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3
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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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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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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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 3R* Guidelines See http://www.dot.state.ia.us/local_systems
*Resurfacing, Restoration, Rehabilitation
Recommended