Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
Lalani (1991)
The City of San Buenaventura, California as part of a Comprehensive Safety Program
introduced 2WLTLs to five areas of the city (Lalani, 1991). Sites were selected because
they were considered high crash locations. The analysis was a naïve before-after analysis
using crash frequency and one-year before and after periods. Details of the treatment are
not reported (i.e., what was the original cross-section? And was reconstruction
required?). The results of the analysis are shown in Table 9.12.
TABLE 9.12: Safety Impacts of 2WLTLs in San Buenaventura, California
Location
Crashes
Before After
CMF
A 18 9 0.50
B 9 5 0.56
C 8 5 0.63
D 3 1 0.33
E 25 11 0.31
Totals 73 31 0.42
Lalani does not account for exposure in the safety analysis but reports that traffic
volumes in the city increase at an average rate of 6% per annum. Furthermore, there is
no information provided on access density.
Bonneson and McCoy (1997)
Bonneson and McCoy (1997) developed crash prediction models to assess the safety
impacts of median treatment on urban and suburban arterial roads in Omaha, Nebraska
and Phoenix, Arizona. Roads included in the dataset shared the following characteristics:
• Annual traffic volume in excess of 7000;
• Speed limit between 30 and 50 mph;
• 350 foot or more signalized intersection spacing;
• Direct access from abutting properties; and
• No more than 3 lanes in each direction
Three years of crash data were used in the analysis. Crashes excluded from the database
included signalized intersection-related crashes, and crashes resulting from extraordinary
circumstances (i.e., driving under the influence, and crashes on snow covered streets).
Overall 126.5 kilometres of street and 6,391 crashes were included in the analysis.
The equations were developed using generalized linear regression assuming a negative
binomial distribution. They are as follows:
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