Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
from the model indicates that the speed limit had no predictive value in determining crash
frequency. In other words, it does not appear as if the speed limit had any impact on
crash frequency.
Haselton (2001)
In a recent study by Haselton (2001), the impacts of speed limits on highways in
California were evaluated. Three groups of roads were studied, as shown in Table 8.6.
Speed Limit
Change
TABLE 8.6: Safety Effects of Speed Limits in California
Years of
Total
No. of
No. of
Crash
Length
Sites
Crashes
Data
(miles)
Road types
65 mph to 70
27 10 270 1315 Rural freeways
mph
55 mph to 65
Urban and rural
112 10 1120 1674
mph
freeways
Retain 55 mph 19 9 171 100 Urban freeways
The crash data used in the Haselton study included crashes that occurred on interchange
ramps, but not the crashes that occurred at ramp-arterial intersections. The metrics
included total crash rate, wet crash rate, dark crash rate, and fatal+injury crash rate.
Several study methodologies were employed, the most reliable one is the before-after
analysis with a comparison group. The generalized results are reported as follows:
• statistically significant increases in all and fatal crashes coincided with the
increase in speed limits from 65 to 70 mph, and 55 to 65 mph;
• the increased speed limit had no effect on crashes fatal+injury crashes and crashes
during wet road conditions; and
• the speed limit increase from 55 to 65 mph produced a statistically significant
increase in dark crashes, while the speed limit from 65 to 70 mph did not.
Wilson et al (2001)
Wilson et al (2001) in a study respecting safety and speed limits on New Brunswick
roadways produced the results shown in Table 8.7 and 8.8. The study used a crosssectional
analysis of 4,459 crashes from 1997 to 1999, inclusive. The results indicate that
a lowering of the speed limit from 100 km/h to 90 km/h would be associated with CMFs
of 0.71, 0.75, and 0.44 for property damage only, injury, and fatal crashes, respectively.
The distribution of crash severities has no appreciable change.
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