GUIDELINES FOR OFFSETTING OPPOSING LEFT-TURN LANES ON FOUR-LANE DIVIDED ROADWAYS

Vehicles turning left at intersections from opposing left-turn lanes often restrict each other's sight distance. Previous research has indicated that collisions between left-turn and opposing through vehicles may result from such sight distance restrictions. However, this problem can be eliminated simply if the opposing lanes are offset so that opposing left-turn vehicles do not interfere with each other's line of sight. Existing design guides do not specify the amount of offset needed. Although they acknowledge the potential problem when medians exceed 18 ft, they do not seem to recognize that it can also occur when medians are narrower than 18 ft. Therefore, a study was conducted to develop guidelines for offsetting opposing left-turn lanes to eliminate the left-turn sight-distance problem. The guidelines presented in this paper specify the offsets required between opposing left-turn lanes at 90-degree intersections on level, tangent sections of four-lane divided roadways with 12-ft lanes. The guidelines provide adequate sight distances for passenger cars opposed by left-turning passenger cars and trucks. A 2.0-ft offset provides unrestricted sight distance when the opposing left-turn vehicle is a passenger car, and a 3.5-ft offset provides unrestricted sight distance when the opposing left-turn vehicle is a truck. All the minimum offsets specified in the guidelines are positive, indicating that the negative offsets typically found at these locations do not provide adequate sight distances for opposing left-turn vehicles.