Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
where: k = 0.766
ADT m = 8,000
ADT s = 4,000
Crash record = 12 crashes in the last 5 years (2.4 crashes/year)
If it has been proposed that the intersection be signalized, the safety impacts of
signalization are determined as follows:
Unsignalized: SPF = 0.0044 (8000) 0.64 (4000) 0.17 = 5.7 crashes/year
w = 0.766/(0.766+(5 x 5.7) = 0.026
Crashes/year = (0.026 * 5.7) + (1-.026)*12/5 = 2.5 crashes/year
If the SPF for a signalized intersection is:
SPF = 0.044 ADT m 0.34 ADT s
0.16
[D6]
where: k = 0.812
Then the expected crash frequency under signalization is:
SPF = 0.044 (8000) 0.34 (4000) 0.16 = 3.5 crashes/year
As there is no crash record associated with this intersection being signalized, the output
from the SPF is the best estimate of the crash record for this intersection.
As a result of the above analysis, it can be expected that signalizing this particular
intersection would result in a 40% increase in total crashes (2.5 crashes/year while
unsignalized, and 3.5 crashes/year expected under signal control).
Integrating Crash Severity
In the above example, the increase in crash frequency may be associated with a change in
the type of crashes, and perhaps the crash severity. If separate SPFs have not been
developed for different crash severities, then it is necessary for the analyst to determine
the distribution of crash severity through other means.
The typical procedure would be to determine the average distribution of crash severities
for the facilities under examination. Again using the above example, it is determined that
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