1. Cross Section Design
Spring 2015

2. Cross Section Design Lane width Side Slopes Shoulders Curbs
Traffic Barriers

3. 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

4. Components of Cross Section
Traveled Way

5. Components of Cross Section
Roadway

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

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

8. Components of Cross Section
Undivided Highways

9. Components of Cross Section
Divided Highways

10. Components of Cross Section
Median
Note: Median measured from edge-of-travel way to edge-of- travel way

11. 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

12. 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

13. 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

14. Cross Slope
Crowned for Each Direction

15. Cross Slope
Unidirectional Cross Slope

16. 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

17. 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

18. Cross Slope
Cross Slope Steeper

19. 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

20. 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

21. Shoulders

22. 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

23. 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

24. 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

25. 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.)

26. 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

27. Curbs

28. Curbs
Curbs: defined by vertical and horizontal configuration

29. 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

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

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

32. 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

33. Slideslopes

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

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

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

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

38. 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)

39. 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

40. 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)