Synthesis of Safety for Traffic Operations - Transports Canada

Forbes and Gill (2000) undertook a review of arterial traffic calming on a road in
Ancaster, Ontario. The road was a two-lane arterial with mainly residential development
and a speed limit of 50 km/h. The treatment was to construct a series of raised islands
between the two lanes of travel, created a narrowing of the pavement, and chicane effect
along the road. The islands were supplemented with trees to further enhance the
narrowing effect.
The study methodology included an examination of mean speed, and the proportion of
traffic exceeding the speed limit using a before-after analysis with a comparison group.
The mean speed dropped 9% in the traffic-calmed section (from 54.0 to 49.3 km/h),
while the mean speed in the control section dropped only 3%. The effects of the islands
on speeding are presented in Table 10.8.
TABLE 10.8: Speed Effects of Traffic Calming in Ancaster, Ontario
Proportion Exceeding the
Section
Speed Limit (%)
Before After
Traffic-calmed 67 47
Control 88 85
Statistical testing indicated that both the change in mean speed, and the proportion of
vehicles exceeding the speed limit are significant at a 99% level of confidence. It might
be questioned, however, if the control section was a suitable comparator since the
“before” proportion exceeding the speed limit is considerably different from the test
section.
Transport Research Laboratory (2000)
The TRL (2000) of the United Kingdom has collected information on the safety impacts
of various traffic calming measures on local streets through the MOLASSES database
(see the section on “Signalization” for more information on MOLASSES). 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 10.9.
The results from the United Kingdom indicate that traffic-calming measures are effective
safety devices. However, caution should be exercised in using the associated CMFs,
since:
• There is no information on why the sites were selected for treatment;
• In some instances the sample size is very small;
• The study methodology is a naïve before-after study of crash frequency and does
not account for regression-to-the-mean; and
• The study methodology does not account for exposure or other possible
confounding factors.