Lec 08 Highway Engineering - Cross Section Elements

Highway Eng.

Cross Section Elements

14 –15

Cross Section Elements of Highways In this lecture; ---------------------

A- General B- Cross Section Elements

The information listed in this lecture is mainly taken from the Policy on Geometric Design of Highways and Streets (AASHTO, 2011), Iraqi Highway Design Manual (SORB, 2005) and Traffic and Highway Engineering (Garber and Hoel, 2009)..

A- General The principal elements of a highway cross section consist of the travel lanes, shoulders, and medians (for some multilane highways). Marginal elements include median and roadside barriers, curbs, gutters, guard rails, sidewalks, and side slopes. The cross section of a road includes some or all of the following elements: - Travelled way: the portion of the roadway provided for the movement of vehicles, exclusive of shoulders. - Roadway: the portion of a highway, including shoulders, provided for vehicular use. Divided highway has two or more roadways. - Median area: the physical or painted separation provided on divided highways between two adjacent roadways. - Bicycle and pedestrian facilities Lecture 08

87

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

- Utility and landscape areas - Drainage channels and side slopes - Clear zone width (i.e., the distance from the edge of the travelled way to either a fixed obstacle or non-traversable slope) Considered as a single unit, all these cross section elements define the highway right of way. The right of way can be described generally as the publicly owned parcel of land that encompasses all the various cross section elements. Figures below show typical cross sections for a two-lane highway and for a multilane highway respectively.

Lecture 08

88

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

More examples of highway cross sections.

Figure 1: Two-lane highway cross section, with ditches.

Figure 2: Two-lane highway cross section, curbed.

Figure 3: Divided highway cross section, depressed median, with ditches.

Figure 4: Divided highway cross section, raised median, curbed. Lecture 08

89

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

B- Elements of Highway Cross Section Following is a brief discussion for several important cross-sectional features: 1) PAVEMENT : Surface Type The selection of pavement type depends on several factors such as the traffic volume and composition, soil characteristics, weather, availability of materials, initial cost, and maintenance cost. Generally, pavements types can be classified as follows: 1- High type pavement: such as asphalt concrete and Portland cement concrete; 2- Intermediate types: such as double bituminous surface treatment;

3- Low type: such as stabilized surface, loose gravel and earth work. 2) PAVEMENT : Cross Slope Undivided travelled ways on tangents, or on flat curves, have a crown (high point in the Middle) and a cross slope downward toward both edges. Unidirectional cross slopes across the entire width of the travelled way may be utilized. The downward cross slope may be a plane or rounded section. The rounded section is advantageous in that the cross slope steepens toward the edge of the travelled way, thereby facilitating drainage. Disadvantages are that rounded sections are more difficult to construct. On divided highways each one-way travelled way may be crowned separately or it may have a unidirectional cross slope across the entire width of the travelled way, which is almost always downward to the outer edge. A cross section with each roadway crowned separately, as shown in Exhibit 4-3A through Exhibit 4-3C, has an advantage in rapidly draining the pavement during Lecture 08

90

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

rainstorms. In addition, the difference between high and low points in the cross section is minimal. Disadvantages are that more inlets and underground drainage lines are needed, and treatment of intersections is more difficult because of the number of high and low points on the cross section.

The recommended ranges for cross slope rates for high-type and low-type surfaces are shown in the table below.

Lecture 08

91

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

3) Width of Travel Lanes. Travel lane widths usually vary from 9 to 12 ft (2.75 – 3.65 m). Most arterials have 3.65 m travel lanes. On two-lane two-way rural roads, lane widths of 3.05 m or 3.35 m may be used, but two factors must be considered when selecting a lane width less than 3.65 m wide. When pavement surfaces are less than 6.70 m, the crash rates for large trucks tend to increase and, as the lane width is reduced from 3.65 m, the capacity of a highway significantly decreases. Lane widths of 3.05 m are therefore used only on low-speed facilities. Lanes that are 2.75 m wide are used occasionally in urban areas if traffic volume is low and there are extreme right-of-way constraints. According to Iraqi SORB, the recommended lane width for major roads is 3.75m. 4) Shoulders. A shoulder is the portion of the roadway contiguous with (adjacent to) the travelled way that accommodates stopped vehicles, emergency use, and lateral support of subbase, base, and surface courses. In some cases, the shoulder can accommodate bicyclists. It varies in width from only 0.6 m on minor rural roads to 3.6 m on major roads where the entire shoulder may be stabilized or paved. The usable shoulder width is that part of the graded (whole width) shoulder that can be used to accommodate parked vehicles. Recommended slopes are 2 to 6 percent for bituminous and concrete-surfaced shoulders, and 4 to 6 percent for gravel or crushed-rock shoulders. Rumble strips may be used on paved shoulders along arterials as a safety measure to warn motorists that they are leaving the traffic lane.

Lecture 08

92

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

All shoulders should be flush (not higher nor lower) with the edge of the adjacent travelled lane and sloped to facilitate drainage of surface water on the travelled lanes. It is desirable that the colour and texture of shoulders be different from those of the travelled way.

