Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
TABLE 3.36: Safety Impacts of An Additional Primary Signal Head
Crash
Crash Rate (/MVE)
Intersection
Frequency
Fatal &
Type
Total
PDO
(/intersection/yr)
Injury
One Primary
Head
23.1 1.30 0.44 0.86
Two Primary
Heads
19.7 1.02 0.40 0.62
CMF 0.78 0.91 0.72
In the before-after analysis, eight intersections in Richmond, British Columbia that are
outfitted with additional 30/20/20 primary signal heads were studied. One or two year
before and after periods was employed depending on date of installation and availability
of data. The researchers employed Empirical Bayes, and Multi-variate Empirical Bayes
methods to assess the effects of the second primary signal head. The results are shown in
Table 3.37.
TABLE 3.37: CMFs for Additional Primary Signal Heads
CMF
Statistical
Fatal & Injury
Method Total Crashes
PDO Crashes
Crashes
EB 0.78 0.79 0.64
Multi-variate
EB
0.72 0.83 0.69
The results of the cross-section and before-after studies are consistent and indicate a
likely reduction in total crashes of 20 to 30%, and a reduction in casualty crashes of 10 to
20%.
Lens Size
Polanis (1998)
Polanis (1998) reviewed the safety impacts of replacing eight-inch traffic signal lenses
with 12-inch lenses at 38 locations in Winston-Salem, NC. None of the locations
warranted the larger lenses, as determined by the MUTCD; sites were selected on the
basis of a pattern of crashes that could be remedied. The study uses a naïve before-after
study of collision frequency with no accounting for exposure.
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