the parabola and the extended arc of the apex radius. Therefore, the apex radius value yields a suitable equivalent radius and an alternative vertical curve constant that can be used to define the size of a parabolic vertical curve.
Table 10.1 shows the effect of grade on vehicle performance and lists road types that would be suitable for these grades. Vehicles can tolerate relatively short lengths of steeper grades better than longer lengths of less steep grades.
10.2.2
Vehicle Operation on Grades
There are three aspects to the design of grades that can be adopted in difficult terrain:
Table 10.1: Effect of Grade on Vehicle Type Grade
Reduction in Vehicle Speed as compared to Flat Grade %
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Uphill
Road Type Suitability
Downhill
Light Vehicle
Heavy Vehicle
Light Vehicle
Heavy Vehicle
0-3
Minimal
Minimal
Minimal
Minimal
For use on all roads
3-6
Minimal
Some reduction on high speed roads
Minimal
Minimal
For use on low-moderate speed roads (incl. High traffic volumes roads)
6-9
Largely unaffected
Significantly slower
Minimal
Minimal for straight alignment. Substantial for winding alignment
For use on roads in mountainous terrain Usually need to provide auxiliary lanes if high traffic volumes
9-12
Slower
Much slower
Slower
Significantly slower for straight alignment. Much slower for winding alignment
Need to provide auxiliary lanes for moderate – high traffic volumes. Need to consider run-away vehicle facilities if proportion of commercial vehicles is high
12-15
10-15 km/h slower
15% max. Negotiable
10-15 km/h Slower
Extremely slow
Satisfactory on low volume roads (very few or no commercial vehicles)
15-33
Very slow
Not negotiable
Very slow
Not negotiable
Only to be used in extreme cases and be of short lengths (no commercial vehicles)
Source: Ref. 66
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RURAL ROAD DESIGN
●
●
The poorer performing vehicles using the road (generally trucks in the lower power ranges) must be able to climb the grade. This limits the maximum grade that can be considered for roads open to the public. It only becomes an acceptable limit in low volume situations, or for special purpose roads, eg to a specific tourist vantage point. Grades cause the need for speed variations, gear changes and braking for all vehicles. This is a quality of service consideration. Flatter grades, which enable a more consistent travel speed, make fewer demands on both vehicle and driver and generally reduce vehicle operating costs.
●
Less important local roads where the costs or impact of achieving higher standards are difficult to justify.
In any case, design options for the road include, on one hand, flattening the grade, and on the other, the provision of auxiliary lanes and/or special facilities for safely controlling runaway vehicles on downgrades (refer Section 13.7). “When adopting maximum grades, side drains need to be considered in respect to the maximum velocity of flow for scour protection. Special lining of the drains may be required to limit damage to the drain and the environment.”
10.2.4 ●
Grades cause speed disparities between vehicle types, leading to increased queuing and overtaking requirements. This is a level of service problem. The increased overtaking requirements and reduced service volumes can give rise to operational and safety problems at higher traffic flows. The problem can arise from cars towing caravans and trailers as well as from heavy commercial vehicles.
10.2.3
Maximum Grades
Grades used in design are, therefore, only controlled at the upper end by vehicle performance. In most designs, the general maximum grade to be sought will be based on level of service and quality of service considerations, modified as appropriate by the severity of the terrain and the relative importance of the road. Table 10.2 shows maximum grades over long lengths of road in various terrain types. The adoption of grades steeper than the general maximum may be justified in the following situations: ●
Comparatively short sections of steeper grade which can lead to significant cost savings;
●
Difficult terrain in which general maximum grades are not practical;
●
Where absolute numbers of heavy vehicles are generally low; and
Length of Steep Grades
To achieve a quality-balanced design, it is necessary to consider the length of the grade. Most standards do not explicitly limit the length of grades, but suggest that it is desirable to limit the length of sections with maximum grades. AASHTO (1994) proposes limiting the maximum length to that which will not exceed the critical length of grade. “The critical length is that which will cause a typical loaded truck (300 pound/horsepower) [5.5 kW/tonne] to operate without an unreasonable reduction in speed. A reduction of 10 mph [16 km/h] is recommended, the reason being the significant increase in accident involvement rate at higher speed reductions.” The length of steep grades is considered in the design of auxiliary lanes with the help of Figure 13.3. However it must be remembered that length of grade can affect safety and capacity. On both the upgrade and down grade, the lower operating speed of trucks may cause inconvenience to cars. Long gradients, for example 5km at 4%, could result in a high risk of serious accidents involving descending vehicles as a result of brake failure. Such gradients could also cause climbing vehicles to slow down to well below the 85th percentile speed.
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All short sections of grade should be checked for appearance.
Table 10.2: General Maximum Grades (%) Operating Speed
Terrain
(km/h)
Flat
Rolling
Mountainous
60
6-8
7-9
9-10
80
4-6
5-7
7-9
100
3-5
4-6
6-8
120
3-5
4-6
-
130
3-5
4-6
-
Note: Values closer to the lower figures should be aimed for on primary highways. Higher values may be warranted to suit local conditions For unsealed surfaces the above value should be reduced by 1%.
10.2.5
Steep Grade Considerations
Although speeds of cars may be reduced slightly on steep upgrades, large differences between speeds of light and heavy vehicles will occur and speeds of the latter will be quite slow. It is important, therefore, to provide adequate sight distance to enable faster vehicle operators to recognise when they are catching up to a slow vehicle and to adjust their speed accordingly. Key considerations are as follows: ●
On any generally rising or falling section of the road, steep grades should be avoided as much as practicable, as these grades reduce vehicle operating efficiency.
●
Where possible, it is preferable to introduce a flatter grade at the top of a long ascent, particularly on low speed roads, but this must not be achieved by steepening the lower portion of the grade.
●
On steep downgrades, it is desirable to increase the 85th percentile speed of the individual geometric elements progressively towards the foot of the steep grade. Where this cannot be achieved and where percentages of heavy
RURAL ROAD DESIGN
55
vehicles are high, consideration should be given to construction of runaway vehicles facilities. Refer to Figure 13.5, 13.6 and Section 13.7 for runaway vehicle facilities.
10.2.6
Minimum Grades
The minimum grade may be zero except in the following situations: ●
●
In cut: In cut, the minimum grade shall normally be 0.5% (absolute minimum 0.33%) for unlined drains. This minimum grade in cut is required to provide adequate fall in table drains. In exceptional cases, where for any reason it is necessary to have a grade flatter than 0.5% this would be acceptable provided that a minimum grade of 0.5% is retained in the table drains. This is done by uniformly widening the drains at their standard slope, thereby deepening them progressively or, alternatively, lining the table drains to permit a flatter grading to be adopted. In medians: On divided roads the type of median drainage proposed may control the minimum grade of the carriageways.
= KA
K =
S2 when S < L 200 (√h1 + √h2 )2
and K =
2S 200 (√h1 + √h2 )2 – when S > L A A2
where: L = length of vertical curve (m) K = is the length of vertical curve in meters for 1% change in grade A = algebraic grade change (%) S = sight distance (m) h1 = driver eye height, as used to establish sight distance (m) h2 = object height, as used to establish sight distance (m) For design purposes the K value may be used to determine the equivalent radius of a vertical curve using R (radius m) = 100K.
10.3.3
Crest Vertical Curves
Curvature of crest vertical curves is usually governed by sight distance requirements. However, the appearance of the road may dictate larger values to provide satisfactory appearance of the curve. These criteria are discussed below.
10.3
Vertical Curves
10.3.3.1
10.3.1
General
At very small changes of grade, a vertical curve has little influence other than appearance of the profile and may be omitted. At any significant change of grade, minimum vertical curves detract from the appearance. This is particularly evident on high standard roads.
The vertical alignment of a road consists of a series of straight grades joined by vertical curves. In the final design, the vertical alignment should fit into the natural terrain, considering earthworks balance, appearance and the maximum and minimum vertical curvature allowed expressed as the K value. Large K value curves should be used provided they are reasonably economical. Minimum K value vertical curves should be selected on the basis of three controlling factors:
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L
●
Sight distance: Is a requirement in all situations for driver safety.
●
Appearance: Is generally required in low embankment and flat topography situations.
●
Riding comfort: Is a general requirement with specific need on approaches to floodway where the length of depression needs to be minimised.
Appearance
10.3.3(a) Crest Vertical Curve
10.3.3(b) Crest Vertical Curve
10.3.2
Forms and Types of Curve
There are various curve forms suitable for use as vertical curves. The parabola has been traditionally used because of the ease of manual calculation and is adopted throughout this Guide. Other forms are equally satisfactory. There are two types of vertical curves. Convex vertical curves are known as summit or crest curves, and concave vertical curves as sag curves. Vertical curve theory and formulae are presented in Appendix B. However, in summary, most vertical curves can be designed using the following equations:
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RURAL ROAD DESIGN
Table 10.3: Length of Crest Vertical Curves – Appearance Criterion when S < L Operating Speed (km/h)
Minimum grade change requiring a crest vertical curve, % (1, 2)
Minimum length of crest vertical curve, m (3)
Minimum K Value (4) S L 200 (√H – h1 + √H – h2 )2
where: H h1 h2 S
= = = =
height of overhead obstruction (m) truck driver eye height (2.4) (m), object height (0.60) (m), stopping sight distance (m), Table 8.3(a).
and K =
2S 200 (h + S tan q) – when S > L A A2
where:
58
10.3.5
Reverse/Compound/Broken Back Vertical Curves
Upright vertical curves with common tangent points are considered quite satisfactory. It is necessary to check that the
RURAL ROAD DESIGN
Figure 10.1: Car Headlight Sight Distance on Curves
Table 10.5: Minimum K Values for Sag Vertical Curves Operating Speed (km/h)
K value a = 0.05g
a = 0.1g
50
4
2
60
6
3
70
8
4
80
11
6
90
14
7
100
17
9
110
20
10
120
24
12
130
28
14
Table 10.6: Minimum Sag Vertical Curve K Value for Headlight Criteria when S < L
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h = 0.60 m, q = 1o Operating Speed (km/h)
Stopping Sight Distance K value Des. Min. R T = 2.5 sec
Abs. Min R T = 2.0 sec
50
10
8
60
14
12
70
19
17
80
25
22
90
32
29
100
41
37
110
50
46
120
62
57
130
72
66
sum of the radial accelerations at the common tangent point does not exceed the tolerable allowance for riding comfort, a
V2 1256
and 0.005
1 1 + K1 K2
It would be desirable to provide a short length of grade between the reverse vertical curves. The desirable length is equal to 0.2V in metres. Where less than the desirable buffer length is available the minimum vertical curves are to conform to the following empirical formula: K
=
K1 + K2 ≤ (1+b) 10,000K1K2
where: K1 & K2
= K values of the two curves being tested
K
= minimum K values listed in Table 10.5 (comfort criteria)
b
= fraction, being the ratio of the actual length between TP’s of the adopted curves to the normally required buffer length, 0.1Vm (absolute) or 0.2Vm (desirable), as the case may be.
Broken Back vertical curves consist of two curves, both sag or both crest curves, usually of different K value, joined by a short length of straight grade. Their use should be avoided when the length of straight grade between curves is less than 0.4Vm (V = operating speed in km/h). Where the length of straight grade exceeds 0.4V m the curves are not then deemed to be broken-backed. Compound curves are made up to two curves in the same direction with the length of straight grade equal to zero.
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59
11. 11.1
●
Vehicle Dimensions: Commercial vehicles are commonly the full legal width of 2.5m. Normal steering deviations as well as tracking errors and pavement imperfections reduce the clearance between vehicles in adjacent lanes. The wider the vehicles and the narrower the lanes, the more significant these reduced clearances become. There is a consensus that 3.5m lanes are appropriate for cars and the 19m prime mover and semi trailer, however, a lane width of 3.6 to 3.75m may be required for significant volumes of larger trucks. The use of 3.5m lanes plus shoulder seals is a more effective use of a given total seal width with regard to both the pavement structure and roadside design.
●
Combinations of Speed and Traffic Volume: When both the operating speed and the traffic volume are high, narrower lane widths should be avoided. When only one of these factors is high, an economic design may frequently dictate narrower lanes. This can be justified on lower volume roads because passing by opposing vehicles occurs less frequently. If the operating speed is high on a low volume road, it would normally be associated with longer sight distances and drivers would have time to adjust speed and position slightly or to increase the level of concentration when passing other vehicles. Such events are relatively infrequent and do not overtax the driver. Even here, however, wider pavements do improve the quality of service of the road.
C R O S S S E CT I O N General
The selection of cross-section elements for rural roads is an iterative process that considers various criteria: safety, environmental impact, economy and aesthetics. The major elements of a cross section are illustrated in Figure 11.1 and discussed below.
11.2
Traffic Lane Width
A traffic lane is that part of the roadway set aside for one-way movement of a single stream of vehicles. Refer Table 11.1. Traffic lane width is based on consideration of: ●
Traffic: Annual average daily traffic (AADT) of the road, and peak hour traffic figures where relevant. Traffic is usually predicted for a future design year. Heavier traffic volumes on a road means frequent passing and overtaking manoeuvres and the path of vehicles as a result is further from the centre line. In these circumstances, wider traffic lanes are preferred. When the AADT increases above 500 (two lane two way), lane width increases from 3.1 to 3.5m.
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Figure 11.1: Typical Cross Sections
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RURAL ROAD DESIGN
Table 11.1: Single Carriageway Road Widths Element
Design AADT 1-150
150-500
500-1,000
1,000-3,000
>3,000
3.5 (1 x 3.5)
6.2 (2 x 3.1)
6.2-7.0 (2 x 3.1/3.5)
7.0 (2 x 3.5)
7.0 (2 x 3.5)
Total Shoulder
2.0
1.5
1.5
2.0
2.5
Shoulder Seal
0.5
0.5
0.5
1.0
1.5
Traffic Lanes
Note: ● Traffic lane widths include centre-lines but are exclusive of edge-lines. ● Shoulder beyond the seal can be lightly constructed, gravel surface suitable for supporting occasional heavy wheel load. ● Short lengths of wider shoulder seal or lay-bys to be provided at suitable locations to provide for discretionary stops. ● Wider shoulder seals may be appropriate depending on requirements for cyclists, maintenance costs, soil and climatic conditions or to accommodate the tracked width requirements for Large Combination Vehicles. ● Full width shoulder seals may be appropriate beside guard barrier and on the high side of superelevation.
The desirable lane width on rural roads is 3.5m. This width allows large vehicles to pass or overtake without either vehicle having to move sideways towards the outer edge of the lane. The lane width and the road surface condition have a substantial influence on the safety and comfort for users of the roadway. In rural applications the additional costs that will be incurred in providing wider lanes will be partially offset by the reduction in long-term shoulder maintenance costs. Narrow lanes result in a greater number of wheel concentrations in the vicinity of the pavement edge and will also force vehicles to travel laterally closer to one another than would normally happen at the design speed.
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Drivers tend to reduce their travel speed, or shift closer to the lane/road centre (or both) when there is a perception that a fixed hazardous object is too close to the nearside or offside of the vehicle. When there is a perceived fixed hazard, there is a movement by the vehicle towards the opposite lane line.
●
There is little or no truck traffic;
●
Finance for road construction is limited; or
●
The alignment and safety records are satisfactory in the case of a reconstructed arterial.
This lane width framework should be supplemented by the consideration of local practice and experience. For prime mover and semi-trailer operation, radii above 300m should be used to avoid lane widening. The use of lanes wider than 4.6 metres as a result of lane widening is not favoured because of the possibility of two cars travelling side-by-side within the lane. If greater width is required for truck tracking, an edge line should be placed at 3.5 m and full pavement depth widening should be provided for the remainder of the width.
11.3
Traveled Way
Alternative lane widths may be considered in some circumstances. Wider traffic lanes should be considered where any of the following apply:
Traveled way is that portion of a carriageway ordinarily assigned to moving traffic (excludes shoulders and parking lanes). Its width depends on design traffic volumes (AADT) and adopted level of service.
●
There is a higher volume of trucks (greater than 80 per day) for the middle lanes of a carriageway as sealed shoulders provide enough space for lanes abutting shoulders;
Where operating speeds are over 80km/h or where the heavy vehicle volume in the traffic flow is high, traveled way width should be based on 3.5m wide traffic lanes.
●
There is a need for widening on horizontal curves;
11.3.1
●
The left lane is to be used by cyclists; or
●
Operation of Type 2 (triple) road trains (or even larger vehicles) is anticipated.
On many roads in Australia, traffic is less than 150 vehicles per day. Some of these are arterial roads passing through sparsely settled flat country where the terrain leads to a high operating speed.
Narrower lanes (suggest down to 3.0m – Ref. 18) should be considered where any of the following apply: ●
The road reserve or existing development form stringent controls preventing wider lanes;
●
The road is in a low speed environment;
Single Carriageways
Where traffic volumes are less than 150 vehicles per day and, particularly, where terrain is open, single lane carriageways may be used. The traffic lane width adopted on such roads should be at least 3.5m. A width of less than 3.5m can result in excessive shoulder wear. A width greater than 4.5m but less than 6.0m may lead to two vehicles trying to pass with each remaining on the seal. This
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61
11.3.1 Single Carriageway
11.3.2 Divided Carriageway
potentially increases head-on accidents. The width of 3.5m ensures that one or both vehicles must have the outer wheels on the shoulders while passing.
but preferably wider to accommodate a broken-down vehicle. Where the shoulder is less than 2 m, opportunity should be taken to provide wider standing areas at regular intervals, by flattening fill slopes on low formations or by widening shoulders at the transition from cut to fill. The widening should be sufficient to allow traffic to pass a stopped vehicle without having to change position in the lane. At the least, the widening should be sufficient to allow traffic to pass a stopping vehicle by changing position in the lane without encroaching into the adjoining lane. Although few rural roads in Australia carry traffic volumes sufficient to require more than four lanes, in designing a rural road it is common to assume that wider carriageways may be required at some future time and to reserve the land required. Table11.2 contains the widths of cross section elements for rural roads.
On two lane sealed roads, total width of seal should desirably be not less than 7.2m to allow adequate width for passing.
11.3.2
Divided Carriageways
A divided rural road has two carriageways separated by a median. The median width is defined as exclusive of any road shoulders where provided. Each of the two carriageways should have at least two traffic lanes so that overtaking is possible. With each carriageway, the shoulder remote from the median should be at least 2 m wide,
Table 11.2: Divided Carriageway Road Widths Element < 20,000
> 20,000
3.5
3.5
Shoulder Left Median
2.5 1.0
3.0 1.0
Shoulder Seal Left Median
1.5 1.0
Traffic Lanes
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Design AADT
(1)
Median (4) Wide, no barrier protection Narrow, barrier protected (5) Verge
3.0 1.0
(2, 3)
15m 3m rigid barrier, 8m flexible barrier Refer Table 11.6
Note: (1) Traffic lane widths include lane lines but are exclusive of edge lines. (2) Wider shoulder seals may be appropriate depending on requirements for cyclists, maintenance costs, and soil and climatic conditions. (3) Full width shoulder seals are appropriate beside guard barrier and on the high side of superelevation. (4) The median widths are exclusive of median shoulders. Refer Figure 11.5. (5) A greater median width will be required to accommodate at-grade intersections.
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RURAL ROAD DESIGN
11.3.2.1
Independent Design of Carriageways
When land is available, economies can be achieved in earthwork volumes on divided roads when the two carriageways are far enough apart for them to be partly or wholly aligned and graded independently of one another. As carriageways are moved further apart, the width of bridging over the road may reduce if bridging of the median can be omitted. Close parallel carriageways, both at the same level, can appear monotonous and have a sleepy effect on the driver. When parallel carriageways are to be relieved, a change in the direction of one relative to the other is best arranged to take place at a curve, either vertical or horizontal, so that any apparent kink resulting from the change can be hidden. Although the distance between carriageways should be such that traffic on one carriageway would not influence driving behaviour on the other, a regular glimpse of the second carriageway is desirable to reassure drivers that they are on a one-way carriageway.
11.3.2.2
Superelevation Issues
On straights, each carriageway may have a single crossfall or the carriageways may be individually crowned. The carriageways may have a common grading such that each is at the same level or they may be individually graded. The two carriageways may be parallel or individually aligned with median width varying. On curved sections, the superelevated lengths of the two carriageways may be in one plane or be in parallel planes or they may be far enough apart to be independent. When the numerous combinations are considered, it becomes impracticable to identify all the issues for the application of superelevation on divided roads. However, the most common issues are discussed for independent and related carriageways.
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Independent Carriageways Usually, carriageways, which are independently aligned and graded, are widely separated with an undisturbed median area. In such cases, a carriageway may be designed as though it were a normal two-lane two-way road or as a twolane road with an auxiliary lane where three lanes per carriageway are proposed. Related Carriageways Where the median is relatively narrow, it is usual for the carriageways to be parallel and at the same level, avoiding difficulties in significant level differential in the narrow median. Commonly, the median would be depressed. Transition curves can be developed along the same principles as for two-lane two way roads, but the superelevation development length will still vary in relation to a number of factors. Such factors would relate to each carriageway: ●
Crowning;
●
Single crossfall;
●
Control line location;
●
Axis of rotation.
