Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
CHAPTER 9: TURN LANES
Left-turn Lanes
Main (1984)
Main (1984) examined the impacts of adding exclusive left-turn lanes and raised medians
at signalized intersections on arterial streets in the City of Hamilton, Ontario. The typical
installation consists of a 3.5 metre wide left-turn lane, and 1.5 metre wide raised median
that is 30 metres long. The approach taper is 20:1 and pavement markings consist of
three left-turn arrows.
Eight locations were selected for analysis. These sites are considered by the author to be
representative of the population. Three years of before, and three years of after crash
data are used in the analysis. The study is a naïve before-after analysis using crash rates.
The results are shown in Table 9.1.
TABLE 9.1: Safety Impacts of Left-turn lanes at Signalized Intersections
Section
Before
After
# AWDT Rate # AWDT Rate
CMF
1 80 29,870 2.55 47 35,130 1.28 0.50
2 71 28,960 2.34 12* 28,960 0.59 0.25
3 66 25,820 2.44 19 29,020 0.62 0.25
4 42 23,160 1.73 31 27,960 1.06 0.61
5 54 25,160 2.05 42 28,880 1.39 0.68
6 56 26,440 2.02 20 28,500 0.67 0.33
7 51 30,670 1.59 45 40,000 1.07 0.67
8 47 23,270 1.93 16* 25,840 0.59 0.31
Average 0.45
* - Only two years of data are used.
Greiwe (1986)
The City of Indianapolis, Indiana evaluated the safety impacts of re-striping intersection
approaches with four lanes (two in each direction), to include opposing left-turn lanes
(Greiwe, 1986). No indication is given as to the site selection process. The study
methodology included a naïve before-after study of crash frequency with two-years of
before, and a minimum of one-year of after data. Target crashes were left-turn, rightangle,
and rear-end crashes. It is noted by the author that traffic volumes remained nearly
the same throughout the study period, so adjustments for exposure were not required.
The results of the analysis are shown in Table 9.2.
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