Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
Metrics
It is generally accepted that the appropriate metric for road safety is motor vehicle crash
(MVC) occurrence and severity. MVCs are system failures that are caused by one or
more of the three elements of the highway system: the environment (the road and its
ancillary devices), the road user, and the vehicle.
Because MVCs are rare and random events using them as the sole measure of
effectiveness is limiting. Many evaluation studies, particularly research into the safety of
vulnerable road users where crashes are very infrequent, use crash surrogates to
determine effectiveness. The most common surrogates are traffic conflicts, violations,
road user behaviour, and speed. The use of surrogates is a reasonable action.
The body of knowledge that is formed by research into the impacts of traffic operations
and control strategies on crash surrogates is formidable and important. Therefore, the
original intent of including only research that used crash frequency and severity as the
primary endpoints was abandoned in favour of including research that measured the
impacts on crash surrogates.
However, in order to be true to the goal of promoting evidence-based road safety,
research that used crash surrogates is included only if there is a reasonable expectation
that the crash surrogate is correlated with crash occurrence or severity.
For instance, operating speed is often used as a measure of the “safety” of a situation.
Measures that reduce actual travel speed are seen to be improving system safety. Since
the collected wisdom on speed is that lowering speeds reduces crash severity, evaluations
that measure the impacts of strategies on speed are included in this Synthesis.
Alternatively, crash surrogates such as “stop sign violations” particularly on local streets,
to the best of our knowledge, have no definitive correlation with crashes and are not
included in this Synthesis.
Chapter 2 includes a description of crash surrogates and provides references to the
research that demonstrates their correlation with crashes.
With respect to terminology, the terms “safety impact” and “safety effect” are more
appropriate than “safety benefits”, as a modification may not always bring about the
anticipated improvement in safety. The term “crash countermeasure” is also avoided, as
traffic operations and control strategies are not always safety-driven. For instance, traffic
signals are most often introduced to better control right-of-way between conflicting
traffic flows, and to reduce delay. Traffic signals are not typically implemented as a
crash countermeasure. Nonetheless, the decision to implement signals will certainly have
an impact on the safety of the intersection.
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