Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
Vogt (1999)
Vogt (1999) in developing crash models for rural intersections examined the safety
impacts of protected left-turn phasing for the major road of signalized intersections with
four approaches. Forty-nine signalized intersections were included in the analysis.
Negative binomial regression analysis was used to model all crashes within 250 feet of
the intersection on the main road, and with 100 and 250 feet of the intersection on the
side road, in California and Michigan, respectively. Three years of crash data were used
in the analysis.
Vogt found that protected left-turn phasing for the major road at a rural, 4-legged,
signalized intersection yields a CMF of 0.51.
Bauer and Harwood (2000)
Bauer and Harwood (2000) using three years of crash data from California developed
crash prediction models for several types of rural and urban intersections. Model
development used statistical sound procedures. With respect to signalized intersections,
only urban intersections with four approaches were modelled. It was found that signal
timing produced the CMFs shown in Table 3.53.
TABLE 3.53: CMFs for Signal Timing Changes
Treatment
CMF
Pretimed to Semiactuated 0.94
Pretimed to Fully actuated 1.75
Two-phase to multi-phase 0.82
Thomas and Smith (2001)
Thomas and Smith (2001) undertook an examination of the safety impacts of left turn
phasing at four intersections in Iowa. The site selection process is not described; the
study methodology is a naïve before-after analysis using crash frequency and severity.
The crash frequency is comprised of three years of before and three years of after data
categorized by severity, and several impact types. The exact change in signal phasing is
not described. The results are shown in Table 3.54.
The results indicate that under a 90% degree of confidence, safety benefits can be
expected for total crashes (CMF = 0.64). The aetiology suggests that the majority of the
safety gains would be a reduction in left-turn crashes. Exposure was not accounted for in
the analysis, as this data was not readily available.
Page 57