Figure 11.2 illustrates typical developments of superelevation for parallel carriageways with single grading and a narrow depressed median. Similar forms of treatment could apply for a raised median. As the median becomes wider, there is more latitude to absorb level differential at the edges of the median over the transition and superelevated lengths, as median slopes can be varied within reasonable limits to maintain a uniform invert grading. The selection of the type of crossfall and the choice of a control line for grading may be influenced by the general road alignment. A section of roadway containing long lengths of straights and few curves may be better suited to carriageways with individual crowns, with the control lines along each of the crowns. This method is generally not used as it is considered that each carriageway should fall away from the median to minimise cross median incidents. A section with a high percentage of curved alignment might be better suited to carriageways with single crossfalls with the control lines along the inner shoulder edges, or even along the centre line of the formation overall.
11.3.2.3
Transitions Between Divided and Undivided Carriageways
A number of situations can arise, either temporarily or permanently, where a transition is made between a divided and an undivided carriageway. This commonly occurs where an existing two-lane two-way road is being duplicated in stages due to varying traffic or level of service conditions along the route, such as a strategy to provide increased overtaking opportunity, or due to funding or construction expedience. A number of short lengths of dual carriageway in close proximity can cause confusion to drivers and special attention needs to be given to traffic signing and road marking provisions. In situations where short lengths of duplication are being used to provide increased overtaking opportunity, a duplication length of at least 3 km is desirable. The transition between divided and undivided roadways should take place in an area where there is good sight distance in both directions. For details of the design of the transition see Ref. 18.
11.4
Pavement Crossfall and its Considerations
Crossfall is the slope of the surface of a carriageway measured normal to the centre line. The purpose of crossfall is to drain the carriageway on straights and curves and to provide superelevation on horizontal curves. Crossfalls flatter than 2% do not drain adequately, and even 2% should only be prescribed for concrete pavements where levels and surface finish are tightly controlled. Unless compaction and surface shape are well controlled during construction, pavements with less than 2.5% crossfall will hold small ponds on the surface, which may cause potholes to develop and hasten pavement failure. Rutting of the pavement
RURAL ROAD DESIGN
63
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Figure 11.2: Changes of Crossfall on Related Carriageways
64
RURAL ROAD DESIGN
Table 11.3: Pavement Crossfall on Straights Type of Pavement
Crossfall (%)
Earth, Loam
5
Gravel, Water bound Macadam
4
Bituminous Sprayed Seal
3
Bituminous Concrete (asphalt) Portland Cement Concrete
2.5-3 2-3
may be a natural or undisturbed median area between them. Where the two carriageways are closer together, the crossfall and drainage of the median may begin to be a control on the relative levels of the two inner carriageway edges.
11.5
Shoulder
11.5.1
Function
Road shoulder carries out two functions: ● Traffic; and ● Structural. The traffic functions of the shoulder are:
is also more likely to hold water, increasing the risk of pavement deterioration and vehicle aquaplaning when the pavement crossfall is less than 3% (Ref. 67).
●
An initial recovery area for any vehicle which may get out of control;
●
A refuge for stopped vehicles on a firm surface at a safe distance from traffic lanes;
●
A trafficable are for emergency use;
●
Space for cyclists;
●
Clearance to lateral obstructions; and
●
For road train routes, the shoulder has the additional function of providing for the additional tracked width associated with road trains. Refer Section 11.2
The pavement crossfall on straights for various pavement types is given in Table 11.3. Generally, on divided roads, two-lane carriageways on straights have a uniform one-way crossfall with the high point of the pavement at the edge nearest the median. Two-way crossfall, with the crown in the middle of the pavement, may come about through one of the carriageways having been or being intended for an initial two-way road. Other factors, which could influence the choice between crowned and oneway crossfalls, would include median treatment and median drainage. One-way crossfalls would be more likely when the median was narrow. A crowned crossfall directs more water towards the median.
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The build up of sheet flow across a wide carriageway can become a safety problem, and three-lane carriageways on straights are usually crowned with one lane falling towards the median. At intersections, the crown position may have to be varied to suit drainage and the grading of the intersecting road. The whole pavement surface area has to drain while retaining satisfactory riding qualities for all traffic movements, having regard to vehicle speeds. Usually, it is desirable to prepare pavement surface contours or profiles to assist design and subsequent construction. On straight sections of divided roads where the crossfall of the pavement is away from the median and the shoulders are not sealed, it would be usual for the crossfall of the median shoulder to be towards the median. With this arrangement, reversed crossfall where pavement meets shoulder, the slope of the median shoulder may be reduced as necessary to give a total change of crossfall, pavement to shoulder, of not more than 7%. Desirably, on curves with superelevated pavements, shoulder crossfall should match that of the running lanes. Where design constraints make this difficult, the friction demand of a vehicle passing onto the shoulder at the design speed should be checked. With superelevated curves on divided roads, the two carriageways may be both in the one plane, or they may be in parallel planes with the difference in levels taken up with the median. Where the two carriageways are far enough apart that they may be graded independently of one another, there
The structural function of the shoulder is to provide lateral support to the road pavement layers.
11.5.2
Width
Shoulder width is measured from the outer edge of the traffic lane to the edge of usable carriageway and excludes any berm, verge, rounding or extra width provided to accommodate guideposts and guard fencing. Wide shoulders have the following advantages: ●
Space is available for a stationary vehicle to stand clear of the traffic lanes; a vehicle standing partly on a shoulder and partly on a traffic lane may be a hazard.
●
Space is available on which vehicles may deviate to avoid colliding with other vehicles and on which a driver may regain control of his vehicle.
●
The resulting wider formations increase driver comfort and the quality of service of the road.
●
They contribute to improved sight distance across the inside of horizontal curves.
Table 11.1 lists shoulder width values for two lane rural roads based on AADT volumes. These widths allow a vehicle to stop, or a maintenance vehicle to operate, with only partial obstruction of the traffic lanes. Provided volumes are not high or sight distances are sufficiently long, this will not present an undue hazard to traffic. A width of 2.5m is needed to allow a passenger vehicle to stop clear of the traffic lanes.
RURAL ROAD DESIGN
65
A width of 3.0m allows a passenger vehicle to stop clear of the traffic lanes and provides an additional clearance to passing traffic. It also allows a commercial vehicle to stop clear of the traffic lanes.
11.5.3 Shoulder Sealing
The cost of maintaining road shoulders does not rise in proportion to their width. However, the cost of the initial construction involves additional earthwork and pavement costs. In reconstruction of older pavements, the provision of wider shoulders may increase the costs extensively. Therefore, an economic balance must be achieved in shoulder width, and in the case of upgrading work this element can be very significant. The aim should be to provide shoulders of 1.5 m to 2.0 m wherever possible, and up to 2.5 to 3 m on higher volume roads. Because most vehicles standing on road shoulders exercise some choice as to the stopping place, it is desirable to take every opportunity to provide areas at intervals where vehicles can stop completely clear of the traffic lanes, such as on low fills where flattening the slopes automatically provides this, or at the transition from cut to fill where minor additional earthworks involved can be made at low cost. On a divided road, refer Table 11.2; with two lanes in each direction, it is desirable to provide shoulders at least 2.5m wide on the left side of each carriageway and 1.0m wide on the median side of each carriageway. If the divided road has three lanes in each direction, it is preferable to have wide shoulders on both sides of both carriageways. This limits the number of lanes a vehicle may have to cross in the event of breakdowns to stop clear of the traffic lanes.
11.5.3
Shoulder Sealing
are desirable at the edge of the traffic lanes. Otherwise, in the case of narrow partial sealing, usage of the additional seal as part of the traffic lanes merely transfers the problem to the new edge. To minimize the effect of wind erosion on shoulder material, a 1.0m seal is often used on roads carrying AADT over 2000 vpd (with 10% heavy vehicles (Ref. 90)). The widths required for the various functions are set out in Table 11.4. A full width seal should be considered under the following conditions: ● ●
Shoulders may be wholly or partially sealed. Sealing of shoulders is frequently done to reduce maintenance costs and to improve moisture conditions under pavements, especially under the outer wheel path.
● ● ● ●
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●
Adjacent to a lined table drain, kerb or dyke; Where a safety barrier is to be provided; On the outer shoulder of a superelevated curve; On floodways; Where rigid pavement is proposed’ Where environmental conditions require it; Where needed to reduce maintenance; and In high rainfall areas.
However, from the geometric design point of view, the shoulder is regarded as being usable by traffic. Partial sealing ensures this by protecting the lane edge against the development of the broken edges or ‘drop offs’ that occur adjacent to the traffic lanes and results in the whole shoulder width remaining usable to traffic up to 2.5m wide.
●
The desirable width of sealed shoulder depends on many factors including:
Table 11.4: Shoulder Width
● ● ● ● ● ●
Traffic composition; AADT; Access; Operating speed; Rainfall; and Shoulder pavement.
While 0.5m wide seals on the shoulders should be considered the minimum when the predicted AADT is less than 2000, more sealed width is often warranted. In some instances, partial shoulder sealing is widened to full width adjacent to concrete gutters and on the topside of superelevated curves. In wetter areas where moisture control is required, shoulder width of 0.5 m is desirable and 1.0 m is preferable. In the case of full or partial sealing of shoulders, longitudinal edge lines
66
RURAL ROAD DESIGN
A contrast in texture or colour between the sealed shoulders and the pavement would assist in defining the limits of the traffic lanes and supplement the edge lines. Where median shoulders are not sealed, depending on median configuration,
Function of Shoulder
Sealed Width (m)
Lateral support of pavement
0.5
Control of water flow path on outside curves
1.0
Initial recovery area
0.5
Discretionary stopping - Cars - Trucks
2.5 3.0
Bicycle demand Source: Ref. 99
2.0/3.0
it may be found that the width of 1 m is not suitable for maintenance using mechanical equipment. A width of 1.5 m or 2 m may, therefore, be adopted.
11.5.4
Crossfalls
Shoulders generally should be steeper than the adjacent traffic lanes to assist surface drainage (marginal increase of 1%). However, where the shoulder consists of full depth pavement and is sealed, its slope may be the same as the adjacent pavement in order to facilitate construction. On straights the shoulder crossfall is shown in Table 11.5 On superelevated sections of roads, the shoulder on the high side and low side must have the same crossfall as the traffic lanes. A cross fall of 5% or more extended across the verge may lead to more frequent maintenance and should be monitored.
The minimum widths for these functions are shown in Table 11.6. It is not intended that verge widths should vary continuously. Designers should apply long sections of appropriate minimum verge width with short transitions where greater or lesser widths are required. Verge and batter toe rounding are of critical importance in minimizing rollover accidents. Verge rounding (see Figure 11.3) enables tyre contract to be maintained and decreases the likelihood of rollover. An errant vehicle may become temporarily airborne where the verge is only 0.5m wide, and the change in slope is greater than 7 per cent. Verges and verge rounding should be provided on unkerbed medians where the lateral change in grade is greater than 10 per cent. Also, rounding at the toe of batter reduces the potential to overturn due to tripping.
11.7
Batters
Table 11.5: Shoulder Crossfall Shoulder Material
Crossfall %
Earth, Loam
5–6
Gravel and crushed rock
4–5 Match traffic lane
Concrete
Match traffic lane
Verge
The main functions of the verge are to provide: Traversable transition between the shoulder and the batter slope;
●
● ● ●
Full depth pavement with bitumen seal or asphalt as wearing course
11.6
Batters are surfaces, commonly but not always of uniform slope, which connect carriageways or other elements of cross sections to the natural surface. Batters may:
Batter slopes are usually defined as the ratio of one vertical on “x” horizontal and are shown as, for example, 1 on 4. The following factors should be considered when selecting batter slopes: ● ● ● ●
A firm surface for stopped vehicles at a safe distance from traffic lanes;
●
● ● ●
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Provide a recovery area for errant vehicles; Be used as part of the landscaped area; and Be used for access by maintenance vehicles.
The results and recommendations of geotechnical investigation; Batter stability; Batter safety (economics of eliminating safety barriers); Future costs of maintaining the adopted slope; Appearance and environmental effects; Earthworks balance; Available width of road reserve; and Landscaping requirements.
●
Support for the boxing edge and shoulder material;
●
●
Space for installation of guide posts and road safety barriers; and
Slopes flatter than the desirable maximum (see Table 11.7) should be used where possible.
●
Provide rounding between the formation cross slope and embankment batter slope to assist controllability of vehicles, which encroach the formation and to reduce scouring due to road storm water run off.
In shallow cuttings (up to about 3 meters depth) it is common practice to flatten cut batters beyond that required for stability purposes for improved appearance. In areas where the batters transition from cut to fill, a catchline treatment (a constant
Table 11.6: Verge Width Function
Width (m)
1
Shoulder support and locate guide posts
2
Traversable transition between the shoulder and the batter slope (depending on how steep the superelevation and/or batters might be and what batter rounding is required)
3
To provide a space for installation of road safety barrier (extra for terminals)
4
To achieve horizontal sight distance, or to balance cut and fill
1.0
1.0 to 6.0 1.5 Where required, 3m to 5m.
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67
Table 11.7: Design Batter Slopes (without safety barriers) CLASSIFICATION
CUT SLOPES
FILL SLOPES
Height
Desirable
Maximum
Height
Desirable
Maximum
H < 3m H > 3m
1 on 3 1 on 2
1 on 2 1 on 1.5
H < 3m H3 - 12m H > 12
1 on 6 1 on 4 1 on 2(2)
1 on 4 1 on 2(2) 1 on 2(2)
1 on 0.5
1 on 0.25
-
-
-
Table Drain Batter
1 on 6
1 on 2
-
-
Median Batter
1 on 10
1 on 6
1 on 10
1 on 6
1 on 3 1 on 2
1 on 2 1 on 1.5
1 on 6 1 on 4
1 on 3 1 on 4
1 on 4
1 on 2
-
-
1 on 2 1 on 2
1 on 1/5(1) 1 on 1/5(1)
1 on 4 1 on 4
1 on 2 -1 on 2(2)
1 on 4
1 on 2
1 on 6
1 on 4
ARTERIAL
RURAL DIVIDED
Batter: Earth
Batter: Rock
RURAL UNDIVIDED Batter
H < 3m H > 3m
Table Drain Batter
H < 3m H > 3m
LOCAL Batter
H < 3m H > 3m
Table Drain Batter
Notes: (1) May be steeper in rock cut. Source: Ref. 99 (2) Batter with roadside safety barrier installed. (3) A benched fill slope batter of 1 on 1.5 may be considered in specific cases. (4) Batter slopes may vary depending on height and geotechnical reports.
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Figure 11.3: Verge Rounding
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RURAL ROAD DESIGN
H < 3m H > 3m
batter offset) may be used to smooth the transition from adjoining cut to fill. It also blends the batters into the surrounding terrain as it follows the natural slope of the surface. Catchlines, or constant batter widths are also applicable, on the grounds of aesthetics, in flat and gently undulating terrain. Where shoulders are near the minimum widths given in Table 11.4, opportunity should be taken to provide pull-off areas at intervals, on low fills (0.5 m) and at the transition from cut to fill. Catchline treatment assists this provision. Where earthwork volumes are significant, maximum batter slopes are dictated by the angle at which the material will stand cut, or at which it can be shaped for a stable embankment. While solid rock cuttings might be stable when vertical, it is unusual to adopt a slope steeper than 1 on 0.25, as otherwise the cutting walls can give the impression of leaning inwards. Accidents can occur where vehicles run off the road and the driver loses control on a steep embankment or the vehicle runs into a cutting wall or drain. The severity of this type of accident may be reduced if the batter slopes are sufficiently flat for the driver to recover control of the vehicle. However, where truck volumes are high (10% and more), embankment slopes flatter than 1 on 6 are desirable, refer also Section 17.3.1.
ends of the bench and discharged on to the natural ground. In some instances, the invert so formed may require lining. The minimum width of bench should be 3m (see Figure 11.4) with a maximum crossfall of 10%. The desirable width of bench for maintenance and drainage purposes is 5m.
11.7.2
Batter Rounding
Rounding of the tops of all cut slopes is essential in order to reduce erosion, especially riling. The size of the rounding is in the range of 1m x 1m minimum up to 6m x 6m maximum, proportional to the height of the batter. Rounding of 1m x 1m shall be applied to the base of all fill batters steeper than 1 on 3, to avoid tripping of errant vehicles.
11.7.2 Batter Rounding
For maintenance purposes (grass mowing) a maximum batter slope of 1 on 2 for side boom slashers is to be used. However, this cannot be achieved in some areas due to geotechnical restraints. A 1 on 4 is the preferred maximum batter slope for a slasher (the most widely used maintenance machine). Mowers and slashes are likely to overturn on a 1 on 3 or steeper batter. Irregularities in the batter face may contribute to overturning. The steepest slope preferred for planting purposes is 1 on 3 and will assist revegetation.
11.7.1
Benches
On high batters (generally exceeding 10m vertical height) or where batters are constructed on unstable material, consideration should be given to the provision of benches.
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Benches can have the beneficial effects of:
11.8
Medians
A median may be defined as a “strip of road not normally used by vehicular traffic, which separates opposing traffic lanes”. Its main function is to separate opposing streams of traffic and to limit conflict areas for turning traffic, thereby significantly reducing the risk of severe collisions and increasing the safety of the road. In addition, medians can:
●
Eliminating the need to flatten the batter slope in the interests of stability;
●
Minimizing the possibilities of rock falling on to the pavement;
●
Reduce conflict with vehicular traffic waiting to turn right (by provision of protected turning lanes);
●
Reducing scour on the batter face;
●
Provide space to shelter crossing traffic at unsignalised intersections;
●
Reducing the amount of water in cuttings to be carried by the table drain;
●
Reduce headlight glare;
●
Providing easier access for maintenance of the batter face;
●
Provide a recovery area for out of control vehicles;
●
Improving the appearance of the cutting;
●
Provide emergency stopping areas;
●
Assisting the re-establishment of vegetation;
●
Reduce air turbulence between opposing traffic;
●
Improving sight distance on horizontal curves.
●
Accommodate level differences between carriageways;
●
Provide scope for improvement of visual amenity by landscaping;
Benches should be sloped away from the roadway and longitudinally so that stormwater can be drained towards the
RURAL ROAD DESIGN
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Figure 11.4: Benches
70
RURAL ROAD DESIGN
●
Provide areas for the location of road furniture on the right hand side of carriageways.
11.8 Median
Medians are usually incorporated in all rural roads of four lanes or more. They may be raised or depressed as shown in Figure 11.5. A depressed median should be of sufficient width to place the invert of the median drain below subgrade level to facilitate drainage of pavement layers. If this cannot be achieved, pavement subsurface drains shall be provided. Subsurface drains may be required as a result of the fill material type, even if a median drain below subgrade level is provided. The absolute minimum width of a depressed median is 10 meters (for drainage reasons), and 15m are a desirable minimum. Numerous studies have shown that wider medians improve safety and that 90% of run off the road incidents deviate less than 15 m from the edge of the carriageway. However, the marginal effectiveness of increased width drops rapidly (80% of these incidents deviate less than 10 m) and, where land is expensive, it is hard to justify widths greater than the minimum. In most rural areas, the additional cost of a wide median is small and widths of 15 m (and more) can be warranted. For Medians less than 15m, roadside safety barriers need to be used to minimise cross-median incidents. Raised medians are sometimes used, especially in cuttings, and have some advantages with headlight glare and a reduction in earthwork costs; however, the cost advantage is somewhat mitigated by additional drainage and safety barrier costs.
The width of a median need not be constant and independently aligned and graded carriageways have much to commend, provided that the opposing carriageway is not out of sight for extended periods. Local widening at intersections may be necessary to accommodate crossing or turning heavy vehicles. Due to numerous factors, usual practice is to widen on the median side for extra lanes and the median width adopted should include provision for future widening. Further discussion on the function and design of medians is provided in Road Medians, AASHTO 1996 Roadside Design Guide, Ref. 2.
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Figure 11.5: Typical Median Cross Sections
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11.9
Roadside Drains
11.9 Roadside Drain
Roadside drains remove water from the road and its surroundings in order to maintain the traffic safety and strength of the pavement. The basic types of roadside drains are: ● ● ●
Table drains; Catch drains; and Median drains.
