Safety and operational analyses of access management treatments : results of managing driveway density and installing raised medians.

This paper describes research sponsored by the Texas Department of Transportation (TxDOT) to investigate the operational and safety impacts of access management techniques--primarily raised medians and driveway consolidation. Operational impacts (travel time, speed, and delay) were investigated through microsimulation on three field test corridors and three theoretical corridors. Safety impacts were investigated along 11 test corridors to estimate relationships between crash rates and access point (driveways and public street intersections) densities, as well as the presence of raised medians or two-way left-turn lanes (TWLTLs). The proposed future conditions (approximately a 20% increase in traffic) with a raised median resulted in a percent increase in travel time from 2 to 57% on two test corridors, and a decrease of 11 to 38% on one test corridor compared to the TWLTL. The travel time increases equated to as much as a 6 mph decrease in speed on one corridor and an increase of 7 mph on another corridor. A similar percent increase with the raised median compared to the TWLTL was found on theoretical corridors--equating to an average speed decrease of 3 mph. The travel time differences are based upon the traffic level and location/number of the raised median openings. This relatively small increase in travel time may 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, there is an increase in crash rates. This trend is irrespective of the median type though the research team found that the relationship is steeper (increases slightly more) on roadways without raised medians. For test corridors where 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 impacts over a broader range of geometric conditions and longer corridors than investigated here. The information provided in this paper is anticipated to be useful for transportation professionals seeking additional information on the potential impacts of these access management treatments.