1. Cross Sections
CE 453 Lecture 22
Iowa DOT Design Manual Chapter 3

2. See also Iowa DOT Standard Plans

3. Objectives 1. Identify cross section components and design criteria
See: (Chapter 6 from FHWA’s Flexibility in Highway Design)

4. Cross Section Elements Roadway Median Roadside Roadway Components
Travel Lanes
Shoulders
Source:

5. Cross Section Elements Roadway Median Border Roadway Components
Travel Lanes
Auxiliary Lanes
Source:

6. Considerations for Design of Cross-Section
Volume and composition (percent trucks, buses, and recreational vehicles) of the vehicular traffic
likelihood of bicyclists and pedestrians using the route
Flexibility in Highway Design - Chapter 6

7. Considerations for Design of Cross-Section
Climatic conditions (storage space for plowed snow, amount of rain)
Presence of natural or human-made obstructions adjacent to the roadway (rock cliffs,etc)
Type and intensity of development along the facility
Safety of the users (speed of traffic)

8. Travel Lanes Function: guidance to drivers and vehicle support
Pavement types:
high (modern standards),
intermediate (surface treatments), and
low (unpaved)

9. Travel Lanes Selection Criteria: Traffic volume and composition
Soil characteristics
Past performance in area
Availability of materials
Energy conservation
Initial cost
Maintenance cost
Overall life-cycle cost

10. Cross Slope Slope perpendicular to flow of traffic on tangent section
Rural – normal crown – uniform slope from center to edge of pavement (cross slope break typically at centerline)
Urban – parabolic shape (gutter capacity)

11. Cross Slope
Rate of Cross Slope = f(drainage, steering, and rollover or cross slope break)
Drivers cross the crown line during passing maneuvers
Difficult to negotiate steep slopes
AASHTO Recommends
High 1.5 to 2% (0.015 ft/ft m/m)
Intermediate 1.5 to 3%
Low 2 to 6%

13. Urban Cross-Sections

14. Urban Cross-Sections

15. HMA Cross-Sections

16. Drainage Considerations

17. Drainage Considerations

18. Roadway Component – Travel Lane
Lane Width Considerations
What is the impact of weather on cross slope design?
Safety: Allow steering adjustment and lateral clearance
Pavement edge crumbling (deterioration) less with wide lane
Cost/Benefit (depends on traffic)
Bicycle Use

19. Lane Width Limited by physical dimension of vehicles 12 ft desirable
11 ft acceptable in urban areas with restrictions
10 ft okay for low speed/urban roadways
9 ft – okay low volume rural and residential roadways
14 ft – shared outside lane with bike
TWLTL 10 – 16 ft (3.0 – 4.8m)
Auxiliary 10 ft. (3.0 m) or more

20. Shoulders Functions: Lateral Support Avoidance Space Emergency Stop
Ped/Bike Use
Turning/Passing at Intersections
Mail Delivery, Buses, etc.
Flexibility in Highway Design - Chapter 6

21. Shoulders Should be flush with roadway surface
Sloped to drain away from traveled way
Should be stable enough to support vehicles in all kinds of weather w/out rutting
Should be visibly different from traveled way

22. Shoulder geometry Paved/Unpaved (earth, turf, gravel)
Graded and usable width depends on foreslope and rounding
Consider function, safety, and capacity impacts
Slope
2 to 6% (paved)
4 to 6% (gravel)
8% (turf)
Min. 2% + lane slope
Max crossover 8%]
Width 0.6 – 3.6 m (2 – 12 ft)

23. Source: A Policy on Geometric Design of Highways and Streets (The Green Book). Washington, DC. American Association of State Highway and Transportation Officials, th Ed.

24. Curbs Control access Control drainage
Type used varies with location and design speed

25. Curbs

26. Curbs

27. Iowa’s Roadway-Related Fatal Crashes
52% of Iowa’s fatalities are related to Lane Departure
39% of Iowa’s fatal crashes are single-vehicle Run-Off-the-Road (ROR) crashes

28. Safety Investment Strategy
Candidate Safety Projects
Paved shoulders
Milled-in shoulder rumble strips
2-lane shoulder widening
High severity crash intersections
High severity crash 2-lane roads
High crash curves
Expressway intersections
Centerline rumble strips
Cross-median head-on crashes

29. 4 Foot Paved Shoulder
US 63

30. Milled Shoulder Rumble Strips

31. Median Function Separate opposing traffic Drainage Aesthetics
Space for future lanes
Recovery
Access control
Minimize headlight glare
Flexibility in Highway Design - Chapter 6

32. Median Types/Geometry
Depressed (rural arterials, 6:1 preferred, 4:1 min)
Raised (urban arterials)
Flush (urban/sub. some rural)
Double yellow to limit access
TWLTL
Width is determined by
Function
Safety
Need for independent design
Flexibility in Highway Design - Chapter 6

33. Iowa DOT Urban Design Aids
See New Construction

34. Iowa DOT Urban 3R* Guidelines
See
*Resurfacing, Restoration, Rehabilitation

35. Rural Design - New Construction