Lec 08 Highway Engineering - Cross Section Elements

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Lecture 08 87

Highway Eng. Cross Section Elements 14 15

Dr. Firas Asad

In this lecture;

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A- General

B- Cross Section Elements

Cross Section Elements of Highways

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


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- 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.


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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.


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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 inthe 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


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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.


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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

Recommended

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.

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.


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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.


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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 ) ), or depressed (). Raised medians are

frequently used in urban arterial streets because they facilitate the control of left-

turn 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.


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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.


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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.


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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.


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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.


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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 out-

of-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.


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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

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

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.


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arterials, they range from about 36 to 91 m, depending on the numbers of lanes and

whether frontage roads are included.

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Lec 08 Highway Engineering - Cross Section Elements

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