11.9.1
Table Drains
Table drains are located on the outside of shoulders in cuttings or alongside shallow raised carriageways in flat country. An unsealed table drain should have its invert level below the level of pavement subgrade for effective drainage of the pavement. This becomes less important where a subsurface drain is provided at the edge of the pavement. Where scour is likely because of the nature of the material or because of the longitudinal grading, some type of protection of the drain invert would be required. This protection could take the form of loaming and grassing, rock lining or concrete. Lining is generally applicable where the material is likely to scour due to velocity. The terminal treatment at the bottom of a steep drain is also important. Consideration may also be given to sealing the outer edges of the pavement, the shoulder verges and the drain lining where siltation or scour could be a problem. Typical table drain details are shown in Figure 11.6. In flat country, the table drain is sometimes used as a source of borrow material. Flat bottom inverts may be adopted where there is a shortage of materials, and this has the additional benefit of reducing scour of the invert. The use of “V” drains should be discouraged due to adverse scouring potential. Table drains in flat country can hold water and cause damage to the pavement in some areas. The side slopes of table drains should be flat enough to minimize the possibility of errant vehicles overturning. Side slopes not steeper than 1 on 4 with a desirable slope of 1 on 6 are preferred.
medians (kerbed) are adopted; normal design practice applies where the kerb acts as a channel.
11.10 Noise Barriers Traffic noise and the need to protect the abutting environment are discussed in Section 6.3. Cross-sectional detail to provide for noise barriers is shown on Figure 11.7.
11.11 Right of Way The clearance to the right of way boundary can be measured from either the batter line or the edge of traffic lanes. It is dependent upon several factors including: ●
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●
11.9.2
Catch Drains
● ●
Catch drains are located on the high side of cuttings clear of the top of batters to intercept the flow of surface water and upper soil seepage water (Figure 11.6). Their purpose is to prevent overloading of the table drain and scour of the batter face. They are generally located at least 2.0 m from the edge of the cuttings in order to minimize possible undercutting of the top of the batter. Catch banks are sometimes used instead of drains to reduce effects of seepage on stability of the batter slopes.
● ● ● ●
Class of road; Landscape planting; Drainage requirements; Access for maintenance vehicles; Batters; Batter rounding; Requirements for services; and Cost, etc, in obtaining additional right of way.
Generally, a clearance of at least 5m to the batter line and 10m to the pavement edge is desirable. Extra clearance may be needed adjacent to high cut batters to prevent the erosion of batters affecting adjacent property.
11.12 Widths of Bridges 11.9.3
Median Drains
Where depressed medians are adopted, the median will be required to perform functions similar to those of a table drain. There are no special considerations required when raised
72
RURAL ROAD DESIGN
A guide to the width of traffic lanes on bridges and the clearance between the outer edge of traffic lanes and structures such as retaining walls, bridge handrails, guard fencing, and subways are set out in the Austroads Bridge Design Code (Ref. 29). Refer also to Section 9.13.
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Figure 11.6: Catch Drains and Table Drains
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73
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Figure 11.7: Noise Barrier Cross-Section Detail
74
RURAL ROAD DESIGN
5 12.
OT H E R D E S I G N C O N S I D E R AT I O N S
P R I N C I PA L FA CTO R S
The principal factors influencing the choice of design standard for a road are as follows.
12.1
other current projects, the effect can be much less than if the longer-term design was adopted in the first instance. One area where this approach is relevant is the high functional class low volume road. Even here, however, one must be aware of committing large amounts of current funds for very long-term options.
Financial Level 13.
The appropriate design standard for a particular road depends on both the overall availability of finance and the state of development of the road network. When the overall network is substantially adequate and finance is available, improvement projects will be directed at operational safety and efficiency, and higher geometric standards are appropriate. When the network is inadequate in terms of traffic demand and funds are limited, geometric standards may be lowered selectively on parts of the road system. The state of the network and the funding position are partly dependent on population, and the developed area over which the network must spread. In Australia, these vary between geographical and administrative regions, and it is reasonable that the appropriate design for individual roads should differ somewhat between regions.
12.2
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Auxiliary lanes are adjacent to the through traffic lanes. They are added to maintain the required level of service on the road and for other purposes supplementary to through traffic movement. They are used to remove traffic that is causing disruption to the smooth flow of traffic in the through lane, to a separate lane. Auxiliary lanes improve the safety, capacity and level of service on the road in question.
13.2
Types of Auxiliary Lanes
Traffic speed and congestion on rural arterial roads are largely determined by two factors: ●
Alignment and standard of a road affects the magnitude and the spread of operating speeds;
●
Interactions between faster and slower vehicles determine the extent of traffic delay and congestion. The effect of these interactions is greatest when the spread of speeds (the difference between the operating speeds of the fastest and slowest vehicles) is largest.
Energy
The total road fleet makes considerable use of liquid fuels and other products derived from crude oil. Grades exceeding about 5% cause greater consumption of fuel by heavy vehicles in the uphill direction than they save in the downhill direction. However, the greatest changes in the energy consumption related to transport spring from questions of appropriate modes for long freight haulage, and from land use distributions in urban areas. At the present stage, flattening of grades can rarely be justified on the basis of energy saving alone.
Of these two, traffic interactions have an increasingly dominant effect on delay and congestion as traffic flows increase. Overtaking opportunities, therefore, have a large effect on traffic operations on rural roads. These can be improved in varying degrees by the following methods: ● ● ● ● ● ●
12.4
General
Safety
Whatever design standard is adopted, safety is a major goal of road design. The theme of enabling the driver to perceive hazards in time to take appropriate action, and of providing geometric parameters appropriate to the likely speed of operation, runs throughout the Guide. Further, vehicles can get out of control, and items like traversable batter slopes, roadside safety barriers, breakaway light poles and sign supports are desirable attributes of what has been described as a ‘forgiving’ roadside.
12.3
13.1
AUXILIARY LANES
Stage Construction
● ●
In a situation of changing land use and growing traffic, no road can ever be regarded as ‘final’. There will always be requirements for future augmentation or modification. Where it is obvious that medium term requirements would alter the best-staged design for a particular road, it is often possible to modify the design slightly to provide better options for future action. While this ties up some funds and prevents their use on
Speed change lanes; Improved overtaking sight distance; Overtaking and climbing lanes; Wide full depth paved shoulders; Four lane wide cross sections; Dual carriageway cross sections; Slow vehicle turnouts; and Descending lanes.
The types of auxiliary lanes discussed in this section are as follows. ● ● ● ●
Speed change lanes (acceleration and deceleration); Overtaking lanes/climbing lanes; Slow vehicle turnouts; and Descending lanes.
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75
In addition, passing bays and emergency escape ramps (runaway vehicle facilities) are included in this category. In this guide, weaving lanes are not treated as auxiliary lanes but as part of the required cross section of a motorway where weaving conditions occur.
13.3
Speed Change Lanes
13.3.1
Acceleration Lanes
Acceleration lanes are provided at intersections and interchanges to allow an entering vehicle to access the traffic stream at a speed approaching or equal to the 85th percentile speed of the through traffic. They are usually parallel to and contiguous with the through lane with appropriate tapers at the entering point. The warrants for this type of auxiliary lane and the desirable road layouts are discussed in Austroads “Guide to Traffic Engineering Practice”, Part 5 – “Intersections at grade” (Ref, 18).
13.3.2
Deceleration Lanes
Deceleration lanes are provided at intersections and interchanges to allow an exiting vehicle to depart from the through lanes at the 85th percentile speed of the through lanes and decelerate to a stop or to the 85th percentile speed of the intersecting road, whichever is appropriate for the circumstances. These lanes are usually parallel to and contiguous with the through lanes with appropriate tapers at the departure point on the through lane. At intersections, the deceleration lane can be placed on either the right or the left of the through lanes, depending on the type of turn being effected. At interchanges, it is preferred that the exit be from the left side for most ramps and the deceleration lane will therefore be on the left in most cases.
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Details of the requirements for deceleration lanes are given in Austroads “Guide to Traffic Engineering Practice, Part 5 – “Intersection at grade” (Ref, 18)
13.4
Overtaking Lanes/ Climbing Lanes
13.4.1
Overtaking Lanes
13.4.1 Overtaking lane
13.4.1.1
Overtaking Demand
The demand for overtaking occurs each time a vehicle catches up with another and the driver wishes to maintain the speed of travel. Provided there is no approaching traffic, this manoeuvre can occur at where there is adequate sight distance. As traffic volume increases the approaching traffic will restrict the available places where overtaking can occur and these will be further limited by the road geometry. If demand is not met the results are: enforced following, the growth of traffic bunches, and driver delay and frustration. In extreme no-overtaking situations very long queues can develop behind the slowest vehicles in the traffic stream. The delay and frustration experienced on grades may be greater due to the slow speed of travel. The proportion of the journey time spent following in bunches is a useful measure of quality of service as seen by the driver. The type of slow vehicle influences the nature of overtaking demand. Some vehicles can be overtaken easily anywhere along a route, while for others an upgrade overtaking opportunity is desirable. In evaluating the need for auxiliary lanes, attention should be given to the type of slow vehicles involved and whether the overtaking demand is continuous along a route or confined to specific problem locations. Types of slow vehicles are:
On two lane two-way carriageways, overtaking lane configurations are shown on Figure 13.1. These overtaking lanes are provided to break up bunches of traffic and improve traffic flow over a section of road. They provide a positive overtaking opportunity and are sometimes the only real chance for overtaking to occur. The desirable layout is based on the start or end of the lane merge location being separated by a 3 second distance of travel time. This distance is to minimise the possibility of conflict between opposing merging vehicles. An acceptable layout, when the geometric considerations do not provide for an alternative is to allow the start of the merges to be opposite one another. The undesirable and unacceptable configurations are shown to highlight the possible conflict areas of late merging vehicles if these two were to be considered. These are not to be used.
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RURAL ROAD DESIGN
● ● ●
Vehicles with fairly high speeds, that slow down markedly on grades; Vehicles with low speeds, not affected by grades; and Vehicles with average speeds, that are seen as slow by those wishing to travel faster.
13.4.1.2
Overtaking Opportunities
On two-lane roads, the availability of overtaking opportunities depends on sight distance and gaps in the opposing traffic stream. As opposing traffic volume increases, overtaking opportunities become restricted even if sight distance is adequate. Sight distance that appears adequate may also be unusable on occasions due to the size of the vehicle in front, particularly on left-hand curves. On an existing road, overtaking opportunities can be increased either by improved alignment or the provision of overtaking
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Figure 13.1: Overtaking Lane Configurations
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Figure 13.2: Development of Overtaking Lane
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RURAL ROAD DESIGN
lanes. Of the two options, overtaking lanes will generally prove to be the most cost-effective in reducing the level of traffic bunching. This is because realignment to provide overtaking opportunities is likely to be a much more expensive option, and even then the opportunities are only available when opposing traffic permits. This has been demonstrated by ARRB simulation studies, which showed that the provision of overtaking lanes at regular spacings often led to greater improvements in overall traffic operations than even major alignment improvements (Ref. 58). A two-lane two way road with overtaking lanes at regular intervals provides an intermediate level of service between two lane two way roads and four lane roads, undivided and divided. The overtaking lanes may delay the need for the provision of dual carriageways. Where a four-lane road has already been provided, and traffic volumes are consistently high, the need for auxiliary lanes on grades may still arise when there are a high proportion of heavy vehicles.
13.4.1.3
Warrants
In deciding whether an overtaking lane is warranted, the evaluation needs to be carried out over a significant route length and not be isolated to the particular length over which the additional lane may be constructed. Overtaking opportunities outside the particular length can affect the result considerably. On multi lane roads, this may not apply since the reason for the extra lane will usually be confined to a specific location. The following guidelines are based on initial ARRB research using traffic simulation and benefit-cost analysis (Ref. 59). Alternatively, the need for an additional lane can be evaluated in terms of level of service. In special circumstances, a more detailed evaluation may be undertaken using traffic simulation or the results of prior ARRB research (Ref. 58).
The basis for adopting an overtaking lane is the traffic volume, the percentage of slow vehicles including light trucks and cars towing, and the availability of overtaking opportunities on adjoining sections. The percentage of road allowing overtaking is described in section 8.4 of this Guide. Table 13.1 gives the current-year design volumes (AADT) at which overtaking lanes would normally be justified. These guidelines apply for short low-cost overtaking lanes at spacings of 3 to 10 km or more along a road in a given direction. If spacing is less than this a specific cost benefit analysis will be needed to justify the construction at the shorter spacing. Development of an overtaking lane is shown in Figure 13.2.
13.4.1.4
Length
Table 13.2 (a) presents the adopted lengths of overtaking lane lengths that are appropriate for both grades and level terrain. On long grades, the values for a lower operating speed should be used. The minimum lengths provide for the majority of movements as single over takings, but may not allow many multiple over takings, or over takings between vehicles with only a small difference in speed. Minimum lengths are generally only appropriate for lower operating speeds or constrained situations. Overtaking lanes may be extended up to the normal maximum length to allow start and termination points to fit in with the terrain. However since bunches generally break up in the first section of the overtaking lane, the additional length is not as well utilised. As a general rule, it is more cost-effective to construct two short overtaking lanes several kilometres apart rather than to construct one long one in excess of the normal maximum length. Even when very long bunches occur at the start of an
Table 13.1 Traffic Volume Guidelines for Providing Overtaking Lanes
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Overtaking Opportunities Over the Preceding 5 km (1) Description
Current-year Design Volume (AADT)
Percent Length Providing Overtaking (2)
Percentage of Slow Vehicles (3) 5
10
20
Excellent
70-100
5670
5000
4330
Good
30-70
4330
3670
3330
Moderate
10-30
3130
2800
2470
Occasional
5-10
2270
2000
1730
Restricted
0-5
1530
1330
1130
0
930
800
670
Very Restricted (4)
Note: (1) Depending on road length being evaluated, this distance could range from 3 to 10 km. (2) See Section 8.4.4. (3) Including light trucks and cars towing trailers, caravans and boats. (4) No overtaking for 3 km in each direction.
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Table 13.2 (a) Overtaking Lane Lengths Operating Speed (kmh)
Overtaking Lane Lengths (excluding taper lengths) (m)* Minimum
Desirable Minimum
50
75
225
325
60
100
250
400
70
125
325
475
80
200
400
650
90
275
475
775
100
350
550
950
110
420
620
1070
Normal Maximum
Note: * (1) Derived from Table VI Ref. 59 (2) Refer Table 13.8 for diverge and merge taper lengths (3) For road train routes, the normal maximum should be the minimum and lengths 1.5 times the ‘normal maximum’ are desirable.
overtaking lane, it is generally preferable to provide several overtaking lanes at regular spacings rather than one very long one. This should break up traffic bunches before they become very long. The length of an overtaking lane on a grade is largely constrained by the choice of appropriate locations for start and termination points. These should be clearly visible to approaching drivers, and be located to minimise speed differences between slow and fast vehicles. These constraints, however, sometimes lead to quite long and/or expensive climbing lane proposals.
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The sight distance to the termination of the overtaking lane is based on the distance for the vehicle in the fast lane to complete or abandon the overtaking manoeuvre. The sight distances required to overtake the various types of MCV’s are shown in Table 13.2 (b). Situations may exist however, where an overtaking lane might end where the sight distance is less than that required to complete an overtaking. In such cases drivers will have to rely on adequate signage of the termination.
13.4.1.5
Location
The location of overtaking sites should be determined after considering the following: ●
●
Strategic planning of the road in question and the long term objectives of that link – the spacing and consequently, expenditure, must be in accord with the strategy to obtain the best use of funds over the whole network; Nature of traffic on the section of road – if queuing occurs all along the route, then overtaking lanes at any location will be useful; if they occur at specific locations where slow vehicles cause the queue, then specific locations should be chosen;
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RURAL ROAD DESIGN
●
Location of grades – may be more effective to take advantage of the slower moving vehicles;
●
Costs of construction of the alternative sites – may get a more cost effective solution by locating on the sites where construction is cheapest;
●
Geometry of the road – when the sites are not on grades, sections with curved alignment and restricted sight distances are generally preferable to long straight sections. These locations will make the location appear appropriate to the driver. However, sections with curves with reduced safe speeds are not suitable for overtaking lanes.
If the conclusion is that the overtaking lane should be located on a grade, the length will be tailored to fit the grade. If the costs of the lane on the grade outweigh the benefits of being on the grade, the lane should be located to minimise the costs. Alternatively, a partial climbing lane could be considered (see “Climbing Lanes” Section 13.4.2).
13.4.1.6
Spacing
The factors already discussed must be taken into account in deciding the spacing of the overtaking lanes on a section. An analysis of the operating conditions over the whole link in the network, combined with the strategy for that link will establish the desired locations and therefore the spacing of the overtaking lanes. In general, if no auxiliary lanes exist, establishing the first ones at a larger spacing will provide better service than placing two lanes in close proximity. In the first instance, a spacing of up to 20km (Ref 98) may be appropriate, depending on the available overtaking opportunities. A more desirable spacing would be from 10 to 15km with the objective of providing overtaking opportunities every 5km in the long term. The intermediate lanes will be provided between the initial installations as required as the traffic grows.
Table 13.2 (b) Merge Sight Distance at End of Overtaking Lane for Cars Overtaking MCV’s Multiple Combination Vehicles Operating Speed (kmh)
Car & Prime Mover Semi-Trailer
B Double
Type 1 Road Train
Type 2 Road Train
50
110
120
130
145
60
135
145
160
180
70
165
180
195
225
80
200
220
245
285
90
250
270
305
355
100
300
330
345
400
110
375
410
410
435
120
430
430
430
435
130
450
450
450
450
There may be cases where the spacing is closer (3km) because of the proximity of long grade sections requiring treatment. A further case where the spacing may be close is where two partial climbing lanes are provided on the same long grade to reduce the total costs involved. In all these cases, the availability of overtaking opportunities on adjacent sections must be taken into account. Further research is needed into the effect of various combinations of configurations, length and spacing, on the traffic operations and level of service of overtaking lanes.
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13.4.1.7
Improvement Strategy For Overtaking Lanes
The goal of any improvement strategy is to identify and plan for staged development that will keep pace with increases in traffic demand, ensuring the availability of overtaking opportunities at regular intervals. A strategy for improving operational performance of two-lane two-way rural roads should consider overtaking lane strategy in the context of potential future road duplication. With an overtaking lane strategy, overtaking lanes should be provided to maintain the desired level of service. Full duplication of the road will not normally be anticipated during the economic life of these improvements. This period of time, typically 20 years, will be used to recover the cost of the improvements. This strategy should be applied when there are no existing overtaking lanes. The proposed spacing (for each direction) will typically be 3 to 10 km. The upper limit for an overtaking lane strategy is 800 veh./hr, if the desired level of service is C. If the desired Level of Service were B, 500 veh/hr would be the upper limit (Ref. 66). For hourly traffic volumes above the suggested limits, a strategy that is compatible with future road duplication should be adopted. In this situation, full duplication will normally
occur within the economic life of the overtaking lane pavement. Sections of duplication 2km long and at 5km spacings are usually warranted. This strategy does not necessarily preclude the use of some overtaking lanes, particularly at the initial stages. However, it is highly desirable to use all improvements in the final road duplication. Further analysis of a particular section of road will be required to determine the optimum combination of overtaking lane length and spacing.
13.4.2
Climbing Lanes
13.4.2.1
General
Climbing lanes can be considered as a special form of overtaking lane but they are only provided on inclines. Where they are provided, they form part of the network of overtaking opportunities and will therefore have an effect on decisions on the location of other overtaking lanes. On multi lane roads, there is no need to take account of the overall overtaking situation, as the effect is limited to the specific location of the grade in question. The decision on whether to add a climbing lane is based on level of service considerations only. Climbing lanes on multilane roads are specifically provided for slow moving vehicles and are therefore treated differently for signing and line marking. Refer Section 13.5.
13.4.2.2
Warrants
Climbing lanes are warranted where: Truck speeds fall to 40km/h or less; and Traffic volumes equal or exceed those in Table 13.3. In addition, climbing lanes should be considered where: ● Long grades over 8% occur; ● Accidents attributable to the effects of the slow moving trucks are significant;
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Table 13.3: Volume Guidelines for Partial Climbing Lanes Overtaking Opportunities Over the Preceding 5km (1)
Current Year Design Volume (AADT)
Description
Percent Length(2) Providing Overtaking
50
110
120
130
5
10
20
Excellent
70-100
4500
4000
3500
Good
30-70
3500
3000
2600
Moderate
10-30
2500
2200
2000
Occasional
5-10
1800
1600
1400
Restricted
0-5
1200
1000
900
0
700
600
500
Very Restricted
(4)
Percentage of Slow Vehicles(3) 145
Note: (1) Depending on road length being considered, this distance can range from 3 to 10km. (2) See section 8.4.4. (3) Including light trucks and cars towing trailers, caravans and boats. (4) No overtaking for 3km in either direction.
