Impact of Lane Drops on Intersection Capacity

Abstract

Lane drops downstream of signalized intersections are found on many urban and suburban streets and highways. The short lane is typically under-utilized at the intersection since drivers avoid using the short lane due to the potential for stressful merges downstream of the signal. This unconventional geometric configuration downstream of the lane drop intersection affects intersection capacity and operations. The current Highway Capacity Manual (HCM) does not take into account downstream lane drop conditions, thereby resulting in significant discrepancy between the HCM defaults and field observed lane utilization factors (LUF).

The primary objectives of this research were to identify whether and by how much lane drops affect performance of actuated signalized intersections under various traffic demand levels and lane drop geometries. The research developed LUF prediction models by intersection type from data on traffic, signal and intersection geometry collected in the field. The developed models imply that the downstream lane length and traffic intensity are positively correlated with the LUF and that other geometric variables at the approach may also influence lane utilization. Also, a traffic simulation model was used to quantify the impact of lane drops on intersection operations with various traffic volumes and geometric conditions. Through the simulation, it was found that additional green time, which should reduce delay, was created because of the overloaded full lane at the approach, however; a higher probability of green time termination with maximum green time dominates the benefit of extra green and the approach gains significant extra delays. Based on the data obtained from the simulation, this research quantified the amount of additional delay for the subject movement and established delay adjustment factors due to account for lane drop effects. Findings from this research are expected to contribute to the assessment of lane drop intersection performance.