Modified Guidelines for Left-Turn Lane Geometry at Intersections

Left-turn vehicles need sufficient sight distance to decide when it is safe to turn left and cross the lane(s) used by the opposing traffic. The current policy of the American Association of State Highway and Transportation Officials (AASHTO) recommends that the adequacy of sight distance for left-turn vehicles should be checked because the opposing left-turn vehicles can block a driver’s view of the oncoming traffic. Previous studies have established guidelines for various intersection geometric elements (offset between opposing left-turn lanes, left-turn lane length, and left-turn lane-line width) to ensure that adequate sight distance is provided for left-turn vehicles. However, these guidelines are based on overestimation of the available sight distance. This results in underestimating the requirements for intersection elements. This paper develops modified analytical models and guidelines for various intersection elements, based on the actual available sight distance. The median opening is also introduced as a variable in the models. The results show that the existing guidelines for minimum offset and left-turn lane length are inadequate generally at low and high speeds, differing from the modified guidelines by more than 100 and 15%, respectively. The existing guidelines for minimum lane-line width are also inadequate for low percentiles (by $0.17\mathrm{m}$ at an offset of $-0.3\mathrm{m}$ ). The modified guidelines ensure that the adequate sight distance for left-turn vehicles is provided at intersections, and therefore should be of interest to traffic and geometric design engineers.