Effect of Dynamic Sight-Distance Problem on Unprotected Left-Turn Movement Capacity

Classical sight-distance problems involve the horizontal and vertical alignment of the road and stationary objects around the roadway that interfere with the motorist's view. Dynamic sight-distance problems concern the sight-distance situation engendered by “nonstationary” objects such as vehicles using the roadway. This paper discusses a theoretical approach for evaluating the operational effect of a dynamic sight-distance problem that is associated with unprotected left-turning traffic at intersections. Specifically, the problem discussed in this paper concerns the effect of left-turning vehicles on the gap acceptance of the vehicles in the queue. Sample test results indicate that the dynamic sight-distance problem considered here can result in larger critical gaps, and hence significantly reduce the capacity of left-turn movements at unsignalized intersections or signalized intersections with permitted left-turn signal phasing. A sensitivity analysis shows that the severity of dynamic sight-distance problems increases with the traffic intensity (i.e., the degree of saturation). The paper concludes that the critical gap value for unprotected left-turning traffic is not only dependent on the number of conflicting traffic movements, but it is also dependent on the traffic intensity and traffic composition (e.g., percent compact vehicles versus percent noncompact vehicles). These factors should be considered in determining reasonable critical gap values that can be used for capacity analysis of unprotected left-turn movements at traffic intersections.