Impact of Leading Vehicles of the Queue on Saturation Flow at Signalized Intersections

Abstract

Saturation flow is an essential component in the design and capacity estimation of signalized intersections. The saturation flow rate is influenced by several factors, including intersection geometry, speed limit, the proportion of heavy vehicles, parking activities, bus stops adjacent to the intersection area, lane utilization, pedestrian volume, etc. Estimation of saturation flow is well-established for homogeneous traffic conditions. However, measuring and modelling saturation flow are challenging in mixed traffic conditions due to the non-lane-based traffic flow and the presence of multiple types of vehicles. Typically, Passenger Car Unit (PCU) adopted should be enough to account for the variation among the vehicles that constitute the queue while estimating saturation flow. However, field observation at Indian intersections indicated that the vehicles at the front of the queue might profoundly influence the queue discharge at the onset of green phase, thereby exposing the shortcoming of PCUs. This influence is clearly visible when heavy vehicles make up the front of the queue and owing to their inferior operational performance and larger size, they deny the following vehicles the opportunity to attain their desired acceleration. To investigate this, the present study simulated the operation of a typical four-legged signalized intersection in VISSIM software. The subsequent analysis revealed a strong relationship between leaders of the queue and the saturation flow of the approach. This result from the simulation was then verified based on field data. The study concluded that the PCU factors adopted at signalized intersections fail to account for the effect of the queue leader’s composition, which has a marked influence on the saturation flow. © 2024, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.