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Highway Safety Information System (HSIS)
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
Highway Safety Information System (HSIS)
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
Highway Safety Information System (HSIS)
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.
Highway Safety Information System (HSIS)
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
Highway Safety Information System (HSIS)
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.
Highway Safety Information System (HSIS)
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
Highway Safety Information System (HSIS)
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.
Highway Safety Information System (HSIS)
“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
Highway Safety Information System (HSIS)
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
Highway Safety Information System (HSIS)
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
Highway Safety Information System (HSIS)
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.
Highway Safety Information System (HSIS)
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