Table 13.4 (a) Grade/Distance Warrant (Lengths (m) to Reduce Truck Vehicle Speed to 40 km/h). Approach Speed (km/hr)
+ve Grade% 4
5
6
7
8
9
10
100
-
-
1050
800
650
550
450
80
630
460
360
300
270
230
200
60
320
210
160
120
110
90
80
Table 13.4 (b): Merge Sight Distance at end of Climbing Lane for Cars Overtaking MCV’s Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
Multiple Combination Vehicles Operating Speed (km/h)
82
Car & Prime Mover Semi Trailer
B Double
Type 1 Road Train
Type 2 Road Train
50
100
100
105
120
60
130
130
135
155
70
150
160
175
205
80
185
200
220
260
90
230
250
280
325
100
285
305
345
400
110
350
350
350
400
120
385
385
385
400
130
400
400
400
400
RURAL ROAD DESIGN
● ●
Heavy trucks from an adjacent industry enter the traffic stream on the up grade; and The level of service on the grade falls two levels below that on the approach on the up grade or to level “E” (Ref 1.).
Development of a climbing lane is shown in Figure 13.2.
13.4.2.3
Length
The length of the grade and the start and end points of the lane dictate the length of the climbing lane. The theoretical start point is taken as the point at which the speed of the truck falls to 40km/h and decelerating. The point at which the truck has reached a speed equal to operating speed minus 15km/h and is accelerating determines the end of the lane. The starting and ending points of the lane should be clearly visible to drivers approaching from that direction. Table 13.4 (a) indicates the lengths on constant individual grades needed to produce a reduction in truck speed to 40km/h. Truck speeds on grades can be assessed using the curves included in Figure 13.3 and the longitudinal section of the road. These curves assume an entrance speed to the grade of 100km/h. This is conservative as modern trucks can operate at highway speeds approaching those of cars. If more precise design is required, the conditions should be analysed using software designed to simulate truck performance and using entrance speeds based on the operating speed at the site.
it will be satisfactory to use a turnout on part of the up grade. A turnout may be appropriate if traffic volumes are low or construction costs are very high. Turnout lengths of 60 to 160 m for average approach speeds of 30 to 90 km/h respectively and a width of 3.7 m is to be used. If a turnout is used, care must be taken to provide adequate sight distance. Signing at the start and merge points are required to better indicate diverge and merge locations. The minimum sight distance should be stopping distance for the Operating Speed.
13.5.2
Passing Bays
On two lane two-way roads a passing bay may be provided as shown on Figure 13.4, for slow vehicle turnouts. On steep grades where truck speeds can reduce to a “crawl” speed less than 20km/h and a full climbing lane can not be provided, passing bays may provide an improvement to traffic flow. A passing bay is a very short auxiliary lane (of the order
13.5.2a Passing bay (sequential)
The sight distance to the termination of the climbing lane is based on the distance for the vehicle in the fast lane to complete or abandon the overtaking manoeuvre. The sight distances required to overtake the various types of MCV’s are shown in Table 13.4 (b). The starting point should be located at a point before the warrant is met to avoid the formation of queues and possibly hazardous overtaking manoeuvres at the start of the lane. If the length of climbing lane exceeds 1200m, the design should be reconsidered. Options include: ●
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● ● ●
Partial climbing lane; Passing bay(s) in extreme conditions; Overtaking lane prior to the grade (where the delays on the grade are not excessive); and Retention of the climbing lane where traffic volumes are sufficiently high.
13.5
Slow Vehicle Turnouts
13.5.1
Partial Climbing Lanes
A turnout is a very short section of paved shoulder or added lane that is provided to allow slow vehicles to pull aside and be overtaken. It differs from an overtaking lane in its short length, different signing, and the fact that the majority of vehicles are not encouraged to travel in the left lane. On dual carriageways a partial climbing lane for slow vehicles can be provided as shown on Figure 13.4. While climbing lanes should preferably be designed to span the full length of the grade, there may be circumstances where
RURAL ROAD DESIGN
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Figure 13.3: Determination of Truck Speeds on Grades.
84
RURAL ROAD DESIGN
of 100m) that allows a slow vehicle to pull aside to allow a following vehicle to pass. The passing bay provides for the overtaking of the slowest vehicles and is only appropriate if all of the following conditions are met: ● ● ● ●
Long grades over 8%; High proportion of heavy vehicles; Low overall traffic volumes; and Construction costs too high for full climbing lanes.
Passing bays must be properly signed to ensure their effectiveness. Normally, 300m advance warning of the location of the bay is required to allow heavy vehicle drivers to prepare for the overtaking manoeuvre and to alert other drivers to the approaching facility.
13.6
Descending Lanes
On steep down grades the speed of trucks will be as low as that on equivalent up grades as shown on Figure 13.3 with a similar effect on traffic flow if overtaking opportunities are not available. A descending lane will be appropriate in these circumstances.
If overtaking sight distance is available overtaking will be readily accomplished and a descending lane will not be needed. Similarly, if a climbing lane is provided in the opposite direction, and the overtaking sight distance is adequate, overtaking slower down hill vehicles can be safely achieved and a descending lane will not be needed. Where the downgrade is combined with tight horizontal curves, a descending lane will be appropriate to provide satisfactory traffic operation. Design details are similar to those of climbing lanes.
13.7
Runaway Vehicle Facilities
13.7.1
General
Where long steep grades occur it is desirable to provide emergency escape ramps at appropriate locations to slow and/or stop an out-of-control vehicle away from the main traffic stream. Out-of-control vehicles result from drivers losing control because of loss of brakes through overheating or mechanical failure or because the driver failed to change down gears at the appropriate time. Experience with the installation and operation of emergency escape ramps has led to the guidelines described below.
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Figure 13.4: Development of Slow Vehicle Turnouts
RURAL ROAD DESIGN
85
13.7.2
Types of Escape Ramps
Figure 13.5: Types of Vehicle Escape Ramps
Figure 13.5 illustrates four types of escape ramps.
13.7.2.1
Sand Pile
The sand pile types are composed of loose, dry sand and are usually no more than 130m in length. The influence of gravity is dependent on the slope of the surface of the sand pile. The increase in rolling resistance to reduce overall lengths is supplied by the loose sand. The deceleration characteristics of the sand pile are severe and the sand can be affected by weather. Because of these characteristics, the sand pile is less desirable than the arrester bed. It may be suitable where space is limited and the compact dimensions of the sand pile are appropriate.
13.7.2.2
Descending Grade
Descending grade ramps are constructed parallel and adjacent to the through lanes of the highway. They require the use of single sized or uniform graded aggregate to prevent compaction in an arrester bed to increase rolling resistance and, therefore, slow the vehicle. The descending-grade ramps can be rather lengthy because the gravitational effect is not acting to help reduce the speed of the vehicle.
13.7.2.3
Horizontal Grade
For the horizontal-grade ramp, the effect of the force of gravity is zero and the increase in rolling resistance has to be supplied by an arrester bed composed of single sized or uniform graded aggregate to prevent compaction. This type of ramp will be longer than those using gravitational force acting to stop the vehicle.
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13.7.2.4
Ascending Grade
The ascending-grade ramp uses both the arresting bed and the effect of gravity, in general reducing the length of ramp necessary to stop the vehicle. The loose material in the arresting bed increases the rolling resistance, as in the other types of ramps, while force of gravity acts downgrade, opposite to the vehicle movement. The loose bedding material also serves to hold the vehicle in place on the ramp grade after it has come to a safe stop. Ascending grade ramps without an arresting bed are not encouraged in areas of moderate to high commercial vehicle usage as heavy vehicles may roll back and jack-knife upon coming to rest. Each one of the ramp types is applicable to a particular situation where an emergency escape ramp is desirable and must be compatible with the location and topography. The most effective escape ramp is an ascending ramp with an arrester bed. On low volume roads of less than approximately 1000 vehicles per day, clear run off areas without arrester beds are acceptable.
13.7.3
Location of Runaway Vehicle Facilities
Runaway vehicle facilities should not be constructed where an out of control vehicle would need to cross oncoming traffic. On divided roadways where adequate space is available in the median, safety ramps can be located on either side of the
86
RURAL ROAD DESIGN
carriageway with adequate advance warning sings prior to the safety ramp exit. For safety ramps to be effective their location is critical. They should be located prior to or at the start of the smaller radius curves along the alignment. For example, an escape ramp after the tightest curve will be of little benefit if trucks are unable to negotiate the curves leading up to it. Vehicle brake temperature is a function of the length of the grade, therefore escape ramps are generally located within the bottom half of the steeper section of the alignment. Lack of suitable sites for the installation of ascending type ramps may necessitate the installation of horizontal or descending arrester beds. Suitable sites for horizontal or descending arrester beds can also be limited, particularly if the downward direction is on the outside or fill side of the roadway formation.
13.7.4
Arrester Beds and Escape Exits
An arrester bed is a safe and efficient facility used to deliberately decelerate and stop vehicles by transferring their kinetic energy through the displacement of aggregate in a gravel bed. An escape exit consists of any surfacing used in the event of an emergency that will allow a runaway vehicle to exit the downgrade off the road and decelerate to a lower speed. For example, escape exits can be side streets, sidetracks or accesses that are not normally signed as a safety ramp. An arrester bed is a particular kind of escape exit. The following
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13.7.4 Arrester bed and Escape Exit Note: One-way carriageway (sequential)
section lists broad guidelines for the design of arrester beds and escape exits.
13.7.4.1
Arrester Beds
From field tests and other research studies, rounded particles such as uncrushed river gravel with uniform gradation produce higher deceleration than the more angular crushed aggregate. This is because the vehicles sink deeper in to the river gravel, transferring more energy to the stones over a shorter length. The use of a material with low shear strength is desirable in order to permit tyre penetration. Sand is not ideal because it consolidates with time and moisture ingress. Crushed stone has been used but is not considered effective as it will require longer beds and will need regular ‘fluffing’ or de-compaction. Nominal 10mm river gravel has been used satisfactorily in testing. The gravel should be predominantly rounded, of uniform gradation, free from fine fractions and with a mean particle size ranging between 12mm and 20mm. In general,
RURAL ROAD DESIGN
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Figure 13.6: Typical Arrester Bed Layout
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RURAL ROAD DESIGN
gravels with a smaller internal friction angle will perform better than those with larger internal friction angles.
maintenance regime to ensure their continued effectiveness.
13.7.4.2 An appropriate crush test such as the Los Angeles abrasion test (or equivalent) should be used to evaluate durability of the stone. Stones with a high crush test will not deteriorate and will therefore not produce fines.
Escape Exits
Lengths will vary depending on the gradient of the facility and the surface material used (specific to the site). Wambold et al (Ref 101) recommend the following formula to determine the length of a truck escape ramp exit.
A typical arrester bed is shown in Figure 13.6. L A gradual or staged increase in the depth of the bed should be provided on the entry ramp. This is to ensure a gradual rate of deceleration when entering the ramp. The first 50 metres of the bed acts as the entry ramp and should increase in depth from 50 mm to 350 mm of suitable material. Over the first 50 m of the arrester bed length the depth increases to 450mm and remains at that depth for the rest of the bed length. A bed constructed to this design would accommodate low speed entries within the 350mm deep section of the bed. Vehicles entering at higher speeds will slow down significantly as they reach the deeper section of the bed, thus reducing the chances of the vehicle being damaged.
= 0.004V2 / (r + G)
where L
= Distance to top, the escape exit (m)
V = Entering velocity (km/h) G = Grade (g1) divided by 100 (m/m) r
= Rolling resistance expressed as equivalent grade (%) divided by 100.
Values of r for several materials given in Table 13.5 The average deceleration achieved in sand or gravel bed is: ●
Sand 350mm deep
2.8m/sec2;
●
Sand 450mm deep
3.4m/sec2;
●
Gravel 350mm deep
3.0m/sec2; and
●
Gravel 450mm deep
3.7m/sec2 (Ref 91)
These decelerations may be used in the following formula to calculate the length of an arrester bed. L
= V2 / (26a + 2.55g1)
Table 13.5: Rolling Resistance Values Surfacing Material
Rolling Resistance (r)
Portland Cement Concrete
0.010
Ashphalt Concrete
0.012
Gravel Compacted
0.015
Earth, sandy and loose
0.037
Crushed Aggregate, loose
0.050
where:
Gravel, loose
0.100
L
Sand
0.15
Pea Gravel (uniform grading)
0.25
= length of full depth arrester bed excluding 50m transition at start (m)
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V = entry speed (km/h) a
= deceleration (m/sec2)
g1 = grade (%) (positive for upgrade, negative for downgrade). A 50m entry ramp provides a satisfactory and safe means of entering the full depth of the arrester bed; this entry ramp is not included in calculations for bed length. Where insufficient length is available at a particular site for stopping the vehicle at the anticipated entry speed, the bed depth should be increased in stages from 350mm up to 450mm. The increasing depth will provide greater deceleration toward the end of the bed allowing the vehicle to stop within the available length. However, each case should be designed on its merits. Sand has problems of drainage, compaction and contamination and should not be used unless alternative materials are unavailable. Beds using sand will require a strict
The design of arrester beds and escape exits is site dependent, and careful consideration of all of the factors discussed in Section 13.7.4.4. For escape exits, careful consideration of the land use adjacent to the exit is required. Local streets should only be used at the top of steep exit grades where the truck has decelerated to a speed equal to the posted speed limit. Existing roads and streets used for property access should only be used where the traffic volume is very low and there is a very low probability of an escaping truck meeting another vehicle.
13.7.4.3
Spacing
For new projects Table 13.6 may be used as a guide when considering the need for escape exits on grades greater than 6% and with numbers of commercial vehicles exceeding 150 per day. The distances in Table 13.6 are not absolute and greater distances could be acceptable, as site location is dependent on factors discussed in Section 13.7.4.4. The need for a facility
RURAL ROAD DESIGN
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Table 13.6:
Grade (%)
Approximate Distance from the Summit to Ramp * (km)
6-10
3
10-12
2.5
12-15
2.0
15-17
1.5
17
1.0
Note: • Actual distances will depend on site topography, horizontal curvature and costs. Table 13.7:
Maximum Speed Decrease between Successive Geometric Elements
Grade (%)
Maximum Decrease in Speed between Successive Geometric Elements (km/hr)
10
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●
Vehicles that enter the ramp will have to be retrieved, as it is likely that they will not be able to remove themselves from the arrester bed. An appropriate service road adjacent to the ramp is required to effect retrieval. An alternative and/or enhancement to the service road is the provision of anchorage points/blocks for winching vehicles out;
●
When the location of the ramp is such that the length is inadequate to fully stop an out-of-control vehicle, a positive attenuation (or ‘last chance”) device may be required. Care is required to ensure that the device does not cause more problems than it solves – sudden stopping of the truck can cause the load to shift with potentially harmful consequences to the driver and the vehicle. Judgement will be required on whether the consequences of failing to stop are worse than these effects. Crash cushions or piles of sand or gravel have been used as “last chance” devices.
8
A Brake Check Area is an area set aside for commercial vehicles at the top of a steep descent. A Brake Rest Area, however, is an area set aside part way down or at the bottom of the decent.
6
Summary of Design Considerations
The length of the escape ramp must be sufficient to dissipate the kinetic energy of the vehicle;
●
The alignment of the ramp should be straight or of very gentle curvature to relieve the driver of undue vehicle control problems; The width should be wide enough to accommodate two vehicles if it is considered likely that a second vehicle will need to use the ramp soon after the first one;
●
The arrester bed material should be clean, not easily compacted or consolidated and have a high coefficient of rolling resistance;
●
The full depth of the arrester bed should be achieved in the first 50m of the entry to the bed using a tapering depth from 50mm at the start to the full depth at 50m;
●
The bed must be properly drained;
●
The entrance to the ramp must be designed so that a vehicle travelling at high speed can enter it safely. A 5º angle of departure or less is required, and as much sight distance as possible should be provided. The leading edge
90
Comprehensive signing is required to alert the driver to the presence of the escape ramp;
13.7.5
●
●
●
10
will be increased if the number of commercial vehicles is more than 250 per day and the maximum decrease in Operating Speed between successive geometric elements is approaching the limits set in Table 13.7.
13.7.4.4
of the arrester bed must be normal to the direction of entry to ensure that the two front wheels of the vehicle enter the bed simultaneously;
Approximate Distance from Summit to Safety Ramp
RURAL ROAD DESIGN
Brake Check and Brake Rest Areas
These facilities should be provided, at least to an unsealed gravel condition, on routes that have long steep downgrades and commercial vehicle numbers of around 100 per day, especially on National Highways and principal traffic routes. These areas, when used, will ensure that drivers begin the descent at zero velocity and in a low gear that may make the difference between controlled and out-of-control operation on the downgrade. It also would provide an opportunity to display information about the grade ahead, escape ramp locations and maximum safe descent speeds. These areas may need to be large enough to store several prime mover and semi-trailer combinations, the actual numbers depending on volume and predicted arrival rate. The location will need good visibility with acceleration and deceleration tapers provided, as discussed in Section 8 and Section 13.8.2. Adequate signage will be required to advise drivers in advance of the facilities. Special signs, specific to the site, will need to be designed for these areas.
13.8
Geometry of Auxiliary Lanes
13.8.1
Starting and Termination Points
The start and termination points of an auxiliary lane should be clearly visible to approaching drivers from that direction. The start point should be prior to the point at which the warrant is met to avoid potentially hazardous overtaking manoeuvres. Visibility to this point should be sufficient for the driver to assess the situation and make a decision on the course of action to take.
The termination of the auxiliary lane should only be at a point where there is sufficient sight distance for the overtaking driver to decide whether to complete or abandon the overtaking manoeuvre. The overtaking sight distances given in Section 8.4.2 Table 8.4 may be used. These distances were adopted from the research (Ref 95) carried out in 1981.
Table 13.8: Tapers for Diverges and Merges Operating Speed (km/h)
It is desirable for the termination point to be on a straight to give drivers a better visual appreciation of the approaching merge. Termination on a left-hand curve should be avoided because slow vehicles are seriously disadvantaged by reduced rear vision. It is also desirable that the termination point be on a downgrade to minimise the speed differential between vehicles. There are however, some examples in the state of Queensland where an auxiliary lane must end where the sight distance is less than that required to complete an overtaking. In such cases, drivers have to rely upon signing. Eye height and object height requirements at least must be achieved. This method should only be used when all other options have been considered.
13.8.2
Tapers
Diverging Taper The widening of the pavement at the start of the auxiliary lane is achieved with a taper. The length of the taper should be sufficient to permit easy diverging of traffic with the slower traffic moving to the left and the faster traffic going to the right lane. This length depends on the speed of the approaching traffic and the width of the through lane. The rate of the lateral movement is assumed to be 1.0m/sec, giving the following formula for taper length: TD = VW/3.6 where: TD = Diverge taper length (m) V = Operating speed (km/h) W = Amount of pavement widening (m)
60
60
100
70
70
115
80
80
130
90
90
150
100
100
165
110
110
180
120
120
200
130
130
210
A “run out” area should be provided through the merge area to accommodate those vehicles prevented from merging as they approach the narrowed section. This can be achieved by maintaining a total pavement width in the direction of travel equal to at least the sum of the full lane width plus a shoulder width of 2.0m over the full length of the taper plus 30m (see Figure 13.2).
13.8.3
Cross Section
13.8.3.1
Pavement Width
The width of the auxiliary lane should not be less than the normal lane width for that section of road.
13.8.3.2
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Merging Taper At the termination of the auxiliary lane, a taper that allows the two streams to merge into one should reduce the pavement width. Since this situation is equivalent to the dropping of a lane, drivers will be less prepared for the merging action than they would be if merging from an acceleration lane. It is therefore necessary to adopt a lesser rate of merging than for the tapers on acceleration lanes and a rate of 0.6m/sec is used. The minimum length depends on the speed of the approaching traffic and the width of the lane and is determined from the following formula: TM
Crossfall
The crossfall of the auxiliary lane will usually be the same as the adjacent lane. Because of the additional width of pavement, the depth of water flowing on the pavement should be checked to ensure that aquaplaning does not occur. It may be necessary to change the crown line to overcome this type of problem.
13.8.3.4
Lane Configurations
The specific circumstances of each design will dictate the preferred treatment for individual locations but the following considerations should be taken into account when deciding on the layout of the design: ●
If duplication is a longer term goal, providing a section of four lane divided road may be a logical first stage;
●
Providing a four lane section of divided road is applicable when the analysis of the road shows that a spacing less than 5km is required and the topography is suitable;
●
The merge areas of opposite overtaking lanes should be in accordance with Figure 13.1;
= VW/2.16
where: TM = Merge taper length (m) V = Operating speed (km/h) W = Amount of pavement widening (m) (This formula has been derived on the basis of a merging rate of 0.6m/sec2 of lateral movement)
Shoulder Width
A shoulder width of 1.0m is often satisfactory because the pavement has been widened over the section with an auxiliary lane. This width will have to be increased in areas of restricted visibility (eg. around curves) and in the merge area at the end of the lane.
13.8.3.3 If convenient, developing the widening around a horizontal curve can improve appearance and contribute to an easier divergence of the traffic into the fast and slow streams
Taper Length (m) Diverge Merge (TD) (TM)
RURAL ROAD DESIGN
91
●
Diverges may occur opposite each other without any special requirements.
