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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/ch06.htm (Chapter 6 from FHWA’s Flexibility in Highway Design)

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Cross Section Elements

1. Roadway

2. Median

3. Roadside

Roadway Components

1. Travel Lanes

2. Shoulders

Source:http://www.fhwa.dot.gov/environment/flex/ch06.htm

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Cross Section Elements1. Roadway2. Median3. Border

Roadway Components1. Travel Lanes2. Auxiliary Lanes

Source:http://www.fhwa.dot.gov/environment/flex/ch06.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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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)

Shoulder geometry

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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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Iowa’s Roadway-Related Fatal Crashes

52% of Iowa’s fatalities are related to Lane Departure

39% of Iowa’s 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 Milled Shoulder Rumble Strips

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Median Function

Flexibility in Highway Design - Chapter 6

► Separate Separate opposing trafficopposing traffic

► DrainageDrainage► AestheticsAesthetics► Space for future Space for future

laneslanes► RecoveryRecovery► Access controlAccess control► Minimize Minimize

headlight glareheadlight glare

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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

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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