Cross Sections

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Cross Sections

• CE 453 Lecture 22

Iowa DOT Design Manual Chapter 3
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See also Iowa DOT Standard Plans
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Objectives

• 1.     Identify cross section components and
  design criteria
•  
• See http//www.fhwa.dot.gov/environment/flex/ch0
  6.htm (Chapter 6 from FHWAs Flexibility in
  Highway Design)

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• Cross Section Elements
• Roadway
• Median
• Roadside

• Roadway Components
• Travel Lanes
• Shoulders

Sourcehttp//www.fhwa.dot.gov/environment/flex/ch
06.htm
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• Cross Section Elements
• Roadway
• Median
• Border

• Roadway Components
• Travel Lanes
• Auxiliary Lanes

Sourcehttp//www.fhwa.dot.gov/environment/flex/ch
06.htm
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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
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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)

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Travel Lanes

• Function guidance to drivers and vehicle
  support
• Pavement types
• high (modern standards),
• intermediate (surface treatments), and
• low (unpaved)

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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

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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)

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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

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Urban Cross-Sections
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Urban Cross-Sections
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HMA Cross-Sections
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Drainage Considerations
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Drainage Considerations
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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

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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

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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
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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

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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)

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Source A Policy on Geometric Design of Highways
and Streets (The Green Book). Washington, DC.
American Association of State Highway and
Transportation Officials, 2001 4th Ed.
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Curbs

• Control access
• Control drainage
• Type used varies with location and design speed

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Curbs
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Curbs
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Iowas Roadway-Related Fatal Crashes

• 52 of Iowas fatalities are related to Lane
  Departure
• 39 of Iowas fatal crashes are
  single-vehicleRun-Off-the-Road (ROR) crashes

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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

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4 Foot Paved Shoulder
US 63
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Milled Shoulder Rumble Strips
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Median Function

• Separate opposing traffic
• Drainage
• Aesthetics
• Space for future lanes
• Recovery
• Access control
• Minimize headlight glare

Flexibility in Highway Design - Chapter 6
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Median Types/Geometry

• Depressed (rural arterials, 61 preferred, 41
  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
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Iowa DOT Urban Design Aids
See http//www.dot.state.ia.us/local_systems
New Construction
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Iowa DOT Urban 3R Guidelines
See http//www.dot.state.ia.us/local_systems
Resurfacing, Restoration, Rehabilitation
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Rural Design - New Construction