13.8.4 13.8.4.1
Line marking and Signing Signs
All forms of auxiliary lane should be signed as Overtaking Lanes and the sign Keep Left Unless Overtaking should be used as specified in relevant standards (see Ref 8). This form of signing encourages maximum use of the auxiliary lane and allows overtaking even between vehicles travelling at similar speeds.
Perceived operational or safety problems on a given road section. The use of more restrictive line markings should not be too widespread, since the presence of apparently unnecessary barrier lines can lead to driver frustration and a reduced quality of service on a road. ●
14. 14.1
The alternative signs for Slow Vehicle Lane and Slow Vehicles Keep Left should only be used in exceptional circumstances where it is specifically desired to encourage a lesser use of the added lane. Passing bays should be specifically signed to alert drivers of their existence. The provision of advance signs for auxiliary lanes promotes road safety and improves the quality of service as perceived by the driver. Having seen such a sign, drivers wishing to overtake may relax their search for overtaking opportunities and are less likely to accept gaps with low safety margins. Advance signs are particularly appropriate when significant bunching occurs for 3 minutes of driving time (at the slow vehicle's speed) before the commencement of an auxiliary lane.
13.8.4.2
Linemarking
General practice for marking overtaking barrier lines on rural roads is described in the relevant standards (see Ref 8). For auxiliary lanes constructed as three-lane road sections, three particular aspects are of relevance:
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●
In (direction 1 of the auxiliary lane traffic) it is normal practice to provide a continuous barrier line over the full auxiliary lane length – including tapers – to prohibit any use by direction 1 vehicles of the third or opposing traffic lane. This also serves to define the centreline of the road and indicate that the centre lane is primarily for direction 1 traffic.
●
For direction 2 (opposing traffic) a barrier line is generally provided adjacent to the auxiliary lane diverge and merge tapers.
●
For direction 2 traffic adjacent to an auxiliary lane in direction 1, AS 1742 (Ref. 8) recommends that the direction 2 lane separation line marking follow normal practice for two lane roads. This means that, if sight distance permits, direction 2 vehicles may be permitted to use the centre lane as an opposing traffic lane provided no vehicles are encountered in that lane.
● ● ●
Short auxiliary lane length Moderate to heavy traffic volumes Sight distances only marginally adequate for overtaking, and
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RURAL ROAD DESIGN
General
Vehicle stopping areas are roadside facilities that are placed beyond the edge of shoulder along a roadway, allowing travellers to safely stop and rest, well clear of the through traffic. Provision of vehicle stopping areas is important for maintaining an efficient and safe movement of vehicles along a route. Vehicle stopping areas can be divided into: Service Centre ● Highway Service Centre; and ● Highway Service Town. Major Rest Area ● Major rest area, catering for light and heavy vehicles combined; ● Major rest area, catering for light and heavy vehicles separated; and ● Welcome Centres. Basic Rest Area ● Basic rest area catering for light vehicles only; ● Driver Reviver sites; ● Truck Parking areas; and ● Truck Changeover areas. Other Areas ● Lay-bys; ● Breakdown bays; ● Bus Bays; ● Telephone bays; and ● Enforcement areas for speed and for overloaded heavy vehicles. Depending on the facility provided, the use will vary for each of these facilities, but will generally take the form of: ● ● ●
Some use of auxiliary lane sections by opposing traffic is allowed, particularly when traffic volumes are low. However there may be cases where more restrictive line marking is appropriate. These will generally arise when there exists a combination of the following factors:
V E H I C L E S TO P P I N G A R E A S
● ● ●
Stopping for fuel and food; Carry out emergency repairs; Change drivers; Rest to alleviate fatigue; Seek emergency assistance; and Pick up and/or set down passengers.
14.2
Service Facilities
14.2.1
Rest Areas
Rest areas are areas clear of the road carriageway, where vehicles may park and where basic facilities such as toilets,
picnic tables, etc. are provided. There are two types of rest areas: ● ●
14.2.1.1 Major Rest Area
Major rest areas; and Basic rest areas.
14.2.1.1
Major Rest Areas
Major rest areas include combined major rest areas, separated major rest areas and welcome centres. a) Combined Major Rest Areas ● These are major rest areas that cater for both heavy and light vehicles. These rest areas should be separated by a distance of three to four hours driving time. ●
Preferably these should be placed at the crest of a hill or in flat areas to allow trucks to enter and leave the site easily.
●
Heavy vehicle parking should be separated from the light vehicle parking areas, and any recreation facilities. Trees or sound absorbing walls should be used for the separation.
●
Major rest areas should include the following facilities: – Parking for cars, cars and caravans and trailers; – Sheltered parking for heavy vehicles; – Covered tables and seats; – Toilets; – Shelter; – Rubbish and recycle bins, if viable; – Water; – Children’s play/exercise areas; – Shade; – Lighting;
– – – – – – – – –
Barbecues or fireplaces, if practical; Emergency telephones; Access to facilities for disabled people; Parking area for more than 10 cars; Parking area for 5 prime movers and semi-trailers; Information board, including local geographic and historical information (no advertisements); Sealed access and parking areas; Acceleration and deceleration lanes on approach and exit respectively; and Turning lanes where site services both carriageway directions.
b) Separated Major Rest Areas ● These rest areas should be spaced at three to four hours driving distance.
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14.2.1.2 Basic Rest Area
RURAL ROAD DESIGN
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●
Where possible a rest area should cater for both directions of travel.
●
These sites should provide 10 or more spaces for light vehicles and 5 or more separate spaces for heavy vehicles.
●
Sign posting is discretionary for these sites.
14.2.1.3
c) Welcome Centres ● These are centres that are designed to attract tourists and are established at the gateway to a major tourist region, featuring tourist and accommodation information. ●
These centres offer services to motorists including toilets, telephones, food and information.
●
Sites should provide a minimum of 15 or more parking spaces for cars (6m x 2.4m) and a minimum of 5 spaces for car and caravans (14m x 2.4m).
14.2.1.2
a) Lay-bys and Breakdown Bays ● The provision of wide shoulders for discretionary parking is both expensive and unwarranted. However, there may be a need to provide lay-bys at regular intervals for vehicles to stand clear of the carriageway, and provision should be made accordingly. Passenger vehicle lay-bys should be a minimum of 4.5m wide from the edge line and 20m long to accommodate two vehicles. Where the predicted AADT exceeds 1000 they should be approximately 10km apart, staggered on alternate sides of the road at 0.5km intervals. Where volumes are less than 1000 AADT, the spacing may be extended to a maximum of 15km. Preferably, lay-bys should be sealed, however a gravelled surface is acceptable. Desirable locations for lay-bys include sags, flat areas near cutting/embankment lines, pick-up points for country school buses, and adjacent to property access points.
Basic Rest Areas
Basic rest areas are provided for light vehicles only. ●
Basic rest areas should be provided at 50km intervals where the AADT exceeds 1000 and the distance between towns having comparable facilities exceeds 50km. They should be provided at reducing intervals of 30 km where the AADT exceeds 2500 and the distance between towns having comparable facilities exceeds 30km (Ref. 78).
●
These sites can be built so that access is from one direction only.
●
Sites should include: – Off road parking for cars, caravans, and trailers; – Covered tables and seats; – Rubbish bins; – Potable water; – Electricity; – Toilets; and – Five or more car parking spaces.
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a) Driver Reviver A formal approach for the placement of these sites must be instituted with the following measures suggested: 1. Placement of driver reviver sites within rest areas; 2. Spacing driver reviver sites at 45 minutes driving time separation. b) Truck Parking Area ● A truck parking area can be any large site separated from the roadway by shrubs or trees to block the headlight glare from passing vehicles but taking into account driver security. The only facility needed at truck parking areas is a regularly emptied covered bin. ●
Truck parking areas should accommodate two or more prime mover and semi-trailer parking spaces.
c) Truck Change-over Area ● A truck changeover area is a small sealed or unsealed area where trucks can safely pull over to change drivers. These are larger than breakdown bays as they must accommodate the largest vehicle using any route.
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Other Areas
●
For heavy vehicles, an area of at least 4.5m in width and 50m in length is to be provided on the near side of each carriageway at intervals of approximately 10km, to allow trucks to stop. The design of these areas is to include measures for the capture of all surface drainage runoff from the lay-by. It is desirable for lay-bys for heavy vehicles to be located on or near a crest.
b) Bus Bays A bus bay is an indented storage area that is provided for buses to pull clear of the through traffic flow in order to stop and to pick up or set down passengers. Shoulders should be widened to provide sufficient width to enable buses to stand clear of the pavement, particularly where sight distance is restricted or where speeds are generally high enough that a stopped vehicle will create a hazard. When a bus bay is provided, it should be designed in such a way to allow free flowing, passenger comfortable movements and ease of manoeuvring for the largest dimensional bus that is likely to use the facility. Bus stops and/or bus bays should be provided at regular intervals along a recognised bus route within rural towns so that users generally do not have to walk more than 400 metres from their dwelling to the bus stop. Adequate provision must be made behind the kerb line, especially at indented bus bays, for sufficient waiting area to allow passengers to assemble and disperse. This may necessitate local widening of the formation/footway area to satisfy pedestrian standing. At schools, where the safety of children is of paramount importance, consideration should be given to the provision of a one-way movement bus zone. c) Emergency Telephone Bays Emergency telephones are installed to provide a
communication facility for the benefit of road users requiring assistance. Controlled access roads create an environment where the availability of outside assistance for a vehicle breakdown, road accidents, etc is restricted. Therefore, emergency telephones are desirable on controlled access roads despite other warrants not being satisfied. There are advantages associated with the provision of emergency telephones, provided they are installed and used correctly. •
They will result in early attention to reported road users’ problems.
•
They will reduce the delay in getting medical attention to injured people, thus reducing the possibility of loss of life.
determine the actual distance between each installation. Vehicle stopping areas between towns should complement stopping opportunities provided by towns, aiming to provide adequate, signposted stopping opportunities at intervals of 80km or less on routes with medium traffic volumes (20005000 AADT), and at intervals of 50km or less on higher volume routes (>5000 AADT). A signposted rest area or service centre facility should be available at not more than twice these intervals. Factors to be considered in locating new vehicle stopping areas should include: ● ● ● ● ●
•
They will reduce the time of exposure to danger by occupants of disabled vehicles.
●
Topography (preference to stop on crests in hilly areas); Width of road reserve; Scenic or aesthetic value (presence of natural features); Potential environmental impact; Volume and type of traffic; Sight distance (to permit safe access to the facility);
14.2.3 Location of emergency telephone facilities should be on the near side of each carriageway, approximately opposite one another. This alleviates the tendency and/or necessity for road users to cross multiple lanes of highspeed traffic to access a facility. On routes where there are three lanes or more, and there is an inner shoulder of sufficient width to accommodate a broken down vehicle, median placed emergency phones can be installed to provide a facility for use by both carriageways, reducing the necessity to cross multiple lanes to use the emergency telephone facility. As a guide, on rural routes, desirable spacing is 2km with a maximum spacing of 5km.
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Emergency telephone facilities should be easily identifiable both during the daylight hours and darkness. If lighting is inadequate, provision must be made to enable night-time use of the facility by road users.
Heavy Vehicle Considerations
Road transport drivers and representative groups should be consulted on the location and facilities for planned new roadside rest areas, changes to existing areas to allow heavy vehicle use or changes affecting heavy vehicle use. Stopping opportunities suitable for heavy vehicles should be provided at 10km intervals. Because of the exposure of long distance heavy vehicle drivers to the dangers of driver fatigue, and legal obligation for heavy vehicle drivers to rest from driving, stopping opportunities for these drivers should be first priority when providing for vehicle stopping. Areas where greater than minimum provision is required should be identified in consultation with road transport industry representatives.
Emergency telephones should be placed to allow easy access to the facility from the carriageway. Normally, emergency telephone facilities are to be provided just outside the shoulder, and not in a position that is vulnerable to errant vehicles.
Modifications to vehicle stopping areas must be driven by user needs but may include provision for heavy vehicle access, with parking separate from other vehicles to prevent conflict during manoeuvring, reduce the disturbance of heavy vehicle drivers’ rest by holiday travellers and meet the requirements for parking of dangerous goods carrying vehicles. Separated vehicle stopping areas may be an alternative. However, driver security should also be considered so that a potentially isolated driver with a valuable cargo does not feel vulnerable.
Careful consideration must be given to the requirements of road users with a disability when determining the location and the height of the installation.
Sealed, bypassed sections of road can be made useful parking areas for heavy vehicles, provided that connections with the through road are designed appropriately.
14.2.2
Location of Vehicle Stopping Areas
For more information refer to “Guide to the provision and signposting of service and tourist facilities” AS/NZ1742.6 1990.
The appropriate locations for vehicle stopping areas should be planned in the design stage so that earthworks, pavement design, conduits, etc., can be installed during the construction stage. When considering suitable spacing for these facilities, predetermined distances cannot be strictly adhered to. Consideration of road user safety, isolation of a stranded vehicle, effect of a disabled vehicle on through traffic, sight distance to the facility, associated earthworks, etc., will
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15.
C O M M U N I T Y C O N S U LTAT I O N
16.
The planning and design process for rural road projects should include consultation with local community and other stakeholders.
16.1
The objectives of such consultation should be to:
●
●
Collect and analyse information on local conditions and items of importance to the local community;
●
Provide information on the proposed project;
●
Obtain the views and responses from the local community; and
●
●
Identify areas of agreement or disagreement and possible compromises.
A variety of consultation methods can be used including: ● ● ● ● ● ●
Public meetings; Discussions with land owners; Direct discussion with affected owners; Meetings with stakeholder groups; Public displays and exhibitions at various stages of the project with provision for community comments; and Distribution of project bulletins.
●
● ●
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●
● ● ● ● ● ●
Clear statements should be made at the beginning of a project on the: – Purpose, nature and extent of the project; – Project timetable, indicating community participation; and – General project procedure. Affected parties should be given the opportunity to participate and to be heard; Flexible project procedures should be able to accommodate the community input, as required; Alternatives, developed and presented in a simple and clear fashion, should be used for discussions with the community; Alternatives agreed to should be developed further; Results and conclusions should be presented to the community; Adequate time for effective participation should be allowed; Quick responses should be provided on community comments; Participation at community activities should be adequately resourced; and Status of all presented materials should be provided to the community.
The extent of community participation will depend on social and environmental factors involved, significance of the road, and on the possible degree of controversy of any proposals likely to result from the project.
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Maintain the natural flow of water that existed prior to road construction; Collect water from the road pavement and convey it to suitable discharge points; Protect the road from overland flow from adjacent areas; and Provide an appropriate level of service.
Drainage structures can also provide access across road corridors for both terrestrial and aquatic fauna. An effective design must balance a number of factors against the construction cost and the proposed level of protection, such as: ● ●
● ●
The following basic principles should be employed to ensure effective community participation:
General
Any road should have an adequate drainage system to:
●
●
DRAINAGE
●
Flooding effects on adjacent properties as a result of road construction; Traffic delays or extra travel distance caused by road closures during floods greater than the design Average Recurrence Interval; Possible structural damage to the road or adjacent facilities due to floods greater than the design ARI; Service life of the proposed drainage systems and the costs of its replacement, improvement, or extension; and Road maintenance cost.
The prime sources of data and methodology for this section are: ● ●
● ● ●
Australian Rainfall and Runoff (Ref. 60); Metric version of technical memorandum No. 61, Water and soil Division, Ministry of Works and Development NZ (Ref. 74); Waterway Design, A Guide to the Hydraulic Design of Bridges, Culverts and Floodways (Ref. 34); Guide to the Design of Road Surface Drainage (Ref. 80); and Road Runoff & Drainage: Environmental Impacts and Management Options, 2001 (Ref.104).
References more suited to local characteristics and practices may supplement or substitute the above reference list.
16.2
Flood Estimation
Runoff flowing towards a road should be returned to its natural course as soon as possible. Estimates of design floods can be based upon either stream flow or rainfall records. Stream flow records are usually held by the regional water authorities and provide the largest flow rate in each year. In the absence of stream flow records, flood flows can be estimated by using mathematical procedures incorporating rainfall data. Australian Rainfall and Runoff (Ref. 60) is recognised in Australia and NZ as the primary reference for the estimation
of design flood flows. In addition to the discussion of the theory of catchment analysis and the estimation of flood flows, it presents rainfall intensity, storm frequency and duration data. Most road authorities have manuals and guides to supplement Ref. 60 for local conditions. For rural catchments flood estimation procedures available to the designer can be divided into those used for gauged and ungauged catchments. For gauged catchments the following methods are generally used:
where: Q = peak discharge (m3/s) C = coefficient of runoff I = average rainfall intensity over the time of concentration for the particular catchment and the selected storm recurrence interval (mm/h) A = catchment area (ha). However, this method has a number of deficiencies. They include: ● ●
●
●
Flood frequency analysis – for catchments with long stream flow records, where the recorded floods are statistically analysed to estimate design floods of a selected probability of exceedance. Unit hydrograph methods – for catchment with limited stream flow records, where the recorded floods and associated rainfall are used to construct a unit hydrograph. Design storms, less losses are applied to the unit hydrograph to obtain the design flood of the same ARI as the design storms.
●
These deficiencies mainly apply in large rural catchments with large proportions of pervious areas. Most road surface drainage catchments are generally: ● ● ●
●
Runoff routing method – for catchments with limited stream flow records, where the recorded floods and associated rainfall are used to derive the catchment model parameters. Design storms, less losses are applied to the model to produce design flood hydrographs of the source ARI as the design storms.
For ungauged catchments, the following methods, commonly known as regional methods, are generally used:
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●
Rational Method – as a probabilistic or statistical method in which a peak flow of a selected ARI is estimated from an average rainfall intensity of the source ARI.
●
Regional Flood Frequency Methods – such as the Index flood method and multiple regression method.
●
Synthetic Unit Hydrograph Methods - using regional relationships for the parameters required to construct the unit hydrograph.
●
Runoff Routing Methods – using regional relationships to estimate the model parameters.
Ref. 60 gives descriptions of each method and details of the factors to be considered when choosing a flood estimation procedure. It also provides guidance on when flood estimation procedures based on rainfall should be used in preference to flood frequency analysis.
16.3
Small enough for the assumption of uniform rainfall to be reasonable; Relatively impervious; and Surface storage is not a major issue.
As the equation does not take channel storage into account, this may require consideration. Earley (Ref 46) examined channel storage. He found that channel storage lessened peak flow prediction, using the equation, by about 2 to 7 percent. It is concluded that refinements to take account of this relatively small difference are not worthwhile, and the deterministic interpretation of the Rational method represents the most appropriate method of estimating peak flows in road surface drainage design (Ref. 80). The coefficient of runoff is the ratio of the peak rate of runoff to the average rainfall intensity during the critical rainfall period for the catchment area under consideration. It is the measure of the peak rate at which water drains from a particular area compared to the average rate at which rain falls on the area. The coefficient of runoff adopted must account for the ultimate future development of the catchment as depicted in the strategic plan of the relevant local authority, but should not be less than the value determined for the catchment under existing conditions. The procedure for the determination of C is in Ref. 60. In cases where portions of a catchment are significantly different, the percentage impervious of separate areas will provide an appropriate C value to be used in the calculation of runoff.
Rational Method
The “Rational Method” is the most commonly used method to estimate design flood flow in road surface catchments, which are generally well defined and relatively small. The design flow estimated using the Rational Method has about 25% accuracy and is described by the following equation: Q
Assumption of uniform rainfall over a catchment; Use of a constant value of C, which assumes that runoff is a fraction of rainfall, rather than the residual after losses have been accounted for; and Inability to take storage effects into account.
=
CIA 360
Intensity is measured in millimetres of rain per hour (mm/h). Data are provided for storm durations (for which the storm continues for the given intensity) between 6 minutes and 72 hours with frequencies (recurrence intervals or return period) between 1 and 100 years. The data are based on 100 rainfall stations located around Australia. Intensity data for New Zealand is presented in Ref. 74.
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16.4
Design Considerations
serviceability requirements of the road in question, and the duration of road closure during times of flooding.
Water must be conveyed away from the road for the following reasons: ● ● ● ●
Maintain adequate pavement skid resistance; Maintain an acceptable level of road lighting performance (Ref. 25); Reduce spray; and Visibility of pavement markings to be maintained.
The selection of the level of serviceability is generally based on the following criteria: ● ● ● ●
The level of service expected by the community; The availability of alternative routes and period of closure; The importance of the road/access to hospitals, airports, etc; and Economic considerations. (Ref 34)
The designer must consider the following issues: ● ● ● ● ● ●
Grading of the roadway with respect to flood levels, ground water levels and tidal levels; Estimated runoff; Maximum permissible flow width on the carriageway; Minimum size of cross culverts and outlet conditions; Subsurface drainage; and Consequences of a storm of greater ARI than the design storm.
