Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
Region of Waterloo (2001)
As part of an ongoing program the Region of Waterloo, Ontario routinely assesses the
street system for locations with an elevated risk for motor vehicle crashes, and
implements appropriate countermeasures. The Region of Waterloo (2001) reports that in
1998 three locations were changed from stop control to signal control with results as
shown in Table 3.19.
TABLE 3.19: Safety Effects of Signalization in Waterloo, Ontario
Location
Crash Frequency
Before After
CMF
A 7 5 0.71
B 8 3 0.38
C 9 4 0.44
Average 8 4 0.50
The Waterloo analysis is a naïve before-after study of crash frequency using one-year of
before and one year of after data. Furthermore, the traffic signals were installed, at least
in part, because these locations had an aberrant crash record. The results are very
unreliable due to a failure to account regression-to-the-mean, the limited sample size, and
the failure to account for exposure.
Pernia et al (2002)
Florida’s investigation into the safety impacts of signalization included developing crash
prediction models for 447 intersections that were signalized between 1990 and 1997
(Pernia et al, 2002). The models included crashes that occurred within three years of
signalization (i.e., three years before, and three years after), and captured all crashes
within 76 metres (250 feet) from the point of intersection along the major road. It is
recognized that regression-to-the-mean bias may be present in the dataset; it was not
addressed in the analysis.
The form of equation used in the analysis was as follows:
N = exp(a + C 1 X 1 + C 2 X 2 + … + C n X n ) [3.15]
Where:
N = Annual number of crashes
a = constant as shown in Table 3.16
X n = Independent variables as shown in Table 3.16
C n = (“value” multiplied by “coefficient”) from Table 3.16
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