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Evaluation of Lane Reduction

“Road Diet” Measures on Crashes and Injuries

Herman F. Huang and Charles V. Zegeer

University of North Carolina Highway Safety Research Center

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What Is a Road Diet?

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Arguello Blvd., San Francisco

After

Before

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Road Diet Benefits

Drivers’ speeds limited by lead vehicle

May reduce left turn delays

Pedestrians have fewer lanes of traffic to cross

Sidewalks and bike lanes can be added

Turn pockets and refuge islands can be added

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Research Design

Four-group design:

Road diet - “before”

Road diet - “after”

Comparison site - “before”

Comparison site - “after”

Obtained crash data from cities

3 yrs of “before” - 3 yrs of “after”

Objective

To investigate the effects of road diets on motor vehicle crashes and injuries.

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Road Diet

Comparison Site

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Analysis

Initially had 30 road diets and 50 comparison sites

Many locations had short lengths, small sample sizes of crashes, low ADTs

Locations used in analysis:

12 road d iets (2,068 crashes)

25 comparison sites (8,556 crashes)

These were assigned into 11 groups

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Study Sites and Crashes Used in the Analyses

NUMBER OF SITES NUMBER OF CRASHES CITY

Road Diet Comparison* Road Diet Comparison*

Bellevue, WA

1

2

134

307

Mountain View, CA

1

2

20

134

Oakland, CA

2

5

443

2,067

San Fran-cisco, CA

2

5

450

1,339

Seattle, WA

5

9

969

4,485

Sunnyvale, CA

1

2

52

224

TOTAL

12

25

2,068

8,556

NOTE: Each road diet had one or more matching comparison sites.

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Analysis Categories

Crash Trends to Validate Comparison Sites

“Before” and “After” Crash Frequencies

B/A Crash Rates

B/A Crash Severity

B/A Crash Type

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Crashes per Month, Oakland

4.42

5.58

6.837.83

16.25 16.0816.67

13.25

8.83

10.3310.25

14.42

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

1993 1994 1995 1996

Year

Cra

sh

es

Pe

r M

on

th

2 Road Diets

4 Nearby Comparisons

2 Faraway Comparisons

Analysis of the trends in Oakland and other cities showed that the comparison sites were a good match to the road diets.

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“Before” and “After” Crash Frequencies

11 road diets and 24 comparison sites

Road diets had about 6% fewer crashes in the “after” period than the comparison sites

Overall increase in ADT - any traffic diversion is limited

ADTs increased by 6.4% on road diets and 8.0% on comparison sites

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Crash Rates(per million vehicle miles)

8 road diets and 14 comparison sites

Crash rates were lower at road diets in both periods.

Regression analysis indicated no differences in road diet and comparison site rate changes in B/A..

Before After Before After Road Diets Comparison Sites

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Crash Severity

10 road diets, 20 comparison sites

About 63% of the crashes were PDO and 37% had at least one injury or fatality

Crash severity was virtually the same at road diets and comparison sites, and did not change from the “before” to the “after” period

Injury % varied somewhat from city to city - different crash reporting practices

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Crash Types

45.3% 45.4%41.2% 42.0%

17.4%24.7%

21.6%26.6%

15.4%

14.1%

15.1%

14.2%

21.9%15.8%

22.2%17.2%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Road Diet - Before(N=1,321)

Road Diet - After(N=722)

Comparison - Before(N=5,010)

Comparison - After(N=3,469)

Site Type

Pe

rce

nt

of

All

Cra

sh

es

Other

Sideswipe

Rear End

Angle

12 road diets and 25 comparison sites

The changes between the “before” and “after” periods were not significant.

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Conclusions

Trend analyses indicated comparison sites were valid

Road diets had 6% fewer crashes in the “after” period than the comparison sites

While crash rates were lower at road diets sites in both periods, relative change in B/A was not significant

Crash severity was the same at road diets and comparison sites and did not change in B/A

Traffic operations and capacity must be considered at each site