Crash Reduction Factors
Transportation planners and engineers across the country utilize Accident Modification Factors (AMFs) to estimate how potential engineering improvements could affect reductions in crashes. HSRC researchers have joined other highway safety experts as part of National Cooperative Highway Research Project 17-25 to examine these AMFs and their effectiveness. HSRC Director David Harkey served as the principal investigator along with research assistance from Dr. Raghavan Srinivasan and Charles Zegeer of HSRC.
The first phase of the study assessed 100 different AMFs within the following categories; intersection treatments, including treatments such as installing turning lanes or traffic signals; roadway segment treatments, such as changing the width of a median or adding rumble strips; and miscellaneous treatments, for example, truck rollover warning systems or weather detection and warning systems.
|Type of Treatment||Treatment||Level of Predictive Certainty|
|Intersection Treatments||Install a roundabout||High|
|Add exclusive left-turn lane||High|
|Add exclusive right-turn lane||High|
|Install a traffic signal||High|
|Remove a traffic signal||High|
|Modify signal change interval||Medium-High|
|Convert to all-way stop control||Medium-High|
|Convert stop-control to yield-control||Medium-High|
|Install red-light cameras||High|
|Roadway Segment Treatments||Narrow lane widths to add lanes||Medium-High|
|Add passing lanes (two-lane roads)||Medium-High|
|Add two-way left-turn lane (TWLTL)||Medium-High|
|Increase lane width||Medium-High|
|Change shoulder width and/or type||Medium-High|
|Flatten horizontal curve||Medium-High|
|Improve curve superelevation||Medium-High|
|Add shoulder rumble strip||Medium-High|
|Add centerline rumble strips||Medium-High|
|Miscellaneous Treatments||Install raised medians at crosswalks||Medium-High|
According to an interim report published in November 2005, of the 100 treatments reviewed, 23 were found to have credible AMFs available. For an AMF to be deemed “credible,” researchers required that the estimate have a high or mid-high level of predictive certainty. A critical factor in determining this credibility was the scientific rigor of the methodology used in the study in which the AMF was developed. For more information, please view the NCHRP Research Results Digest 299.
The research on AMFs will continue,
as another goal of the study is to develop additional recommended
AMFs to fill the knowledge gaps identified to date. Work is underway to
develop AMFs for the installation of signals at rural intersections, skid
reduction treatments at intersections and 4-lane to 3-lane conversions (i.e.,
Further research will also take place in Winston-Salem, N.C., to develop AMFs within an urban setting for both signalized and unsignalized intersections.