Accident Modification Factors for Medians on Freeways and Multilane Rural Highways in Texas

Abstract

With the growing demand for safer streets and highways, state and national transportation agencies are investigating the relationships between roadway characteristics and crashes. The objective of this study was to develop accident modification factors (AMFs) for median characteristics on urban and rural freeways and on rural multilane highways. Data available for use in the evaluation included 458 mi of with-barrier segments (primarily urban, with some rural sites), 359 mi of urban without-barrier segments, and 436 mi of rural without-barrier segments. A series of negative binomial regression models was used to determine the effects of independent variables on crashes. Variables considered in developing the base models included average daily traffic, left-shoulder width, barrier offset, median (with shoulder) width, and pole density. Crashes were examined in relation to median crashes for 5 years (1997 to 2001). An AMF represents the change in safety when a particular geometric design element changes in size from one value to another. In this project, the AMFs were estimated directly from the coefficients of the models. This approach for AMF development assumes that (a) each AMF is independent because the model parameters are assumed to be independent, and (b) the change in crash frequency is exponential. AMF equations were developed for urban and rural medians with rigid barriers, urban medians without barriers, and rural medians without barriers.