Average recurrence interval (ARI) The average recurrence interval is the average interval of time during which a storm event will be equalled or exceeded once. When selecting the average recurrence interval for a design, the following factors should be considered: ● ● ●
● ●
Consequence of flooding (potential damage to property, road and structures); Additional cost of providing for a larger ARI; Capacity of underground or outfall drainage systems into which the road surface drainage components will discharge; Level of serviceability to traffic; and Consistency of flood immunity along other sections of the road.
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Table 16.1 Contains values of ARI, used successfully in Australia and New Zealand in the past 20 years, and may be used for preliminary design. The ARI to be used for final design must be selected after evaluation of the factors listed above, and consideration of local road authority practice. Level of Serviceability to Traffic The level of serviceability will depend upon the ARI of the flood for which the stream crossing will be passable to traffic, the
Table 16.1: ARI for Road Design Location
ARI, years
Major waterway structures
100
Water bypass around water treatment facilities
100
Cross road drainage
50
Road with landlocked areas (at a sag in cut)
50
Road surface drainage
10
Bridge deck drainage
10
Road surface drainage at wide flat pavement
1
Water quality treatment (wetlands, etc)
1
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In addition, the requirements of local authorities, environmental agencies, and those responsible for navigation and flood control, will also influence type of waterway structures and hence impact on the level of serviceability provided. Typical levels of serviceability are as follows: ●
Arterial roads generally designed to pass the 50 or 100 years ARI without interruption to traffic. However, for arterial roads in remote areas, a reduced standard is commonly adopted where traffic densities are low.
●
Minor roads are generally designed to pass the 20 (or less) year ARI. The level of their serviceability depends upon: – The importance of the road; – Interruption to traffic significance; and – Economics of providing a higher level of serviceability.
Trafficability Trafficability will depend upon the combination of depth and velocity of flow over a floodway, when the frictional resistance between a vehicle’s tyres and the floodway surface is overcome and the vehicle loses stability. Road closure is normally assumed when the total head (static plus velocity) on a carriageway with a two-way crossfall or across the highest edge of a carriageway with a one-way crossfall exceeds 300mm (Ref. 34). Longitudinal drainage A desirable minimum longitudinal grade of 1.0% and an absolute minimum grade of 0.3% are to drain water effectively from the traffic lanes. However, where flat terrain prevents these grades being achieved, the carriageway itself can have a zero longitudinal fall, provided that water can drain away from the road formation. Pavement drainage Pavement drainage is achieved by providing minimum crossfall on the pavement of 3.0%. However, on wide smooth pavements with flat grades there may be difficulty in maintaining satisfactory drainage. Special problems (aquaplaning) may occur at intersections and superelevation transitions. With flat grades, water is shed directly towards shoulder but problems can arise on steep grades when water tends to flow longitudinally down the pavement. The resulting sheet of water can cause drivers to lose control. Further consideration of drainage of wide flat pavements is set out in the publication Drainage of wide flat pavements (Ref 77). The formula for depth of flow is:
d
=
0.15(TXD)0.11 L0.43 I10.59 – (TXD) S0.42
where: d = is depth of flow measured from the top of the surface texture (mm) L = is flow path length (m) I1 = is a rainfall intensity for 1 year ARI S = is the average flow path slope (%) TXD = is the texture depth measured by the sand patch or silicon putty method (mm)
There are various methods of reducing the depth of water and length of flow paths that are available to the designer. These may include use of two-way crossfall on one-way pavements, use of artificial crown lines, and modification of superelevation development. Special drainage provisions such as slotted channels or grated trenches may also be considered.
Indicative values of (TXD) are:
More research and trials are required to determine the relationship between speed, rainfall intensity, water depth and the occurrence of aquaplaning. In the meantime, water depths should be limited to 4mm for a rainfall intensity of 1 year ARI, Table 16.1.
Burlap drag concrete Grooved concrete Dense asphalt Size 14 stone seal * Open grade asphalt
Roadside drains Roadside drains include: ● Table drains; ● Catch drains; and ● Median depressed drains.
0.05mm 1.2mm 0.9mm 3.7mm 1.2mm
Note: * There is some doubt about the drainage properties of open graded asphalt in the long term. However, it may be used to improve safety at critical locations in the short term.
Details of depth, width, gradient, and capacity of the drains can be obtained by reference to “Guide to the design of road surface drainage” (Ref 80) and Section 11.9.
16.5 Aquaplaning Aquaplaning is the complete loss of traction and directional control of the vehicle as a result of a fluid film between the tyres and the road surface. The texture of the road surface and the tread on vehicle tyres provide drainage channels for water to escape from beneath a vehicle tyre. If these channels are inadequate and water does not escape there is a risk of partial or full aquaplaning.
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Partial aquaplaning occurs at quite low speeds as a result of some intrusion of a water film between the tyre and road surface. As the vehicle speed increases the water will have less time to escape through the tyre and road surface drainage channels. As a result of this, a tyre’s contact area and skid resistance will be reduced. Full dynamic aquaplaning occurs when a tyre is completely separated from the road surface by a film of water. Fluid pressures can build up within the contact zone between the tyre and the pavement to the point where the hydrodynamic uplift equals the downward force exerted on the tyre. At this point, the tyre is aquaplaning or completely supported by the water layer. As a result, there is almost complete loss of traction and directional control since the fluid film cannot develop the necessary shear forces for braking or steering manoeuvres.
Water Quality
Rural storm water management plans associated with rural road projects should consider the treatment of runoff to meet the water quality requirements as per legislation and of the local environment protection agency or catchment management authority. Water quality treatment facilities should be provided to meet the requirements of the catchment management authority. Details of methods to manage erosion and treat storm water can be obtained by reference to: Urban Storm Water: Best Practice Environmental Management Guidelines (Ref. 43) Austroads document, Road Runoff and Drainage: Environmental Impacts and Management Options, 2001 (Ref. 104) and Water Sensitive Road Design-Design Options for Improving Storm Water Quality of Road Runoff (Ref. 103). Refer table 16.1 for ARI for design purposes. 16.5 Water Quality
Various research projects have been undertaken aimed at predicting water depth as a function of rainfall intensity and physical conditions. Guide to the Design of Road Surface Drainage (Ref. 80) describes some of these relationships. Oliver (Ref. 84) has concluded that currently available skid resistance specifications or recommendations are not suitable in their present form for use in calculating a maximum water film depth consistent with safety for different classes of road. Oliver (Ref. 84) also concludes that full aquaplaning will be a rare event. It was found that a progressive reduction in tyre friction occurs as water depth increases from just wet condition to a film thickness of 4 mm. It was also noted that if pavement rutting occurs, deep films of water could be present in the wheelpath area under conditions of light rain.
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17.
ROADSIDE SAFETY
● ● ●
17.1
Safety Objectives
● ●
Road environment factors are one of the three elements that contribute to road crashes, the others being driver behaviour and vehicle characteristics. It is estimated that some 30% of crashes relate to roadside environment. While road environment factors are often not the single cause of a crash they can contribute to their severity. This section is based on AASHTO “Roadside Safety Barriers” and work done by Troutbeck to adjust AASHTO detail to Australian conditions. Austroads are currently reviewing the 1987 NAASRA “Safety Barriers – Considerations for the Provision of Safety Barriers on Rural Roads”. Further reading may be obtained by reference to the AASHTO “Roadside Safety Barriers” 1996. Austroads Guide to Traffic Engineering Practice, Part 4 – Road Crashes (Ref. 17) outlines various issues to be considered in the design process that can reduce the potential for crashes. In particular it describes the fundamentals of good design. These include: ● ● ● ● ● ● ● ●
Designing for all road users; Pavement surface; Intersection design; Intersection control; Pavement markings and delineation; Pedestrian crossing facilities; Street lighting; and Signing including guide posts.
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Therefore, the following safety objectives are to be adopted when designing a road: ●
Separate potential conflict points and reduce potential conflict areas;
●
Control the relative speeds of conflicting vehicles;
●
Clearly identify the path to be followed;
●
Ensure that the needs of all road users are considered;
●
Provide a roadside recovery area that forgives a driver’s errant or inappropriate behaviour; and
●
Ensure that roadside furniture is located safely.
17.2
On-Road Safety
17.2.1
Intersections
● ● ● ●
Channelisation; Intersection control; Friction or pavement skid resistance; Turning radii; Traffic lane and shoulder widths; Property access; Signposting; Approach speed; and Lighting.
In general, an intersection should be obvious and unambiguous and allow good visibility of traffic control devices and other road users (Ref. 83). It is appropriate to increase intersection control with an increasing ratio between minor and major flow. Capacity considerations will also govern the type of intersection control required. The various types of intersection control in order of increasing standard (and safety) are provided in Figure 17.1. The following actions should be taken in designing for on-road safety: Intersections ● Define and minimise the number of conflict points; ● Separate conflicts; ● Give priority to major movements through alignment, delineation and traffic control; ● Provide a clear indication of priority; ● Control the angle of conflict – crossing streams of traffic should intersect at right angles while merging traffic should intersect at small angles; ● Define vehicle paths; ● Provide adequate sight distance; ● Control approach speeds to major intersections through alignment and geometry; and ● Provide suitable lighting.
Figure 17.1: Scale of increasing safety of intersection controls Uncontrolled Intersection Rely on priority rules
Assigned Priority Giveway signs
Assigned Priority Stop signs
Roundabout Intersection design and control is a major factor in improving road safety. The main factors in intersection safety include: ● ● ● ● ●
Number of legs; Angle of intersection; Sight distance; Alignment; Auxiliary lanes;
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Signals Filter turns
Signals* Fully controlled turns Note: *Rural intersections are unlikely to be controlled by traffic signals.
17.2.2
Mid Block
Mid block on-road safety should also be considered during the design phase. The factors that influence mid block safety include: ● ● ● ● ● ● ● ● ● ●
Pavement surface; Delineation; Shoulder width; Verge rounding; Horizontal and vertical geometry; Degree of access control; Overtaking opportunities; Sight distance; Speed differential between vehicles; and Vehicle speeds.
17.3.1
It is not feasible to provide width adjacent to the carriageway that will allow all errant vehicles to recover. Therefore it is necessary to reach a compromise or level of risk management. The most widely accepted form of risk management for roadside hazards is the ‘clear zone concept’. The clear zone is the horizontal width measured from the edge of the traffic lane that is kept free from hazards to allow an errant vehicle to recover. The clear zone is a compromise between the recovery area for every errant vehicle, the cost of providing that area and the probability of an errant vehicle encountering a hazard. The clear zone should be kept free of non-frangible hazards where economically possible; alternatively, hazards within the clear zone should be shielded. The clear zone width is dependent on: ●
The following actions should be taken in designing for on-road safety:
● ● ●
Mid block ● Define vehicle paths, especially where there are changes in geometry; ● Minimise headlight glare; ● Provide appropriate access control for the function of the road; ● Provide overtaking opportunities including passing lanes or bays; ● Provide truck escape bays on roads with steep grades; ● Minimise major changes in road geometry; ● Minimise adverse or severe crossfall; and ● Provide a smooth road surface with an appropriate level of skid resistance.
17.3
Recovery Area
Roadside safety typically relates to the area adjacent to the traffic lane where an errant vehicle can recover. Providing a safe roadside involves removing or treating likely hazards that may contribute to the severity of a crash.
Clear Zone
Speed; Traffic volumes; Batter slopes; and Horizontal geometry.
It should be noted that the clear zone width is not a magical number and where possible hazards beyond the desirable clear zone should be minimised. Clear zone widths vary throughout the world depending on land availability and design policy. The concept originated in the United States in the early 60’s and has progressively been refined and updated. For a typical high-speed road the clear zone width varies between 4.0m (France, South Africa) to 10.0m (Canada, USA). More recent studies have found that the first 4.0-5.0m provides most of the potential benefit from clear zones. Figure 17.2 provides an indication of appropriate clear zone widths for a straight section of road with trafficable batters The clear zone width increases where there is sub-standard horizontal geometry, especially on the outside of a curve or where non-trafficable batter slopes are present. Non-trafficable batter slopes refers to batter slopes of steeper than 1 on 4.
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Figure 17.2: Clear Zone Widths on Straights
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17.3.1 Clear Zone
The clear zone width on the outside of curves increases by a factor Fc , which depends on the operating speed and the radius of the curve. Fc ranges between 1.0 to 1.9. Figure 17.3 provides guidance on adjustment factors for clear zones on the outside of curves. Where batter slopes are steeper than 1 on 4 (that is non trafficable) designers should give consideration to the provision of a road barrier (refer to Section 17.4). A guide for the installation of roadside safety barriers on embankment is shown on Figure 17.4. Figure 17.5 indicates the variation of clear zone widths on batters steeper than 1 on 6 to give an effective clear zone width to be used in design.
17.3.2 1(a) Existing Hazards Within a Clear Zone
17.3.2
Existing Hazards Within a Clear Zone
Common existing roadside hazards in a rural environment include: ● ● ● ● ● ● ●
17.3.2 1(b) Existing Hazards Within a Clear Zone
Poles – power poles or sign posts; Trees; Batters; Dams and water courses; Drainage and associated infrastructure like culverts and endwalls; Fences; and Bridge piers.
The most desirable action is to remove or relocate hazards although this is not always possible due to road reservation or economic and environmental constraints. Where hazards cannot be relocated then they should either be shielded or made ‘more forgiving’. It is becoming increasingly common for light poles and signposts to be provided with frangible bases. This is an attempt to provide a forgiving roadside while still providing the necessary roadside infrastructure. Common types of frangible poles include: ● ●
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● ● ●
Slip base poles; Impact absorbent poles; Steel frangible posts; Aluminium frangible assemblies; and Wooden frangible posts.
The support connection of a slip base pole shears on impact with the pole landing close to the point of impact. Impact absorbent poles crumple and bend around the vehicle. Slip base poles can usually be re-used after an impact and for this reason tend to be more common. However, they can only be used where there will not be a conflict with overhead services in the event of an impact, and where the risk to other road users, particularly pedestrians, is minimised. Steel frangible posts fail on impact as a result of shear failure planes. Aluminium assemblies collapse due to shear pin action. Frangible wooden signposts have holes drilled at the base creating a plane of weakness that permits the posts to collapse on impact. Other measures to make roadside hazards more forgiving include:
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Figure 17.3: Adjustment Factors for Clear Zones on Curves
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Figure 17.4: Warrants for Guard Fence on Embankment
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Figure 17.5: Effective Clear Zone Widths on Batters
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Table 17.1: Test levels for longtitudinal barriers (TL- 0 TO TL- 6) and test levels for terminals and crash cushions (TL- 0 TO TL- 3) Test Level
Vehicle mass (kg) and type
Speed (km/h)
Angle degrees
Height of Centre of gravity (mm)
0
820 C
50
20
550
1 600 C
50
25
550
820 C
50
20
550
2 000 P
50
25
700
820 C
70
20
550
2 000 P
70
25
700
820 C
100
20
550
2 000 P
100
25
700
820 C
100
20
550
8 000 S
80
15
1 250
820 C
100
20
550
36 000 V
80
15
1 850
820 C
100
20
550
36 000 T
80
15
2 050
1
2
3
4
5
6
Note: (1). Refer NCHRP350 for Test Level Procedure (2) TL- 3: High-speed arterial roads TL- 2: Local and collector roads TL- 0 and 1: Work zones and low speed roads TL- 4 to 6: Truck and other heavy vehicles
Legend: C = small car P = four wheel drive or utility truck S = single-unit van truck T = tanker type semi-trailer V = van type semi-trailer
●
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● ●
Considering the mature trunk size of trees prior to planting; Installing driveable culvert end walls; and Extending culvert walls to beyond the clear zone width.
17.4
Safety Barriers
Safety barriers are used to shield hazards that cannot be relocated or made more forgiving. The barrier itself is a hazard and accordingly should only be used when it is less of a safety concern than the hazard the designer is trying to shield. Roadside safety barrier systems may be considered for use only after they have been satisfactorily crash tested, computer simulated or designed by other professionally acceptable methods that demonstrate acceptability to meet AS/NZS 3845:1999.
1 & 2 procedures. The tests by CEN do provide an equivalent set of tests to compare systems with NCHRP350. Acceptance of the roadside safety barrier systems is based on an evaluation of its performance in an idealised crash test (vehicle in tracking mode; approach surface flat, paved and free from obstructions such as kerbs) for a specific weight and type of vehicle at designated speeds and impact angles. In accordance with NCHRP350 procedures, there are six test levels, refer Test Levels in Table 17.1, so as to provide for a range of restraint requirements (vehicle size) and impact severity conditions (speed and angle). The evaluation criteria, refer Table 17.2, on impact of the vehicle with the barrier system is based on the: ● ●
The crash test procedures to be adopted are based on the AASHTO, National Cooperative Highway Research Program Report Number 350. The European Committee for Normalisation (CEN) has established performance criteria for safety barriers and crash cushions as set out in CENprEN 1317-
●
Structural adequacy of the barrier system; Occupancy risk and the impact velocity and ride down acceleration limits; and Vehicle trajectory after impact.
The designer should be aware that the site of installation will often be different from the test condition, the errant vehicle
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Table 17.2: Safety Evaluation Guidelines
EVALUATION FACTORS
EVALUATION CRITERIA
A. Test article should contain and redirect the vehicle; the vehicle should not penetrate, under ride or override the installation although controlled lateral deflection of the test article is acceptable. Structural Adequacy
B. The test article should readily activate in a predictable manner by breaking away, fracturing or yielding. C. Acceptable test article performance may be by redirection, controlled penetration or controlled stopping of the vehicle.
D. Detached elements, fragment or other debris from the test article should not penetrate or show potential for penetrating the occupant compartment or present an undue hazard to other traffic, pedestrians or personnel in a work zone. Deformations of, or intrusion into, the occupant compartment that could cause serious injuries should not be permitted. E. Detached elements, fragments or other debris from the test article or vehicular damage should not block the driver’s vision or otherwise cause the driver to lose control of the vehicle. F. The vehicle should remain upright during and after collision although moderate roll, pitching and yawing are acceptable. Occupant Risk
G. It is preferable, although not essential, that the vehicle remain upright during and after collision. H. Occupant impact velocities should satisfy the following: Occupant Impact Velocity Limits (m/s) Component Preferred Maximum Longitudinal and lateral 9 12 Longitudinal 3 5 I.
Occupant ride down accelerations should satisfy the following:
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Occupant Ride down Acceleration Limits (G’s) Component Preferred Maximum Longitudinal and Lateral 15 20 J. (Optional Hybrid III dummy. Response should conform to evaluation criteria of Part 571.208, Title 49 of Code of Federal Regulations, Chapter V (10-1-88 Edition)
K. After collision it is preferable that the vehicle’s trajectory not intrude into adjacent traffic lanes. L. The occupant impact velocity in the longitudinal direction should not exceed 12 m/s and the occupant ride down acceleration in the longitudinal direction should not exceed 20 G’s. Vehicle Trajectory M. The exit angle from the test article preferably should be less than 60% of test impact angle, measured at time of vehicle loss of contact with test device. N. Vehicle trajectory behind the test article is acceptable.
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Consideration of these issues and the discussion of various barrier systems below provide general guidance on the most appropriate system for a particular situation. Further information should be obtained from system suppliers or the relevant road authority.
17.4 Safety Barriers
Concrete safety barriers are best suited to situations where there is little room between the barrier and the hazard. Typically this occurs in narrow medians or in areas of restricted road crosssection. The greatest concern with concrete safety barriers is the method of termination. Available options include: ●
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●
Steel guardrail terminal assembly to shield the end of the concrete barrier in association with a bridge approach assembly; Burying the end of the barrier in an adjacent embankment; and Shielding the barrier system with an impact attenuator/ crash cushion system.
will not be in tracking mode and ground conditions for the support of posts will be different from the test site. Judgement must, therefore, be exercised in the application of test results and the performance of safety devices monitored in the field to ensure they operate as intended.
●
Test level 3 is considered to be the rating by which roadside safety barriers systems are designed. They will perform for the car and pick-up truck at 100 km/h at a nominal angle of 20 degrees. The work zone systems can be designed for test levels 0, 1, 2 & 3 at nominal speeds of 50, 70 & 100 km/h respectively and 20 degrees nominal angle. Roadside safety barrier systems and the equivalent test level category of each are listed. The test level rating of a barrier system can be increased by raising the height of the top of the system and proven by acceptable methods:
Concrete safety barriers may be considered on high volume roads as they retain full functionality after impact, provide excellent whole of life costs and minimise the risk to maintenance workers, as maintenance is minimal after an impact.
Rigid System ● F-Shape concrete barrier (adopted by AS/NZS 2845/1999) ● New Jersey concrete barrier ● Sloping face concrete barrier ● Vertical face concrete barrier ● High containment concrete barrier
Test Level 3 to 4
Semi-Rigid System ● W-beam steel barrier ● Thrie-beam steel barrier ● Hollow box steel barriers ● Wire rope safety barriers - four wire rope
Test Level 3 to 4 3 to 4 3 3
Work Zone System ● F-shape concrete precast barriers ● Water filled barriers ● Truck mounted attenuators
Test Level 3 to 4 0 to 4 3
3 3 3 5
to to to to
4 5 5 6
All these systems have specialised terminals, which will provide control led deceleration. Terminals provide deceleration below recommended limits and ensure that the vehicle is not speared and is not vaulted, snagged or rolled on impact.
