Synthesis of Safety for Traffic Operations - Transports Canada

Traffic Signs
determined through a comparison of equivalent property damage only crashes, using
monetary conversations deemed appropriate by the investigation team. Three years of
before and three years of after crash data were used in the analysis.
The results of the analysis are presented in Table 4.2.
TABLE 4.2: Safety Impacts of Traffic Signing in South Dakota
No. Crashes
EPDO Crashes*
Improvement
of
Type
Before After CMF Before After CMF
Sites
Traffic
6 1403 1330 0.95 48860 41588 0.85
Signing
* EPDO crashes were calculated as (1300*F)+(90*I)+(18*N)+(9.5*P)+PDO
where: F = fatal crash
I = incapacitating injury crash
N = non-incapacitating injury crash
P = possible injury crash
PDO = Property damage only crash
The Tople analysis possesses many potentially serious flaws. Most importantly, the sites
were selected for treatment as part of a safety program. This means that the crash record
was likely abnormally high, and there is a great potential for regression to the mean
artefacts. This shortcoming is likely offset somewhat by a failure to account for changes
in exposure. Traffic volumes were not controlled for, but typically volumes tend to
increase which would lead to a higher “after” count of crashes. In the end, the South
Dakota results are based on a limited number of sites and weak analyses.
Transport Research Laboratory (2000)
The TRL (2000) of the United Kingdom has collected information on the safety impacts
of signing through the MOLASSES database (see the section on “Signalization” for more
information on MOLASSES). Specific information on whether “signing” means to
incorporate new signs, improved signing, or some combination on both is not provided.
Therefore, the results are generalized and are only adequate to provide cursory guidance
on the magnitude of the potential for safety improvement. The results are shown in Table
4.3.
TABLE 4.3: Safety Effects of Signing Modifications in the United Kingdom
Number of Number of Crashes
Setting
Locations Before After
CMF
Urban 222 1536 1044 0.68
Rural 136 879 521 0.59
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