CONVERSION FROM PERMISSIVE TO EXCLUSIVE/PERMISSIVE LEFT-TURN PHASING: A BEFORE-AND-AFTER EVALUATION (DISCUSSION AND CLOSURE)

A before-and-after study was conducted to determine the effects of converting left-turn signal phasing from a permissive condition to an exclusive/permissive condition. Data collection was conducted in April 1984 and February 1985. Time-lapse photography was used to collect data on the number of vehicles already stopped, stopping, or not stopping at 5-sec intervals. Each movement (left-turn and through) and direction were separately recorded. These data, in turn, were analyzed to determine traffic volumes, average and total amounts of vehicle stopped delay, the percentage of vehicles stopping, and the percentage of left-turn vehicles. Mean values of these factors for before and after data were compared to determine the significance of any differences. The results showed that left-turn volumes increased significantly in the after phase. However, when these volumes were expressed as a percentage of total volume (which also increased), the increases were not significant. The percentage of vehicles that stopped increased dramatically from 43 percent of all vehicles in the before phase to 71 percent of all vehicles in the after phase. Average delays to southbound through traffic more than quadrupled in the after phase, whereas those to northbound through traffic more than tripled. Average delay to left-turn vehicles decreased to 82 percent of the before values; not a statistically significant amount. The conversion resulted in 87.9 veh-hr of additional delay per day. This delay converts to a cost of $398,587/year in additional vehicle operating, travel time, and vehicle emissions costs. Longer cycles, loss of progression, and inefficient use of green time increased the number of stopping vehicles and vehicle delay. The improvements in processing left-turn vehicles were obtained at the expense of inconveniencing the through movement. A comparison of before- and after-accident experience was not included in this study.