Site characteristics will determine the most appropriate type of termination/ attenuation to use.
Steel W-beam barriers are perhaps the most common barrier and are used extensively in urban and rural areas. The effectiveness of W-beam is dependent on its length and offset from the main carriageway. W-beam termination is also of concern and standards are continually developing to improve end terminals. Most road traffic authorities have detailed guidelines on the installation of W-beam and end terminals. Care should be taken in meeting these requirements. The impact behaviour of the W-beam and terminals should also be considered to ensure that the selected system is appropriate for the intended location. Wire rope safety barrier works through high-tension cables. An errant vehicle bends the supporting posts and the rope deflects with the vehicle before directing it back towards the direction of travel. Wire rope safety barriers are the most forgiving on the errant vehicle of the three methods. The deflection width must be a design consideration for the offset of features behind the barrier. AS/NZS 3845 and relevant road traffic authority guidelines should be referenced to establish installation requirements and the acceptability of these systems. The location of safety barrier in the vicinity of kerb and channel is to be considered very carefully. If kerb and channel is essential in high-speed locations, the line of kerb shall be located: ● ● ●
Crash cushion systems are also used to shield hazards in confined locations, such as the junction of concrete barriers, at ramp noses and other rigid hazards. A discussion of issues to be addressed in the specification of safety barrier and crash cushion systems is included in AS/NZS 3845:1999 – Road Safety Barrier Systems (Ref. 13).
●
At least 3m from the face of concrete safety barrier types; At least 3m from W-beam barrier or wire rope safety barrier for barrier kerbs; Between 0.0 and 1.0m or at least 3m from W-beam barrier or wire rope safety barrier for semi-mountable kerbs; and In areas where the Operating Speed is less than 70km/h an offset of 200-300mm can be used to minimise nuisance damage to vehicles.
Note: Semi-mountable kerb should be 100mm maximum height to minimise dynamic jump.
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Work zone barriers come in various forms and can be precast concrete with impact attenuator/crush cushion terminals or water filled plastic systems. These systems must be considered during the design phase. Truck mounted attenuators can be used for short term or mobile work areas.
17.5
provide protection for pedestrians from errant vehicles. Local specific guidelines should be referenced to determine the provision of crossing facilities.
17.8
Landscaping
In relation to safety, landscaping on road verges or within the clear zone should:
Appropriate traffic management requirements for construction sites are described in the Field Guide for Traffic Control at Works on Roads, SAA HB 81.1 – 816 (Ref. 92).
Not obstruct sight distance; Be frangible; and ● Not obstruct sight lines to signs, delineation and traffic control devices. It is common for medians to be landscaped to reduce headlight glare from opposing traffic.
Construction and maintenance operations should not inhibit traffic and, where possible, separation should be achieved through diversion routes. Studies conducted in the United Kingdom have identified high accident rates through work zones where proper warning and delineation has not been achieved.
Generally, trees with a mature trunk diameter less than 100 mm (subject to tree species) are considered to be frangible. Trees with small trunk diameters (< 100 mm diameter mature) may be used for medians and borders, while for traffic islands, low level vegetation or trees with high canopies are appropriate (subject to the trunks being frangible or outside the clear zone). Provision for adequate sight distance for all road users must be considered.
In reality, there will always be a requirement for some traffic movement through work zones on existing roads. Where this is necessary, clear and positive guidance approaching and through the work zone is a crucial element in the overall safety of the site.
● ●
The designer must consider the balance between landscape and road safety objectives.
17.6
Lighting
Work zone barriers can be used to shield vehicles from hazards and provide a safer work zone. Work zone barriers need to be approved by each road authority against the appropriate work speed zone and NCHRP 350 Test Level before they can be used on site, refer Section 17.4. Careful consideration of the following factors is required for traffic through work zones, particularly in relation to heavy vehicles:
The benefits of a high level of street lighting, especially at intersections, are well documented with a strong correlation to night-time accident reduction.
●
At complex intersections an appropriate level of street lighting should be considered. The lighting should be substantial enough to provide the driver with a clear view of the road alignment.
● ●
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Temporary Works During Construction
AS/NZS 1158.1.1, Road Lighting – Vehicular Traffic (categories V1, V2 and V3) Lighting – Performance and Installation Design Requirements (Ref. 12) has recently undergone a review of acceptable lighting levels. This should be the main reference for street lighting. The Austroads Guide to Traffic Engineering Practice, Part 12 – Roadway Lighting (Ref. 25) is a useful background document.
●
17.7
●
Pedestrians and Cyclists
In designing a safe road environment consideration must also be given to non-vehicular road users. The needs of pedestrians (including those with disabilities) and cyclists are discussed in the relevant Austroads guides (Ref. 26 and 27). The safety issues to be addressed for on-road cyclists are very similar to those relating to motor vehicles. It is important for the road surface to be smooth, to minimise conflicts and to provide appropriate delineation. Consideration should be given to the provision of sealed shoulders on major rural roads. Pedestrian safety relates to the number of controlled crossing points and the provision of footpaths close to the traffic lanes. It is desirable for footpaths to be set back from the kerb to
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● ● ● ●
Alignments should desirably be designed in accordance with the geometric guidance in this publication. They should be designed to operate safely for the chosen reduced speed limit (i.e. no surprises for the driver such as adverse crossfall, abrupt changes in direction, etc); Reduced lane widths; Median crossovers; Reduced number of lanes; Clearances to hazards (trucks travel closer to hazards because of their overhang); Short merge zones; Height of flashing lights (often these affect visibility for the driver); Provision for surface drainage, especially where pavement works are staged; and Provision of work zone safety barriers and truck mounted attenuators for short term or mobile work areas.
17.9
Road Safety Auditing
Road safety auditing, especially during the design stage, serves to identify opportunities to reduce the incidence and severity of crashes. As defined by Road Safety Audit (Ref. 33), a road safety audit is: "A formal examination of a traffic project, or any project that interacts with road users, in which an independent, qualified examiner reports on the project’s accident potential and safety performance.” A road safety audit should be conducted:
● ● ● ● ●
At the feasibility stage; At the draft design stage; At the detailed design stage; Pre-opening; and On existing roads.
The earlier a road is audited within the design and developments process the better. The need is as important for new works as it is for retrofit projects.
The approach visibility angle must not exceed 95º to the left of the crossing and 110º to the right of the crossing as shown in Figure 18.1. Occasional obstructions such as posts, small trees and sparse vegetation can be considered acceptable if their size and spacing would not obscure the driver’s vision of a train. (Also refer to Figure C2) Crossing visibility is deemed to be adequate when an area of unrestricted visibility exists for each approach and the following conditions are met: The driver of a stationary vehicle, positioned at a stop line, has a clear view of approaching trains to a distance along the tracks such that a train appearing in the driver’s field of view at the point where the vehicle begins to move would reach the crossing after the vehicle has cleared the crossing.
1 8 . R A I LWAY L E V E L C R O S S I N G S This section outlines geometric guidelines for at-grade railway/road level crossings and provides guidance for a safety review of existing level crossings. The guidelines have been developed for typical situations. They are intended to aid but not replace sound engineering judgment based on particular local conditions and requirements of the rail authority (Ref. 66).
For the purpose of calculating the visibility triangle, the following figures should be used: ●
At-grade railway level crossings present a potential for severe accidents. Designers should aim to eliminate, improve, or grade separate existing crossings and to avoid the introduction of any new at-grade railway level crossings where possible. The derivation of sight distance requirements at railway level crossings is discussed in Appendix C. These requirements do not apply where other factors such as the level of train and vehicle exposure may require that flashing lights be installed at the crossing. For crossings controlled by lights, the sight distance requirements relate to the ability of a driver to see the signals, not the train. AS 1742 Part 7, 1993 specifies the use of railway crossing warning signs which prompt drivers to ‘Look for Trains’ when approaching a crossing. Refer to the standard for detail warning signage and visibility requirements and details indicated in this section.
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18.1
Horizontal Alignment
Approach and crossing visibility are the primary features affecting safety of the at-grade railway level crossings. The approach visibility is deemed to be adequate when an area of unrestricted visibility exists for each approach as shown on Figure 18.1. Approach visibility is adequate when the following conditions are met: The driver of an approaching vehicle, travelling at the 85th percentile speed (VV) can see a train travelling at maximum operating speed (VT), when the vehicle and the train are at distances S1 and S2 respectively from the crossing, such that the vehicle can either safely stop short of the crossing, or clear the crossing before the train reaches it. Appropriate values of VT should be obtained from the rail authority.
● ●
Distance from the driver’s eye to the rail, whilst at a standstill, is 5.0m; Height of the driver’s eye above the road is 1.05m; and Height of train above the rails is 2.3m.
For a given vehicle, the crossing visibility must be adequate for trains approaching from either direction. The crossing visibility angle must not exceed 110º to the left of the crossing (see Figure 18.2) and 140º to the right of the crossing (see Figure 18.3). If there is a choice of crossing angle, 90º are preferred. (Also refer to Figure C3) Many railways run parallel to adjacent roads and motorists on such roads may be unaware of a train travelling just behind the vehicle in the same direction. In these cases where the road then crosses the rail or a side road crosses the rail, distances S1 and S2 must be checked (unless there is stop control on the crossing with advance warning signs) at the design speed of the main road. It is essential that the visibility angles for S1 and S2 fall within the prescribed limits (see Figure 18.4).
18.2
Vertical Alignment
18.2.1
Road Grading
The railway grading is usually a control on the road. As a general guide, for rural roads the road surface shall not be more than 75mm above, nor more than 150mm below, the projection of the top of the rail pair at a distance of 10m from the nearest rail. The maximum level difference between road and rail when the track is below the road level is 10mm. On rural roads, the rail level should not protrude above the surface, although this may not always be achievable. The maximum permissible protrusion above the road surface is 10mm.
Distance S1 shall not be less than truck stopping sight distance. For a given vehicle, the approach visibility must be adequate for trains approaching from either direction.
The protrusion of the rail level above the road level is more of a problem when the angle between the road and the rail is acute, particularly for cyclists and motorcyclists.
For a given vehicle the approach visibility must be adequate for trains approaching from either direction.
Where a road crosses multiple railway lines at a level crossing, a smoother crossing can be achieved by adjusting the relative
RURAL ROAD DESIGN
109
Figure 18.1: Approach Visibility Angles
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Figures 18.2/3: Crossing Visibility Angle for Driver Looking Left and right
110
RURAL ROAD DESIGN
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Figure 18.4: Road Parallel to Roadway
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111
grade of the railway lines to more closely match the longitudinal grade of the road.
18.2.2
1 9 . C O M P U T E R S O F T WA R E FO R ROAD DESIGN
Cross Section
Width The minimum clear width provided through level crossings should be equal to the traffic lanes plus 1.5m each side; that is, the carriageway width plus 3 m. On duplicated roads, the 1.5m are added to the outer edge of each carriageway. Crossfall At the level crossing, the pavement slope should match the grade line of the railway. This could present a potential hazard where the road is on a curved alignment. The road curvature and superelevation should be selected with superelevation matching the rail grading, so that crossfall does not reduce in the direction of travel along the curve.
The geometric design of rural roads involves many calculations that can be performed by computers. They can quickly and accurately handle large quantities of data, saving considerable design time and cost. Importantly, they enable many more alternatives to be examined and evaluated and as a result they can assist in producing optimum solutions in a reasonable time and at reasonable cost (Ref. 97). Various computer software packages are available and are widely used for designing roads, bridges and multilevel overpasses. They help designers to work faster or reach proof of concept sooner in the design process. Each software package has its own advantages and disadvantages and those interested in pursuing this topic further should contact their relevant road authority. The commercial suppliers of the software packages will provide specific information. The primary functions of road design software include: ● ● ● ● ●
Horizontal and vertical alignment design; Coordination of horizontal and vertical alignments; Creation and viewing of digital terrain model (DTM); Automated calculation of quantities; and Production of plans and profiles.
Software should have a smooth user interface that gives designers full access to all geometric design data, non-graphic information, and criteria at any point in the project cycle. This supports rapid decision-making and design changes. Threedimensional model-viewing capabilities further assist the decision process, as well as enhancing the designer’s ability to present the work. Various computer design aids can: ● ● ● ●
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● ● ●
Stratify and organise project documentation; Record project histories; Work in 2D and generate 3D; Edit digital terrain models with real-time movement of points (rubber banding), allowing contours to move dynamically; Generate cross sections automatically from any data source, for any situation; Link cross section elements to plan view elements; and Alter cross sections and automatically update earthworks quantities.
The use of three-dimensional models can enhance a project to the community especially those non-technical persons. In summary, modern technology opens the door for efficiency, cost savings and better-informed judgement.
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Road Safety Barrier Systems, Standards Association of Australia, Sydney, Australia.
14 Austroads (1988)
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3
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4
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7
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44 Cox R L (1998)
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74 Ministry of Works and Development (NZ)
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76 NAASRA (1973)
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57 Hempsey L and Teply S
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58 Hoban C J (1983)
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71 McLean J R (1988) Speed, Friction Factors and Alignment Design Standards, Research Report ARR No. 154, Australian Road Research Board, Australia.
Overtaking Lane Practice in Canada and Australia, ARR 144, Australian Road Research Board, Australia.
60 Institution of Australian Rainfall and Runoff, Vol 1, Engineers, Australia A guide to flood estimation, Institution (1998) of Engineers Australia, Canberra. 61 Kanellaidis G (1999) Road curve superelevation design: current practices and proposed approach, Vol 8, No 2, June 1999, Road and Transport Research. 62 Krammes R A, Brackett R Q, et al (1993)
State of the Practice Geometric Design Consistency, Final Report, Federal Highway Administration, U.S. Department of Transportation, U.S.A.
63 Lay M G (1985)
Source Book of Australian Roads, Third Edition, ARRB, Melbourne.
64 Lee R E (1963)
Driver Eye Height, Australian Road Research, Vol 1, No 6.
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65 Mai and Sweetman Articulated Vehicle Stability - Phase II (1984) Tilt Tests and Computer Model, ARRB Internal report, AIR 323-2. 66 Main Roads, QLD
67 Main Roads, WA (1997)
68 McCormick E J and Human Factors in Engineering and Sanders M S (1982) Design (Fifth Edition), McGraw Hill, New York, USA. 69 McLean J R (1978) Review of the Design Speed Concept, Australian Road Research, Volume 8, No. 1, March 1978, Australia. 70 McLean J R (1983) Speeds on Curves: Side Friction Factor Considerations, Research Report No. 126, ARRB Transport Research Ltd, Australia.
82 Nicholson A (1998) Superelevation, side friction and roadway consistency, Journal of Transportation Engineering, Vol 124, No 5 American Society of Civil Engineers. 83 Ogden K W (1997) Safer Roads, A Guide to Road Safety Engineering, Avebury Technical, England.
RURAL ROAD DESIGN
115
84 Oliver V W (1979)
Skid resistance reduction in wet weather due to hydroplaning of vehicle tyres, Australian Road Research Board, Australia.
85 Olson et al (1984)
Olsen P.L., Cleveland D.E., Fancher P.S., Kostyniuk L.P., Schneider L.W., Parameters Affecting Stopping Sight Distance, NCHRP Report No. 270.
86 Pape M (1990)
Rural Road Alignment Design Procedure, Technical Note TN/1, VicRoads, Melbourne, Australia.
87 PIARC (1995)
XXth World Road Congress, Interurban Roads, C4, Report of Committee, Montreal.
88 PIARC (1995)
XXth World Road Congress, Interurban Roads, C10, Report of Committee, Montreal.
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89 Prem H et al (1999) Prem H, Ramsay E, Fletcher C, George R, Gleeson B, Estimation of Lane Width Requirements for Heavy Vehicles on Straight Paths, Research Report ARR 342, ARRB Transport Research Ltd, Australia. 90 RTA (1989)
Road Traffic Authority NSW, Road Design Guide, RTA, NSW, Australia, 9 Sections 1989 to 2000.
91 QLD DoT (1992)
Development of Design Standards for Steep Downgrades (DSB02), Queensland, Australia.
92 SAA HB 81.1 – 81.6 (1996)
Field Guide for Traffic Control at Works on Roads
93 TRB (1994)
Transportation Research Board, Highway Capacity Manual, HCM 2000, National Research Council, Washington, D.C., USA, 2000.
94 Triggs T J and Harris W G (1982)
Reaction Time of Drivers to Road Stimuli, Human Factors Report HFR12, Monash University, Melbourne.
95 Troutbeck (1981)
Overtaking Behaviour on Australian two-lane rural highways, ARRB, Special Report 20.
96 Underwood R T (1990)
Traffic Management – An introduction, Hargreen Publishing Co., Melbourne.
97 Underwood R T (1991)
The Geometric Design of Roads, Melbourne.
98 VicRoads (1997)
VicRoads, Road Design Guidelines Part 2 Horizontal and Vertical Geometry, VicRoads, Victoria.
116
RURAL ROAD DESIGN
99
VicRoads (1997)
VicRoads, Road Design Guidelines Part 3 – Cross Section Elements, VicRoads, Victoria.
100 VicRoads (1997)
Traffic Engineering Manual, Volume 1, Chapter 6, Edition 2, Melbourne, Victoria.
101 Wambold et al (1988)
Wambold JC, Rivera-Ortiz LA, Wang MC . A field and Laboratory Study to establish truck escape ramp design methodology. Commonwealth of Pennsylvania. Department of Transportation, Report No FHWA-PA86-032+83-26
102 WITS (1997)
Water Industry Technical Standards, Volumes 1 and 2, Melbourne Water, Victoria.
103 Cooperative Water Sensitive Road Design – Research Centre Design Options for Improving Storm for Catchment Water Quality of Road Runoff. Hydrology (2000) 104 Austroads (2001) Road Runoff and Drainage: Environmental Impacts and Management Options. 105 AASHTO (1993)
American Association of State Highway and Transportation Officials, Recommended Procedures for the Safety Performance Evaluation of Highway Features.
106 Cox R.L. (1995)
Analysis in Traffic and its Effect of Service for the Justification of Overtaking Lanes and Future Road Duplication. Queensland Department of Main Roads.
A
APPENDIX
1.
CHARASTERISTICS OF THE EULER SPIRAL ( C LOT H O I D )
BASIC PROPERTIES OF THE C LOT H O I D T R A N S I T I O N C U R V E
Transition curves connecting a circular curve to two straights are shown in Figure A1. Typical standard notation for transition curves is as follows (See Figure A1):
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Figure A1: Transition Curve Details
RURAL ROAD DESIGN
117
R = radius of the circular curve in metres; IP = intersection point, or the point at which the two straights join; TS = start transition, or the point at which a straight and a transition curve join; SC = start circular curve, or the point at which a transition and a circular curve join; PC = the point on the circular curve (extended) at which the radius if extended would be perpendicular to the straight; I = intersection angle, or the angle between the two straights in degrees; øs = spiral angle in degrees; T = tangent distance in metres; S = secant distance in metres; LP = length of transition curve from TS to SC in metres. Lc = length of circular curve from SC to SC in metres; l = distance in metres along the transition to any point B and TS; x = abscissa of any point B on transition with reference to the straight and TS in metres; y = ordinate of any point B on transition corresponding to the abscissa x in metres; p = the shift, which equals the offset from the PC to the straight in metres;
2.
T S Lc
BASIC RELATIONSHIPS FOR CLOTHOID TRANSITION CURVES
I +K 2 I = (R + p) sec –R 2 π = (I – 2øs ) R 180 = (R + p) tan
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The expressions for x, y, p and k are approximations only and normally are satisfactory for practical use. More precise expressions may be seen in any standard books on surveying. x
=
y
=
p
=
øs = K =
l5 l – 40(RLp)2 l7 l3 – 336(RLp)3 6(RLp) Lp2 Lp4 – 24R 2688R3 180 Lp π 2R L3p Lp – 2 240R2
As the clothoid has a constant rate of change of curvature it gives a constant rate of change of lateral acceleration at constant speed. For a vehicle travelling at a constant speed of v m/s, the lateral acceleration increases from zero at the start of the transition to v2 R at the start of the circular curve. This increase in acceleration takes place over a length LP metres or over a time t (seconds) where:
118
RURAL ROAD DESIGN
t
=
Lp V
Thus, the rate of change of lateral acceleration, A m/s3 v 2 / Lp V R 3 = v m/s3 RLp =
If v m/s is converted to V km/h, this equation becomes: 3 A m /s3 = 0.0214V where V is in km/h RLp
B
APPENDIX
V E R T I C A L C U R V E FO R M U L A E
where:
1.
