Transcript HIGHWAY FUNCTIONS: Systems and Classifications

Spring 2015
Cross Section Design
Lane width
Side Slopes
Shoulders
Curbs
Traffic Barriers
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Important Definitions:
◦ Roadway: The portion of a highway, including
shoulders, for vehicular use. For instance, a divided
highway has two or more roadways
◦ Traveled Way: The portion of the roadway used for the
movement of vehicles, exclusive of shoulders
Traveled Way
Roadway
Right-of-Way: land owned by a government entity for
public use (state, local, etc.)
Clear Zone: Area located between the edge-of-travel
way and the location of fixed objects (trees, posts,
etc.)
Undivided Highways
Divided Highways
Median
Note: Median measured from edge-of-travel way to edge-oftravel way
Lane Width
Range: 9 ft to 12 ft
12 ft: most commonly used on rural and urban highways;
adequate for most of the heavy vehicle
11 ft: acceptable in urban areas (to accommodate
pedestrians crosswalk)
10 ft: acceptable on low-speed highways
9 ft: acceptable on local rural or urban streets (residential)
CL
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Undivided highways (tangent): crown in the
middle and a cross slope downward toward both
edges
Downward cross slope can be either a plane or a
rounded section or both
Rounded sections are usually parabolic:
◦ Advantage: cross slope steepens toward edges
◦ Disadvantage: difficult to construct; cross slope may
be excessive in outer lanes
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Divided Highways: each one-way traveled
way can be crowned separately or can be
unidirectional cross slope across the entire
traveled way
Crowned roadways have the advantages of
rapidly drain water during rainstorms
However, need to install additional inlets and
underground drainage lines
Preferred in region of high rainfalls
Crowned for Each Direction
Unidirectional Cross Slope
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Unidirectional cross slope provide more comfort
to drivers when they change lanes
Drainage away from median may effect savings
in drainage structures, minimize drainage the
inner, high-speed lanes
Drainage toward the median will increase costs,
but will minimize water on most used lanes of
highways
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The rate of cross slope is an important element
in cross-section design
Tow-lane Highway: 1.5 to 2 percent is
acceptable; drivers will have difficulty perceiving
this rate
For three or more lanes: start at 0.5 percent and
increase by about 0.5 to 1 percent for each
successive pair of lanes
Cross Slope Steeper
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Cross slope steeper than 2 percent for highspeed highways (high type pavement) with a
central crowned design should be avoided:
◦ Abrupt change in centrifugal force
◦ Trucks may sway (high center of gravity)
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Exception: 2.5% in areas of intense rainfall
Three or more lanes in each direction: 4%
max in areas of intense rainfall
For cases where maximum cross slope is
used: consideration should be given for
grooving and open-graded mixes
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The shoulder is the portion of the roadway
contiguous with the traveled way that
accommodates stopped vehicles, emergency
use, and lateral support of subbase, base and
surface courses.
It can vary from 2 ft to 12 ft
Two types:
◦ Graded width: measured from edge of traveled way
to intersection of the shoulder slope
◦ Usable width: actual width that can be used when
a driver makes an emergency stop
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Shoulders may be surfaced either full or
partial width to provide a better all-weather
load support than that afforded by native
soils
Materials for shoulders: gravel, shell, crushed
rock, mineral and chemical additive, concrete
and asphalt pavements
Shoulders on minor roads: serves essentially
as structural support
Narrow lane width + narrow shoulders are
usually associated with higher number of
crashes
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Advantages of well-designed shoulders (rural
and urban highways, freeways):
◦ Space provided for vehicles in the event of an
emergency
◦ Space for motorist to stop occasionally to consult road
maps or for other reasons (although illegal in many
states)
◦ Provide space for evasive maneuvers
◦ Structural support for pavement
◦ Space for pedestrians and cyclists in urban areas
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Width of shoulders:
◦ Desirably, provide 1 ft to 2 ft between edge of traveled
way and vehicle
◦ 2 ft (low volume) to 12 ft (high speed/high flow)
◦ Wider may encourage unauthorized use of shoulder
◦ Shoulders should be continuous
◦ Might want to provide wider shoulders every mile or
kilometer on elevated structures
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Shoulder cross section:
◦ Shoulder and traveled way must be jointly
connected
◦ Cross Slope:
 Asphalt/concrete: 2 to 6 percent
 Gravel: 4 to 6 percent
 Turf: 6 to 8 percent
◦ Avoid severe cross section break on horizontal
curves
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Desirable to use different color and texture
between shoulders and traveled way (night,
inclement weather, etc.)
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Curbs are used for:
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Drainage control
Roadway edge delineation
Right-of-way reduction
Delineation of pedestrian walkways
They are mainly used on low-speed roads
They should be avoided on high-speed highways
Curbs: defined by vertical and
horizontal configuration
6 to 8 in
Vertical curb:
-Prevent from vehicles to leave the
roadway
-Should not be used on high-speed
highways
-May be used along walls and
tunnels
Horizontal sloping curb:
-Mountable by vehicles
-Rounded edge allow ease to be
mountable during emergency
-Will scrap undersides of some
vehicles
Horizontal extruded curb:
-Mountable by vehicles
-Design to be mountable
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Sideslopes are the area adjacent to the roadway
used for drainage purposes
They are also used to ensure roadway stability
and allow for recovery for an errant vehicle
Elements of sideslopes:
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Hinge point
Foreslope
Ditch bottom
Backslope
May contribute to loss of steering control (vehicle
become airborne) (possibility of rollover)
Area where the driver can try to recover for loss-ofcontrol vehicle
The area where the vehicle will most likely end up if
the driver cannot regain control of vehicle
Need to design the transition between foreslope and
backslope adequately (to prevent the vehicle to
come to an abrupt stop)
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Foreslopes should not be steeper than 1V:3H
(general rule: flatter the better, but more
costly)
If foreslope is steeper than 1V:3H, need to
shield the slope with a guardrail or barrier
Same kind of rules apply for backslope
Need to ensure that transition points are
rounded
Be aware that different material used on
foreslopes (turf, soil, etc.) can have a
different effects on the probability of a
rollover (known as tripping mechanisms)
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Barriers are used for shielding hazardous to
prevent vehicles leaving the traveled way to hit
to an object that has a greater severity potential
than the barrier itself
They should be used carefully since they are a
source of crash potential themselves
There is a lot of research performed on barriers.
Thus, you should be aware of the latest
development on this topic
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Traffic barriers include two components:
◦ Longitudinal barriers: the primary function is to redirect
errant vehicles
◦ Crash cushions: the primary function is to decelerate
errant vehicles
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Two types of barriers:
◦ Flexible: allows for deflection (cable, guardrail)
◦ Rigid: no deflection is allowed (concrete)