Paper Details

TRB 91st Annual Meeting (January 22-26, 2012)

Event Number:	648
Event Title:	Highway Safety Performance
Event Date:	Jan 24 2012 7:30PM- 9:30PM
Event Location:	Marriott, Salon 2
Event Description:
Event Agenda:

Crash Modification Factors for Changes to Left-Turn Phasing (12-2521)

This study estimated crash modification factors (CMFs) from before-after evaluations of two treatments targeted at left turn crashes at signalized intersections: (1) changing from permissive to protected-permissive phasing, and (2) implementation of flashing yellow arrow (FYA) for permissive left turns. Results of the first evaluation, which was based on 59 intersections from Toronto and 12 from North Carolina, indicated a substantial reduction in left turn opposing through crashes, especially at intersections where more than one leg was treated, and a small percentage increase in rear end crashes. For the second evaluation (FYA), which was based on data from 51 signalized intersections from Oregon, Washington, and North Carolina, results indicated a safety benefit at locations with some kind of permissive left turn operation before, and a disbenefit where there was protected only operation before. A key aspect of the study was the estimation of the standard deviation of the distribution of the CMF in addition to the conventionally estimated standard error of the mean CMF value. For several CMFs, the standard deviation of the distribution was larger than the standard error of the mean value of the CMF, indicating substantial variation in the treatment effect across sites. This indicates the need for further research into the development of crash modification functions instead of crash modification factors, and for the use of large treatment databases to undertake more extensive disaggregate analysis of safety effects. Equally important, it emphasizes the importance of providing more explicit consideration of CMF variability in future editions of the Highway Safety Manual.

Authors
     Srinivasan, Raghavan , University of North Carolina, Chapel Hill
     Lyon, Craig , Persaud and Lyon Inc., Canada
     Persaud, Bhagwant , Ryerson University, Canada
     Baek, Jongdae , University of North Carolina, Chapel Hill
     Gross, Frank , Vanasse Hangen Brustlin, Inc.
     Smith, Sarah , University of North Carolina, Chapel Hill
     Sundstrom, Carl Andrew, University of North Carolina, Chapel Hill