Cross Section Design Spring 2015

Cross Section Design Lane width Side Slopes Shoulders Curbs Traffic Barriers

Cross Section Elements 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

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 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) C L

Cross Slope 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 Crowned for Each Direction

Cross Slope Unidirectional Cross Slope

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 Steeper

Cross Slope 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

Shoulders 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.)

Curbs Curbs are used for: They are mainly used on low-speed roads 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: defined by vertical and horizontal configuration

Curbs 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

Curbs Horizontal sloping curb: -Mountable by vehicles -Rounded edge allow ease to be mountable during emergency -Will scrap undersides of some vehicles

Curbs Horizontal extruded curb: -Mountable by vehicles -Design to be mountable

Slideslopes 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 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)

Slideslopes 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)

Barriers 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)