Cross Section Design

Spring 2015

Cross Section Design

Lane width

Shoulders

Side Slopes

Curbs

Traffic Barriers

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

Components of Cross Section

Traveled Way

Components of Cross Section

Roadway

Components of Cross Section

Right-of-Way: land owned by a government entity for public use (state, local, etc.)

Components of Cross Section

Clear Zone: Area located between the edge-of-travel way and the location of fixed objects (trees, posts, etc.)

Components of Cross Section

Undivided Highways

Components of Cross Section

Divided Highways

Components of Cross Section

Median

Note: Median measured from edge-of-travel way to edge-of- travel way

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

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

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

Cross Slope

Crowned for Each Direction

Cross Slope

Unidirectional Cross Slope

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

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

Cross Slope

Cross Slope Steeper

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

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

Shoulders

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

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

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

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

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

Curbs

Curbs

Curbs: defined by vertical and horizontal configuration

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

CurbsHorizontal sloping curb:

-Mountable by vehicles

-Rounded edge allow ease to be mountable during emergency

-Will scrap undersides of some vehicles

CurbsHorizontal extruded curb:

-Mountable by vehicles

-Design to be mountable

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

Slideslopes

Slideslopes

May contribute to loss of steering control (vehicle become airborne) (possibility of rollover)

Slideslopes

Area where the driver can try to recover for loss-of-control vehicle

Slideslopes

The area where the vehicle will most likely end up if the driver cannot regain control of vehicle

Slideslopes

Need to design the transition between foreslope and backslope adequately (to prevent the vehicle to come to an abrupt stop)

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

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

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