Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
There is a great potential for the results of this research to mislead the practitioner. First
and foremost, the study methodology is a naïve before-after study and therefore does not
account for confounders or regression to the mean effects (although the site selection was
not undertaken on the basis of a high incidence of collisions, therefore regression to the
mean effects are likely not as troublesome).
A speed limit reduction is indicated on the roadway by a change in speed limit signing.
While this is certainly a change to the “road” part of the road-driver-vehicle system, it is
unlikely that the revised signing by itself has any impact on safety. For the speed limit
reduction to have any effect on safety, the signs must evoke a change in driver behaviour.
The impetus for the modified behaviour is of no real concern in this discussion, but it
suffices to say that the threat of being a larger fine may be the causal chain. At any rate,
for the new speed limit to have an impact on safety, the new speed limit must also have
an impact on driver behaviour. The conventional wisdom in this respect is that the
average speed of the traffic stream is not affected, but the variation in speeds among the
travel stream is affected. As the speed limit is moved closer to the 85 th percentile speed,
the speed variance decreases, and as the speed limit is moved away from the 85 th
percentile speed, the speed variance increases.
Belanger (1994)
Belanger (1994) studied the safety of unsignalized intersections with four approaches
including an examination of the impacts of the main road speed limit on intersection
safety. Belanger examined the safety of 149 intersections in eastern Quebec with average
annual daily traffic volumes ranging from 388 to 15,942. Crashes that occurred within 30
metres of the intersection, or were recorded as intersection-related were included in the
analysis. Regression-to-the-mean effects were accounted for through the application of a
multivariate Empirical Bayes technique.
Belanger developed simple SPFs for unsignalized, cross intersections for main road speed
limits of 50 km/h and 90 km/h. The SPF is shown in Equation 8.1, and the SPF
parameters are as shown in Table 8.4.
N = a AADT 1 b AADT 2
c
[8.1]
where:
N = Crashes/year
AADT = Average annual daily traffic
a, b, c = model parameters (see Table 8.4)
TABLE 8.4: Safety Impacts of Speed Limits at Intersections
Speed Limit
(km/h)
a b c k
50 0.003906 0.34 0.49 3.10
90 0.001230 0.41 0.59 5.10
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