GENERAL A a
The parabola has traditionally been used in road design for crest and sag vertical curves because: ● the vehicle undergoes a constant vertical acceleration; ● the length of curve is directly proportional to the grade change; ● a parabola retains its basic shape when the scale is changed whereas a circle takes the form of an ellipse when a change is made to one of the scales. ● The calculation of vertical and horizontal ordinates in relation to any point on a parabola is a simple matter. Gravity makes the use of vertical ordinates more convenient in construction. Other curves such as circular curves may be used if required for a specific reason. The K value equivalent radius R = 100 K.
2.
V E R T I C A L C U R V E FO R M U L A E
Parameters used in formulae for parabolas are shown on Figure B1.
g1, g2 e h1 h2 K L L1 SL
S V x xhp xlp y
= g2 – g1 = Algebraic grade change (%) = Vertical acceleration of vehicles on parabolas (m/sec2) = Grade (%) = Middle ordinate (m) = Eye height – for use with sight distance (m) = Object height – for use with sight distance (m) = Length of vertical curve for a 1% change in grade (m) = Length of vertical curve (m) = Length over which the grade is less than a specified slope SL (m) = Slope of the tangent to the curve at any point (%) Low or high points occur where SL = 0 = Sight distance (m) = Speed (km/h) = Distance from tangent point to any point on curve (m) = Distance from tangent point to high point (m) = Distance from tangent point to low point (m) = Vertical offset from tangent to curve (m)
NOTE: A rising grade with increasing chainage carries a plus sign and a falling grade carries a minus sign.
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Figure B1: Vertical Curve Nomenclature
RURAL ROAD DESIGN
119
Figure B2: Eye Height and Object Height
The general formula for the parabola used in road design is: y
=
x2 (g2 – g1) x2 = 200K 200L
...
K=
x2 200y
In road design most parabola can be designed using the following three equations: L
= KA
L
= K(g2 – g1)
K =
S2 200 (√h1 + √h2)2
An explanation of the use of K is included in Section 10.3. Other equations that may be used include: a e e L1 x xhp y
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y
AV2 1300L L = (g2 - g1) 800 ElevTP1 + ElevTP2 = 0.5 ElevIP 2 L = 2SL g2 – g1 (SL – g1) = L g2 – g1 Lg1 = g2 – g1 ex2 = (0.5L)2 ex2 = 4 (L)2
=
120
RURAL ROAD DESIGN
C
D E R I VAT I O N O F S I G H T D I S TA N C E R E Q U I R E M E N T S AT R A I LWAY L E V E L C R O S S I N G S
APPENDIX
Figure C1: Influence of Slope on Stopping Distance
1. GENERAL Before detailing the procedures used in the derivation of the formulae used in this Guide, it is important that users note that sight distance requirements at railway level crossings have historically varied from State to State. It is necessary to consider two scenarios in the evaluation of sight distance requirements at railway level crossings. Case 1 address the sight distances required for an approaching vehicle considering two critical situations (necessary to establish whether the Give Way Control is adequate); and case 2 addresses the sight distance along the railway for a vehicle stopped at a STOP sign (necessary to establish the adequacy of STOP sign control). The geometry and associated notation for cases 1 and 2 are depicted on Figures C2 and C3 respectively. Thus, to stop on level ground, we require:
2.
CASE 1: Sight Distance Required for Give Way Control
Case 1 allows a motorist approaching the crossing at distance S1 to sight a train at distance S2 from the crossing and either: Case 1(i)
Decelerate and safely stop at the stop or holding line; or
RTVv + Vv2 + Ld + Cv (1) 3.6 254F The influence of slope on the stopping distance component of this equation can be derived using simple physics as shown on Figure C1.
S1
≥
The influence of grade on vehicle deceleration can be derived as follows: ●
Braking distance
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Case 1(ii) Proceed and clear the crossing with an adequate safety margin.
(Vv / 3.6)2 Vv 2 Vv2 = = 2gF 254F metres 2a
When motorists reach a crossing and see a train approaching, they must decide whether to decelerate and stop, or proceed and clear the crossing. There is a finite distance required between the vehicle and the rail in order to reach a decision and act in safety. This distance, assuming a level grade crossing site, comprises four components:
●
Component of vehicle mass acting down the slope = mgsin0 (g = acceleration due to gravity = 9.81m/sec2);
●
For small angles sinq = tanq = x/y = G (m/m) (grade is expressed as ratio, negative for downhill);
The distance travelled during the perception/reaction time
●
Force acting down the slope . mgsinq . mgtanq = mgG; Effective deceleration = gF + gG = g(F + G); and Therefore effective deceleration = g(F + G)
●
RTVv = ●
●
RTVv metres 3.6
●
Braking distance 2 v
2
2
V (Vv / 3.6) Vv = = metres 2a 2gF 254F
In order to stop on sloped ground, equation 1 subsequently becomes: S1 ≥
where:
RTVv 3.6
+
Vv2 254 (F+G)
+ Ld
+ Cv
(2)
where: g = acceleration due to gravity = 9.81m/sec2; ● ●
Distance of the driver from the front of the vehicle (Ld metres); and Clearance from the vehicle stop or holding line to the nearest rail (Cv metres).
S1 =
minimum distance of an approaching road vehicle from the nearest rail when the driver of the vehicle can see an approaching train (m);
RT =
perception/reaction time (general case assumption = 2.5 sec);
RURAL ROAD DESIGN
121
Vv
= the 85th percentile road vehicle speed in the vicinity of the crossing. The road speed limit plus 10% is a reasonable approximation where the 85th percentile speed is not known (km/h); = coefficient of longitudinal friction (refer to Table 8.2); = distance from the driver to the front of the vehicle (general case assumption = 1.5 m); = clearance from the vehicle stop or holding line to the nearest rail (general case assumption = 3.5 m); and = grade, negative for downhill, positive for uphill (m/m).
F Ld Cv
G
3. CASE 1(i): Decelerate and Safely Stop at the Stop or Holding Line The time required for a motorist (at a distance S1 from the nearest rail) to stop at the stop or holding line, comprises: ● ●
Perception/reaction time (RT); and Braking time Vv / 3.6 Vv 2 Vv gF 35.3 a = = metres
Sight Distance S 2L Adjustment For the case of a train approaching the crossing from the left, the sight distance S2 is calculated from the left edge line of the road (or the road pavement if there is no edge line). In order to measure distance S2L from the referenced datum point, an adjustment needs to be incorporated in the S2 equation. The datum point referenced in the field survey is the intersection of the centre line of the road and the mid point of the rail tracks at the crossing. Adjustment for S2L equation =
0.5WR sinZ
In the case of a train approaching the crossing from the right, the sight distance S2R is equal to that adopted for S2, as the potential point of impact is at the datum point. The minimum distances, S2L and S2R, where an approaching train is first sighted in order for a driver of an approaching vehicle to safely stop at the stop or holding line, are calculated from equations 4 and 5 respectively. The minimum distance for a train approaching from the left of the crossing, to enable the driver of a road vehicle to decelerate and safely stop at the stop or holding line is: S2L(l)
≥
0.5WR V Vv + T RT + sinZ 3.6 35.3F
(4)
(g = acceleration due to gravity = 9.81 m/sec2). Therefore, for the motorist to safely stop, the train would have to be sighted at a minimum distance, S2 from the crossing: VT Vv 3.6 35.3F S2 ≥ RT + (3)
The minimum distance for a train approaching from the right of the crossing, to enable the driver of a road vehicle to decelerate and safely stop at the stop or holding line is: S2R(1)
≥
VT Vv R + 3.6 T 35.3F
(5)
where: S2
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VT RT Vv
F
= minimum distance of an approaching train from the point of impact with a road vehicle, when the driver of the road vehicle first sees a train approaching in order to safely stop at the stop or holding line (m); = the speed of the train approaching the crossing (the allowed operating speed of trains, as advised by the rail authority) (km/h); = perception/reaction time (general case assumption = 2.5 sec); = the 85th percentile road vehicle speed in the vicinity of the crossing. The road speed limit plus 10% is a reasonable approximation where the 85th percentile speed is not known; and = coefficient of longitudinal friction (refer to Table 8.2).
Note that the distance S2 is measured from alternate datum points which are contingent upon whether a train approaches from the left or right. For a train approaching from the left, the point of impact is at the road edge line, whilst, for a train approaching from the right, it is at the road centre line. For a field survey, distances S2L and S2R are required to be calculated separately as a common datum point is referenced.
122
RURAL ROAD DESIGN
The calculated distances S2L and S2R are then compared to the distances obtained in the case of a driver of a road vehicle safely proceeding and clearing the crossing ( Case 1 (ii): The larger value is adopted as the critical case.
4. CASE 1(ii): Proceed and Clear the Crossing with an Adequate Safety Margin It is also important to consider the case in which a motorist at distance S1 from the crossing decides to proceed (even though he/she could safely stop) and attempt to clear the crossing prior to the arrival of the train. Referring to Figure C2, the distance a motorist has to travel to clear the crossing is: S1 +
WR WT + + Cv + CT + L – Ld tanZ sinZ
Substituting S1 from equation 2, this becomes: VV2 WR WT RTVV + + + + 2Cv + CT + L 254 (F+G) tanZ sinZ 3.6
Therefore, the distance travelled by the train for the motorist to precede and clear the crossing: VV2 VT RTVT WR WT + + + + 2Cv + CT + L Vv 3.6 254(F+G) tanZ sinZ
S2 =
(6)
The minimum distance (S2R) of an approaching train from the intersection of the centre line of the road and the mid point of the rail tracks, when the driver of the road vehicle first sees a train approaching from the right, in order to proceed and clear the crossing is:
where:
VV2 W W VT RTVT + + R + T +2Cv + CT + L Vv 3.6 254(F+G) tanZ sinZ
S2R = S2 = minimum distance of an approaching train from the point of impact with a road vehicle, when the driver of the road vehicle can first see the train approaching the crossing in order to proceed and safely clear the crossing (m); VT = the speed of the train approaching the crossing (the allowed operating speed of trains, as advised by the rail authority) (km/h); Vv = the 85th percentile road vehicle speed in the vicinity of the crossing. The road speed limit plus 10% is a reasonable approximation where the 85th percentile speed is not known; RT = perception/reaction time (general case assumption = 2.5 sec); Cv = clearance from the vehicle stop or holding line to the nearest rail (general case assumption = 3.5 m); CT = clearance or safety margin from stop or holding line on departure side of the crossing (general case assumption = 5 m); F = coefficient of longitudinal friction (refer to Table 8.2); L = length of road vehicle, refer to Table C1; WR= width of the travelled way (portion of the roadway allocated for the movement of the vehicles) at the crossing (m); WT = width, outer rail to outer rail, of the rail tracks at the crossing (1.1 m for single track, 5.1 m for double track); and Z = angle between the road and the railway at the crossing (degrees).
(8)
In order to obtain the critical sight distances, S2L and S2R, the larger distances from Cases 1(i) and (ii) should be adopted. Source: Ref 66
5. CASE 2: S i g h t D i s t a n ce R e q u i re d fo r S TO P S i g n C o n t ro l When motorists are stationary at a crossing controlled by a STOP sign, they require adequate sight distance to determine whether or not it is safe to cross the tracks before the train arrives. Referring to Figure C3, it presents a method by which the time taken to complete this manoeuvre can be ascertained. The time comprises: ● ●
Perception time and time required to depress clutch (J); and Time to clear the crossing by a ‘safe’ distance
WR + WT + 2C + C + L V T sin Z 2 tan Z a
1/ 2
The distance travelled by the train during this time:
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Table C1: Vehicle Lengths Vehicle Route Vehicle Type and Length Roads not on nominated route Medium car 5m Prime mover and semi-trailer 19 m B-double route B-double 25 m Road train route – Type 1 Type 1 road train 33 m Road train route – Type 2 Type 2 road train 50 m
As discussed in Case 1(i), distance S2 is measured from alternate datum points to correspond with the potential point of impact for the left and right train approaches. In order to carry out a detailed survey of a crossing, distances S2L and S2R are required to be calculated separately, as a common datum point is utilised. The minimum distance (S2L) of an approaching train from the intersection of the centre line and the mid point of the rail tracks, when the driver of the road vehicle first sees a train approaching from the left, in order to safely proceed and clear the crossing (considering the sight distance S2L adjustment indicated in Case 1(i)) is: S2L =
0.5WR VT RTVT VV2 W W + + + R + T +2Cv + CT + L (7) sinZ Vv 3.6 254(F+G) tanZ sinZ
S3 =
WR + WT + 2C + C + L V T VT sin Z J +GS 2 tan Z 3.6 a
1/ 2
(9)
Field testing has confirmed that the influence of grade on vehicles accelerating from a stationary position is not accurately modelled by the application of simple physics principles (Lay 1990:571). American literature (AASHTO Policy on Geometric Design of Highways quoted in MRD (WA) 1991:16) provides the grade correction factors in Table C2. Equation (9) subsequently becomes:
S3 =
WR + WT + 2C + C + L V T VT sin Z J + GS 2 tan Z 3.6 a
1/ 2
(10)
where:S3 =
minimum distance of an approaching train from the point of impact with a road vehicle, when the driver of the road vehicle must first see an approaching train in order to safely cross the tracks (m);
RURAL ROAD DESIGN
123
VT =
the speed of the train approaching the crossing (the allowed operating speed of trains, as advised by the rail authority (km/h); J= sum of the perception time and time required to depress clutch (general case assumption = 2 sec); GS = grade correction factor, refer to Table C2; L= length of road vehicle, refer to Table C1 (m); CV = clearance from the vehicle stop or holding line to the nearest rail (general case assumption = 3.5m); CT = clearance or safety margin from stop or holding line on departure side of the crossing (general case assumption = 5m); WR = width of the travelled way (portion of the roadway allocated for the movement of the vehicles) at the crossing (m); WT = width, outer rail to outer rail, of the rail tracks at the crossing (1.1m for single track, 5.1m for double track); Z= angle between the road and the railway at the crossing (degrees); and a= average acceleration of vehicle in starting gear (general case assumption = 0.5 m/sec2, refer to Table C3).
Table C2: Grade Correction Factors (AASHTO Policy on Geometric Design of Highways) Percentage Grade –4 –2 +2 +4
Grade Correction Factor GS 0.8 0.9 1.2 1.7
Sight Distance S 3L Adjustment A sight distance adjustment is necessary to calculate S3L for the common datum point used in the field survey. The datum point referenced in the field survey is the intersection of the centre line of the road and the mid point of the railway tracks at the crossing.
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Adjustment for S3 L equation =
0.5WR sin Z
Therefore, the minimum distance of an approaching train from the intersection of the road centre line and the mid point of the rail tracks, when the driver of a road vehicle must first see a train approaching from the left in order to safely cross the track from a stopped position is:
S3 L =
WR + WT + 2C + C + L V T 0.5WR VT + J +GS 2 tan Z sin Z sin Z 3.6 a
1/ 2
(11)
The minimum distance of an approaching train from the intersection of the road centre line and the mid point of the rail tracks, when the driver of a road vehicle must first see a train approaching from the left in order to safely cross the track from a stopped position is:
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RURAL ROAD DESIGN
S3 R
WR + WT + 2C + C + L V T VT tan Z sin Z = J +GS 2 3.6 a
1/ 2
(12)
Table C3: Heavy Vehicle Speed/Acceleration Performance (RTA, 1990 and QT, 1993) Type of Vehicle
Laden Rigid Truck (RTA 1990) Laden Semi Trailer (RTA 1990) Laden B-double (RTA 1990) Laden Road Train (RTA 1990) Laden 19m Semi-Trailer (QT Mt Cotton Facility 1993) Laden 19m Semi-Trailer (QT Mt Cotton Facility 1993)
Distance Time Average Average Travelled (sec) Speed Acceleration (m) (m/sec) (m/sec) 22.4
9.3
2.4
0.50
28.9
12.6
2.3
0.36
34.4
13.6
2.5
0.37
46.4
21.3
2.2
0.29
27.5
11.3
2.4
0.43
8.7
3.2
0.73
13.8
2.5
0.36
10.8
3.2
0.59
34.5
NOTE: In addition to the data provided in Table C3, limited data collected by ARRB (Barton 1990:6) suggests the average speed of a heavy vehicle commencing from a stopped position equals 3.3 m/sec over a typical crossing distance. The Main Roads Department (Western Australia) (1991:13) quotes values of acceleration obtained from American literature ranging from “45 m/sec2 for the acceleration of trucks in first gear, to 0.54 m/sec2 over a distance of around 12m, then gradually back down to a value of 0.5 m/sec2 for a distance of around 50”. For the required crossing visibility at the critical case, they subsequently recommend the adoption of a heavy vehicle acceleration value of 0.5 m/sec2 to “be on the conservative side”, and indicate that this value has been shown “to be acceptable by measuring the acceleration rates of a number of fully laden trucks, which resulted in values between 0.55 m/sec2 and 0.90 m/sec2”.
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Figure C2: Approach Visibility: At Grade Railway/Road Crossings
Case 1(i) Motorist approaching crossing sights train, decelerates and stops at the holding line.
VV
Case 1(ii) Motorist approaching crossing sights train, proceeds and safely clears the crossing.
CV
Notation (units and/or general case assumptions are shown in brackets): S1 S2 S2L
S2R
VT
Minimum distance of an approaching road vehicle from the nearest rail when the driver of the vehicle can see an approaching train (m); Minimum distance of an approaching train from the point of impact with a road vehicle, when the driver of the road vehicle first sees a train approaching (m); Minimum distance of an approaching train from the intersection of the road centre line and the mid point of the rail tracks, when the driver of the road vehicle first sees a train approaching from the left (m). Minimum distance of an approaching train from the intersection of the road centre line and the mid point of the rail tracks, when the driver of the road vehicle first sees a train approaching from the right (m). The speed of the train approaching the crossing (the allowed operating speed of trains, as advised by the rail authority (km/h).
CT L Ld WR WT X1L
X1R
Z=
The 85th percentile road vehicle speed in the vicinity of the crossing. The road speed limit plus 10% is a reasonable approximation where the 85th percentile speed is not known. Clearance from the vehicle stop or holding line to the nearest rail (general case assumption = 3.5 m). Clearance or safety margin from the vehicle stop or holding line on the departure side of the crossing (general case assumption = 5 m). Length of road vehicle (m). Distance from the driver to the front of the vehicle (general case assumption = 1.5 m). Width of the travelled way (portion of the roadway allocated for the movement of the vehicles) at the crossing (m). Width, outer rail to outer rail, of the rail tracks at the crossing (1.1 m for single track, 5.1 m for double track). Vehicle driver viewing angle measured from distance S1 on the road centre line, where a driver must first see a train approaching from the left at distance S2 from the crossing. Vehicle driver viewing angle measured from distance S1 on the road centre line, where a driver must first see a train approaching from the right at distance S2 from the crossing. Angle between the road and the railway at the crossing (degrees).
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Figure C3: Crossing Visibility: At Grade Railway/Road Crossings
Case 2 Motorist stopped at crossing requires adequate time to accelerate and safely clears the crossing.
Ld
Distance from the driver to the front of the vehicle (general case assumption = 1.5m).
Notation (units and/or general case assumptions are shown in brackets):
CV
Clearance from the vehicle stop or holding line to the nearest rail (general case assumption = 3.5m).
S3
Minimum distance of an approaching train from the point of impact with a road vehicle, when the driver of the road vehicle must first see an approaching train in order to safely cross the tracks.
CT
Clearance or safety margin from the vehicle stop or holding line on departure side of the crossing (general case assumption = 5m).
WR S3L
Minimum distance of an approaching train from the intersection of the road centre line and the mid point of the rail tracks, when the driver of a road vehicle must first see a train approaching from the left in order to safely cross the track from a stopped position at the stop or holding line (m).
Width of the travelled way (portion of the roadway allocated for the movement of the vehicles) at the crossing (m).
WT
Width, outer rail to outer rail, of the rail tracks at the crossing (1.1m for single track, 5.1m for double track).
X2L
Vehicle driver viewing angle measured from at the STOP line to a train approaching from the left at distance, S3 from the crossing.
X2R
Vehicle driver viewing angle measured from at the STOP line at the road centre line to a train approaching from the right at distance, S3 from the crossing.
Z=
Angle between the road and the railway at the crossing (degrees).
S3R
Minimum distance of an approaching train from the intersection of the road centre line and the mid point of the rail tracks, when the driver of a road vehicle must first see a train approaching from the right in order to safely cross the track from a stopped position at the stop or holding line (m).
VT
The speed of the train approaching the crossing (the allowed operating speed of trains, as advised by the rail authority) (km/h).
L
Length of road vehicle (m).
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RURAL ROAD DESIGN
RURAL ROAD DESIGN
127
Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
128
RURAL ROAD DESIGN
Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
A Guide to the Geometric Design of Rural Roads ISBN: 0 85588 655 2 AP-G1/03
AUSTROADS ROAD DESIGN SERIES AUSTROADS
Accessed by AUSTROADS - GHD PTY LTD on 10 Apr 2006
Rural Road Design A Guide to the Geometric Design of Rural Roads
Rural Road Design
