lec 08 highway engineering - cross section elements
TRANSCRIPT
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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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