Turn Speeds and Crashes within Right-Turn Lanes

Right-turn lanes are used to provide space for the deceleration and storage of turning vehicles and to separate the turning vehicles from the through movement. When larger corner radii are used at the right turn, vehicles can turn at higher speeds (thereby minimizing the speed differential between turning and through vehicles) and can more efficiently merge with the cross-street traffic. A concern with the higher operating speed is the challenge it provides pedestrians attempting to cross the street. Equations are available for predicting speeds on a horizontal curve; however, these equations should not be used for predicting speeds in a right turn. This project analyzed the impact of right-turn lane treatments on vehicle speeds and vehicle safety using nine intersections for the safety study and 18 approaches for the speed study. Each approach for the speed study had an exclusive right-turn lane that was separated from the through lane with either a lane line or with a raised corner island. The corner radii ranged between 27 and 86 ft. The speed study included only free-flow right-turning vehicles. The 85th percentile speed near the middle of the right turn ranged from 13 to 21 mph while on the approach it ranged from 17 to 29 mph. Speed prediction equations were developed. For the nine intersections included in the crash study, the monthly crash rate for a shared lane with island (0.67 right-turn crashes per approach per year) was the highest of the treatments studied. The next highest was the right-turn lane with island design with 0.21 right-turn crashes per approach per year.