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.

Treatments with AMFs that have a Level of Predictive Certainty of High or Medium-High
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
Install/upgrade guardrail 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., road diets).
Further research will also take place in Winston-Salem, N.C., to develop AMFs within an urban setting for both signalized and unsignalized intersections.