Spring 2015. Cross Section Design Lane width Shoulders Side Slopes Curbs Traffic Barriers.
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Transcript of Spring 2015. Cross Section Design Lane width Shoulders Side Slopes Curbs Traffic Barriers.
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Cross Section Design
Spring 2015
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Cross Section Design
Lane width
Shoulders
Side Slopes
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
Cross Section Elements
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Components of Cross Section
Traveled Way
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Components of Cross Section
Roadway
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Components of Cross Section
Right-of-Way: land owned by a government entity for public use (state, local, etc.)
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Components of Cross Section
Clear Zone: Area located between the edge-of-travel way and the location of fixed objects (trees, posts, etc.)
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Components of Cross Section
Undivided Highways
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Components of Cross Section
Divided Highways
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Components of Cross Section
Median
Note: Median measured from edge-of-travel way to edge-of- travel way
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Lane WidthRange: 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
Cross Slope
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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
Cross Slope
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Cross Slope
Crowned for Each Direction
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Cross Slope
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
Cross Slope
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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
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Cross Slope
Cross Slope Steeper
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Cross slope steeper than 2 percent for high-speed highways (high type pavement) with a central crowned design should be avoided:◦ Abrupt change in centrifugal force◦ Trucks may sway (high center of gravity)
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
Cross Slope
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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
Shoulders
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Shoulders
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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
Shoulders
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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
Shoulders
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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
Shoulders
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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
Desirable to use different color and texture between shoulders and traveled way (night, inclement weather, etc.)
Shoulders
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Curbs are used for:◦ 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
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Curbs
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Curbs
Curbs: defined by vertical and horizontal configuration
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CurbsVertical curb:
- Prevent from vehicles to leave the roadway
- Should not be used on high-speed highways
- May be used along walls and tunnels
6 to 8 in
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CurbsHorizontal sloping curb:
-Mountable by vehicles
-Rounded edge allow ease to be mountable during emergency
-Will scrap undersides of some vehicles
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CurbsHorizontal 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:◦ Hinge point◦ Foreslope◦ Ditch bottom◦ Backslope
Slideslopes
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Slideslopes
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Slideslopes
May contribute to loss of steering control (vehicle become airborne) (possibility of rollover)
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Slideslopes
Area where the driver can try to recover for loss-of-control vehicle
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Slideslopes
The area where the vehicle will most likely end up if the driver cannot regain control of vehicle
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Slideslopes
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)
Slideslopes
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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
Barriers
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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 Two types of barriers:
◦ Flexible: allows for deflection (cable, guardrail)◦ Rigid: no deflection is allowed (concrete)
Barriers