Synthesis of Safety for Traffic Operations - Transports Canada

Intersection Control
TABLE 3.32: Night-time Crash Frequency at Locations with Modified Signal
Heads in BC
Crash type Before
After
Year 1 Year 2 Year 3
Angle 1 3 0 1
Left turn 3 1 1 0
Right turn 1 1 0 0
Rear end 7 2 3 0
Overtaking 0 1 0 0
Off road 0 1 1 1
Unknown 2 5 0 1
Total 14 14 5 3
Traffic volumes were not used in the analysis, but showed an average increase of 2% per
year. The researchers note that the sample size is small, and the analysis methodology is
suspect. Nonetheless, there are apparently overall benefits, particularly in reducing rearend
crashes.
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 two locations were provided with new signal heads and revised signal timings to
improve safety. The results as shown in Table 3.33.
TABLE 3.33: Safety Effects of New Signal Heads in Waterloo, Ontario
Location
Crash Frequency
Before After
CMF
A* 24 12 0.50
B 34 13 0.38
Average 29 12.5 0.43
* - Site A also had a right-turn lane added to one approach
The Waterloo analysis is a naïve before-after study of crash frequency using one-year of
before and one year of after data. It is impossible to separate out the effects that may be
attributed to the new signal heads, from the effects that may be attributed to the revised
signal timing. Furthermore, the changes were made, 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.
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