Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
2WLTL
Hoffman (1974)
Hoffman (1974) examined the safety effects of constructing 2WLTLs on four four-lane
arterial streets in Michigan. All of the streets had strip commercial development. The
2WLTL were provided with signs and pavement markings as prescribed by the standards
of the day. Traffic volumes on the four streets ranged from 15,000 to 30,000 ADT. A
total of 6.58 miles of 2WLTL were evaluated. The study methodology was a naïve
before-after study using crash frequency as the measure of effectiveness. The results are
shown in Table 9.9. Only one year of before and one year of after crash data were used
in the analysis.
TABLE 9.9: The Safety Impacts of 2WLTL in Michigan
Crash Type No. Crashes
Before After
CMF
Head-on 94 52 0.55
Rear-end 238 90 0.38
Right-angle 92 105 1.14
Sideswipe 42 39 0.93
Other 66 70 1.06
Total 532 356 0.67
The analysis does not account for exposure. Hoffman does mitigate this weakness by
stating that the traffic volumes in the after period were about seven percent higher than
the before period, suggesting that the safety benefit may be even greater. However, it is
not clear why these sections of road were selected for treatment, and there may be a
significant regression-to-the-mean bias. Furthermore, without a control group it is
difficult to apportion to crash reduction to the 2WLTL installation.
Main (1984)
Main (1984) examined the effects of 2WLTLs on arterial streets in the City of Hamilton,
Ontario. The 2WLTLs were five metres wide, marked and signed in accordance with the
then current version of the Transportation Association of Canada’s Manual of Uniform
Traffic Control Devices. All 2WLTLs were constructed on four lane roads, resulting in a
five-lane cross-section. At the time only four sections of 2WLTL were available for
analysis. The study methodology was a naïve before-after study using crash rate as the
metric. Three years of before data were compared to one and two year after periods. The
results are shown in Table 9.10.
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