Estimating the Safety and Operational Impact of Raised Medians and Driveway Density

Abstract

This paper describes research sponsored by the Texas Department of Transportation to investigate the operational and safety impact of raised medians and driveway consolidation. Operational effects (travel time, speed, and delay) were investigated through microsimulation on three field test corridors and three theoretical corridors. Safety effects were investigated along 11 test corridors to estimate relationships between crash rates and access point densities as well as the presence of raised medians or two-way left-turn lanes (TWLTLs). The research demonstrates that access management effects are case specific and that microsimulation can assess these unique operational effects. For the case studies investigated, replacing a TWLTL with a raised median resulted in an increase in travel time on two test corridors and a decrease on one test corridor. Small increases in travel time were found with the theoretical corridors as well. The travel time differences are based on the traffic level and location and number of the raised median openings. When present, the relatively small increases in travel time, and subsequent speed and delay, appear to be outweighed by the reduction in the number of conflict points and increased safety. Detailed crash analysis on 11 test corridors indicated that as access point density increases, crash rates increase. This trend holds regardless of the median type. For test corridors in which crash data were investigated before and after the raised median installation, a reduction in the crash rate was always found. Finally, future research needs are identified, including the need to investigate operational and safety impact over a broader range of geometric conditions and longer corridors than investigated here.