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

This paper presents a methodology for critical gap analysis at signalized intersections with unprotected opposing left-turn movements. Highway capacity manual (HCM) methodology for estimating potential capacity uses base critical gap and base follow-up time, which are adjusted to reflect specific conditions of each intersection. That methodology assumes an unobstructed line-of-sight for drivers while executing left-turn maneuvers. However, the line-of-sight is not always unimpeded. Previous studies have shown that leading noncompact (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 HCM 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, we show 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, which are subjects of future research.