Offsetting Opposing Left-Turn Lanes for Intersections on Horizontal Curves

Left-turn vehicles need sufficient sight distance to decide when it is safe to turn left crossing the lane(s) used by the opposing traffic. Current AASHTO policy recommends that the adequacy of sight distance for left turns should be checked for the reason that the opposing left-turn vehicles can block a driver’s view of oncoming traffic. Previous studies developed guidelines for offsetting opposing left-turn lanes to overcome this problem. However, these guidelines are only applicable to intersections with no curvature. This paper presents a mathematical model for calculating the required minimum left-turn lane offset and the median width (to accommodate the offset), when the intersections are located on horizontal curves. The provision of required offset ensures that the left-turn vehicles have unobstructed required sight distance. An application of the model is presented for divided highways with median width of 4.88 m assuming general values of other variables. The model is translated into an Excel worksheet in which the calculations for the required minimum offset and median width can be performed for any geometric configuration (e.g., curvature of major road, number of lanes, median and lane widths of the minor and major roads, etc.). Other design factors may be also input based on field observations, including design speed along the major road and longitudinal and lateral positioning of vehicles.