Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
Hadi et al (1995)
Hadi et al (1995) in developing crash prediction models for nine types of roadways in
Florida, included the speed limit as a potential independent variable. Speed limit was
included because of the availability of the data, and the expectation that it would correlate
with crash occurrence. Four years of crash data were used in the analysis. Crash
prediction models were developed using negative-binomial regression. The equations
were disaggregated by crash severity, the resulting CMFs for changes in the speed limit
are shown in Table 8.5.
TABLE 8.5: CMFs for Changes in the Speed Limit in Florida
Setting Class No. Lanes Median
CMF for a 1 mph increase in speed
Total Injury Fatal
2 U 0.97 0.98 1.00
Highway
Rural
4 D 1.00 0.98 1.00
Freeway 4 and 6 D 1.00 1.00 1.00
2 U 0.97 0.98 1.00
4 U 0.95 0.96 1.00
Highway
4 D 1.00 1.00 1.00
Urban
6 D 1.00 0.97 1.00
Freeway
4 D 0.97 0.98 1.00
6 D 0.97 0.97 1.00
The regression equations that produce these CMFs demonstrate that an increase in the
posted speed limit is associated with either no change or a decrease in the number of
crashes.
Liu and Popoff (1997)
Liu and Popoff (1997) examined the relationship between travel speed and casualty crash
occurrence on Saskatchewan highways. Property damage only collisions were excluded
from the analysis, because of changes in the reporting threshold over time. The authors
produced the following equations relating casualties and casualty rates to speed
measurements.
C = -17126.1 + 190.71 V avg R 2 = 0.81 [8.2]
C = -12473.8 + 133.88 V 85 R 2 = 0.85 [8.3]
where:
C = number of casualties
V avg = average travel speed (km/h)
V 85 = 85 th percentile speed (km/h)
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