Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
there appears to be some correlation between conflicts and crashes, and therefore they are
suitable crash surrogates.
In a separate study conducted by Salman and Al-Maita (1995) traffic conflicts and
crashes were correlated at three-leg unsignalized intersection in Jordan. The researchers
used the traffic conflict technique as developed by the Federal Highway Administration
(Parker and Zeeger, 1989). Eighteen sites were studied and were selected because of the
availability of crash data, and because all of the sites had low pedestrian volumes,
approaches that were two-way streets, no visibility restrictions, no turn restrictions, no
parking restrictions, and no appreciable approach grades.
Crash data spanned a three year period, and crashes that occurred during wet weather, at
night, on the weekend, and those that involved a pedestrian were excluded from the
analysis. Traffic conflict studies were conducted between 07:00 hrs and 18:00 hrs on
weekdays during the summer.
A linear regression of crashes and conflicts yielded a statistically significant correlation
as follows:
N = 0.744 + 0.0116X [2.1]
where:
N = Annual number of crashes
X = Mean hourly conflict count
Together the Hamilton Associates and the Salman and Al-Maita provide reasonably
sound evidence that traffic conflicts and collisions are correlated.
Speed
While the link between speed and crash occurrence is somewhat undecided, there is
nonetheless a strong correlation between speed and crash severity [IBI Group, 1997].
While the debate over speed and crash occurrence continues, it is enough to know that
speed is correlated with severity, therefore it will be considered as a suitable crash
surrogate.
While conflicts and speed are determined to be suitable crash surrogates, their direct
application to safety management can be troublesome. The statistical correlation between
the surrogate and the crash occurrence/severity needs to be established for the surrogates
to be used in a meaningful quantitative manner. A linear correlation would permit the
pseudo-CMF for the surrogate to be used directly as the CMF. However, non-linear
relationships would not permit direct application of the surrogate results.
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