MITIGATION OF SIGHT-DISTANCE PROBLEM FOR UNPROTECTED LEFT-TURNING TRAFFIC AT INTERSECTIONS

Sight-distance problems at intersections are often discussed in the context of visual obstructions caused by permanent objects such as trees, parked vehicles, and buildings. Very little investigation has been done on sight-distance problems engendered by queued vehicles. This type of problem poses safety and capacity deficiencies, particularly for unprotected left-turn movements at intersections. The available sight distance for a left-turning motorist on a major approach is shown to be dependent on the offset for opposing left-turn lanes, that is, the distance from the inner edge of a left-turn lane to the outer edge of the opposing left-turn lane. The model developed can be used to determine the minimum offset required for ensuring an adequate sight distance for left-turning traffic, given the design speed and the configuration of the intersection. A sensitivity analysis was undertaken to investigate the effects of changes in the noted offset on the available sight distance for left-turning traffic. It was determined that there was a strong correlation between the offset and the available sight distance for left-turning traffic. It is, therefore, possible to resolve this problem by incorporating in the intersection design some considerations for the minimum offset for opposing left-turning lanes.