Methodology for Critical Gap Analysis at Intersections with Unprotected Opposing Left-Turn Movements

摘要

This paper presents a methodology for critical gap analysis at intersections with unprotected opposing left-turn movements. Currently, the Highway Capacity Manual (HCM) methodology uses base critical gap and follow-up time which are adjusted to reflect specific conditions of each intersection. The methodology assumes an unobstructed line-of-sight for drivers executing left-turn maneuvers. However, the line-of-sight is not always unimpeded. Previous studies have shown that leading non-compact (larger and taller) vehicles and vehicles in the opposing left-turn lane may impede the line-of-sight. Specifically, the studies have demonstrated that the impedance may result in a capacity reduction. In order to quantify the capacity reduction, we present a methodology to modify the critical gap and follow-up time model when the line-of-sight of unprotected left turn maneuver is obstructed. We introduce new adjustment factors to account for vehicles in the opposing left-turn lane obstructing the line-of-sight during the left turn maneuver. Using field data from Baltimore and Annapolis, Maryland it is shown that the obstruction increases the left-turn critical gap and the left-turn follow-up time, and hence decreases the potential capacity for left-turns at intersections with unprotected left-turn movements. This capacity reduction may be a significant contributor to systemwide delay during rush hour, and may influence dilemma zone and red light running behavior at intersections equipped with a red light camera.