Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
increasing an average of 11%. The researchers re-examined the crash frequency 13
years after stop sign installation and found the crash frequency had increased to 3.3
crashes/intersection/year (CMF = 0.15). The traffic volumes at this time are not reported.
Laplante and Kropidlowski also were concerned about crash migration to the streets
surrounding the treated neighbourhood. An examination of the crash frequency at the 10
stop controlled intersections on the four peripheral streets (arterials or collectors)
decreased from 27.9 crashes/intersection/year to 19.7 crashes/intersection/year. Again
traffic volume changes are not reported. However, the analysts make note that the total
number of crashes in Chicago increased during the study period.
The results of the Laplante and Kropidlowski study are questionable based on a likely
regression-to-the-mean bias. Site selection is not detailed in the documentation.
However, a crash frequency of 21.3 crashes/year at a low-volume intersection is certainly
considered abnormally high. It would appear that these intersections were selected based
on their high crash frequency. Moreover, the sample size (nine intersections) is very
small and a lack of statistical analysis does not provide the reader with any information
on the variability of the results.
Intersection Control Beacons
Pant et al (1999)
Pant et al (1999) undertook a cross-section, and before-after study of six stop-controlled
intersections and seven stop-controlled intersections that were supplemented with
beacons. The sites were rural intersections located in Ohio. The study sites were
selected because of the availability of complete traffic and crash data for four years but
were matched on the following geometrics:
• The angle of intersection was approximately 90 degrees;
• All legs had a single approach lane;
• All intersections were located in a rural area and had no substantial development
around the intersection; and
• The posted speed limit was 55 mph in the major direction.
The researchers speculate that the intersections that are supplemented with beacons were
done so because of an abnormal crash record.
The results of the cross-section study are shown in Table 3.26.
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