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Chapter 4: Roadways

Roadways Overview

• Road standards• Stopping sight distance• Cross section• Roadway surface• Curves• Pavement edge drop-offs

Appropriate Standards

• AASHTO “Green Book”• AASHTO low-volume roads book• Adopt your own

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Consistency

Consistency 2

Sight Distance

• Stopping sight distance • Decision sight distance• Intersection sight distance

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Stopping Sight Distance

Reaction distance+ Braking distance

Stopping sight distance

Stopping Sight Distance

Driver eye height = 42 inObject height = 24 in

Stopping Sight Distance on Horizontal Curves

Roadcenterline

Centerline of near lane

S

Line of sight

Sight obstruction

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Cross Section Elements

• Cross slopes• Lane width• Shoulders and curbs• Roadside slopes

ShoulderWidth

ShoulderWidthTraveled Way Width

Roadside Slope

Curb

S S

S is the pavement cross-slope

Lane Widths

• Higher functional class roads wider lanes– 9 ft for low volume low speed– 12 ft for arterials and freeways

• Add 1 to 2 ft next to curb

Safety Effects of Lane Width

Average Daily Traffic (veh/day)

Acc

iden

t Mod

ific a

tion

Fact

or

1.40

5001.00

1.10

1.20

1.30

1,000

1.50

1.60

1.70

1.30 10-ft lanes

1.00 12-ft lanes

1.05 11-ft lanes

2,5001,500 2,000

1.50 9-ft lanes

1.011.021.05

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Shoulders

• Crash reduction– High-volume roads– Bicyclists and

pedestrians• Support pavement• Improve drainage

Safety Effects of Shoulders

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

500 1,000 1,500 2,000 2,500

1.50 0-ft shoulders

1.30 2-ft shoulders

1.15 4-ft shoulders

1.00 6-ft shoulders

0.87 8-ft shoulders

1.10

1.07

1.02

0.98

Acc

iden

t Mod

ifica

tion

Fact

or

Average Daily Traffic (veh/day)

Edge Drop-Offs

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Edge Drop-Offs 2

Pavement Edge Drop-Off Video

Road Surface

• Wet weather crashes increased by– Poor drainage– Slippery pavement

• Rough pavement decreases tire/road contact• Pavement management

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Slippery pavement

Slippery Pavement 2

Slippery Pavement 3

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Curves

• Common curve crash types– Run-off-road– Head-on– Opposite direction sideswipe

• Lane-following problems

Curve Safety Example

Curve Signing

• Curve warning signs• Chevrons or arrows• Delineators• Pavement markings

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Curve Signing Example

Safety Widening

• Widen lanes on sharp curves• Cheaper than widening whole road

Curve Widening Example

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Curve Widening Example 2

Roadways Summary

• Road standards• Cross section• Surface• Curves• Pavement edge drop-offs