Highway projects involving access management strategies are among the most hotly debated transportation issues, particularly in regards to the choice of midblock left turn treatment. The two main competitors for midblock left turn treatment on four-lane arterials are raised medians with openings and two-way left turn lanes (TWLTL). This research focused on determining the safety effects of medians on midblock road segments and the adjacent signalized intersections. For the segment study, predictive collision models were calibrated using geometric, volume, land use, and collision data for 143 midblock segments. Analysis showed that collisions were significantly related to cross-section type, average daily traffic, segment length, land use, and approach density (two-way total). For predominantly residential and industrial land uses, the raised median design was always associated with fewer collisions than the TWLTL design. For predominantly business and office land uses, the raised median design had a safety advantage for low approach densities. For higher driveway densities, the raised median was slightly safer at high traffic volumes and the TWLTL was slightly safer at lower traffic volumes. To investigate the safety effects on adjacent intersections, the study examined a set of 78 intersection approaches in North Carolina. Although the group of study sites was purposely biased toward sites with high U-turn percentages, the study found that 65 of the 78 sites did not have any collisions involving U-turns in the three-year study period, and the U-turn collisions at the remaining 13 sites ranged from 0.33 to 3.0 collisions per year.
Abstract