Accident and Capacity Comparisons of Leading and Lagging Left-Turn Signal Phasings

Many traffic engineers nationwide are reluctant to implement lagging left-turn phasing plans along arterial streets, even though signal progression can sometimes be significantly improved. An opinion frequently expressed by traffic engineers is that lagging left turns are contrary to driver expectancy and cause an increase in start-up lost time. To prevent the “left-turn trap” in which drivers making left turns may be caught unaware of the extended opposing lagging phase, many traffic engineers do not allow permitted left turns at an intersection with lagging phasing. This study focuses only on protected left-turn phasing in which the trap does not occur. Saturation flow rates, start-up lost times, fourth-vehicle crossing times, and accident rates are compared between protected-only leading and lagging phasings. All four measures were found to be better for locations with lagging phasing than for those with leading phasing, although neither saturation flow rates nor accident rates were found to be significantly better at the 95 percent confidence level. Although further studies are needed, these findings suggest that a mix of leading and lagging protected-only phasing may be used to gain signal coordination advantages without compromising safety or capacity.