5) Median A median (physical or painted) is the portion of a divided highway which separates opposing directions of the travelled way. Medians are highly desirable on arterials carrying four or more lanes. Median width is expressed as the dimension between the edges of travelled way and includes the left shoulders, if any. The key functions of a median include: • As a recovery area for out-of-control vehicles and stopping areas at emergencies. Lecture 08

93

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

• Separating opposing traffic. • Providing refuge for pedestrians. • Reducing the effect of headlight glare. • Providing storage areas for left-turning and U-turning vehicles. Medians can either be raised, flush (‫)ﻣﺳ�ﺗﻭﻳﺔ‬, or depressed (‫)ﻣﻧﺧﻔﺿ�ﺔ‬. Raised medians are frequently used in urban arterial streets because they facilitate the control of leftturn traffic at intersections by using part of the median width for left-turn-only lanes. Flush medians are commonly used on urban arterials. They can also be used on freeways, but with a median barrier. To facilitate drainage of surface water, the flush median should be crowned. Depressed medians are generally used on freeways and are more effective in draining surface water. The general range of median widths is from 1.2 to 24 m or more. In general, the wider the median, the more effective it is in providing safe operating conditions and a recovery area for out-of-control vehicles; cost should be taking into account.

Lecture 08

94

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

6) Curbs Curbs are raised structures made of either Portland cement concrete or bituminous concrete that are used mainly on urban highways to delineate both pavement edges and pedestrian walkways. Curbs are also used to control drainage, improve aesthetics, and reduce right of way. Curbs can be generally classified as either vertical (barrier) or sloping (mountable). Vertical curbs range in height from 15 to 20 cm and are designed to prevent vehicles from leaving the highway. Sloping curbs are designed so that vehicles can cross them if necessary. The Figure below illustrates typical highway curbs.

Lecture 08

95

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

7-a) Gutters or Drainage Ditches Gutters or ditches are usually located on the pavement side of a curb to provide the principal longitudinal drainage facility for the highway. They are sloped to prevent any hazard to traffic, and they usually have cross slopes of 5 to 8 percent and are 0.30 to 1.8 m wide. Gutters can be designed as V-type sections or as broad, flat, rounded sections.

7-b) Drainage Channels Drainage channels perform the key function of collecting and conveying surface water from the highway right-of-way. Roadside channels should have adequate capacity for the design runoff and be located and shaped to provide a safe transition from the roadway to the backslope. The primary purpose for construction of roadside channels is to control surface drainage. The most economical method of constructing a roadside channel usually envolves the formation of open-channel ditches by cutting into the natural roadside terrain to produce a drainage channel. Lecture 08

96

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

8-a) Median & Roadside Barriers A median barrier is defined as a longitudinal system used to prevent an errant vehicle from crossing the portion of a divided highway separating the travelled ways for traffic in opposite directions. Roadside barriers, on the other hand, protect vehicles from obstacles or slopes on the roadside. They also may be used to shield pedestrians and property from the traffic stream.

Lecture 08

97

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

8-b) Guard Rails Guard rails are longitudinal barriers placed on the outside of sharp curves and at sections with high fills. Their main function is to prevent vehicles from leaving the roadway. They are installed at embankments higher than 2.4 m and when shoulder slopes are greater than 4:1.

9) Sidewalks Sidewalks are usually provided on roads in urban areas. Generally, sidewalks should be provided when pedestrian traffic is high along main or high-speed roads in either rural or urban areas. Sidewalks should have a minimum clear width of 1.2 m in residential areas and a range of 1.2 to 2.4 m in commercial areas.

Lecture 08

98

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

10) Side Slopes Side slopes are provided on embankments and fills to provide stability for earthworks. They also serve as a safety feature by providing a recovery area for outof-control vehicles. When being considered as a safety feature, the important sections of the cross slope along the roadside are the top of the slope (hinge point), the foreslope, and the toe of the slope (intersection of the foreslope with level ground or with a backslope, forming a ditch). The following Figure illustrates these three regions. Slopes of 3:1 (horizontal:vertical) or flatter are generally used for high embankments. Retaining walls should be considered where space restrictions would otherwise result in slopes steeper than 2H:1V.

11) Frontage Road Frontage roads are generally parallel to the travelled way. They may be used to control access to the arterial, function as a street facility serving adjoining properties, and maintain circulation of traffic on each side of the arterial. Frontage roads segregate local traffic from the higher speed through-traffic and intercept driveways of residences and commercial establishments along the highway. Lecture 08

99

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

12) Outer Separation The area between the travelled way of a through-traffic roadway and a frontage road or street is referred to as the “outer separation.” Such separations function as buffers between the through traffic on the arterial and the local traffic on the frontage road and provide space for a shoulder for the through roadway and ramp connections to or from the through facility. 13) Right of Way The right of way is the total land area acquired for the construction of a highway. The width should be sufficient to accommodate all the elements of the highway cross section, any planned widening of the highway, and public-utility facilities that will be installed along the highway. The right of way for two lane urban collector streets should be between 12 and 18 m, whereas the desirable minimum for two-lane arterials is 25 m. Right-of-way widths for undivided four-lane arterials vary from 20 to 33 m, whereas for divided Lecture 08

100

Dr. Firas Asad

Highway Eng.

Cross Section Elements

14 –15

arterials, they range from about 36 to 91 m, depending on the numbers of lanes and whether frontage roads are included.

==============================

Lecture 08

101

Dr. Firas Asad