Passenger car units and saturation flow models for highly heterogeneous traffic at urban signalised intersections

Abstract

Signalisation is a traffic control strategy to ease the competition by providing right-of-way in a cyclic manner to conflicting traffic at intersections. Saturation flow is a major component in the design of signals, and is influenced by a variety of factors like vehicle composition, intersection geometry and driver's behaviour. The highway capacity manual (HCM) has recommended a saturation flow model primarily for homogeneous conditions, with limited ability to address heterogeneity. But the traffic in many parts of the world is highly heterogeneous and hence, defining a unified saturation flow concept is a challenging task. The variability in vehicle types necessitates the use of passenger car units (PCUs). This article proposes a methodology to develop a saturation flow model based on dynamic PCUs by a microscopic analysis. Field data from intersections of three Indian cities – Jaipur, Bangalore and Trivandrum – is used for the study. PCU values are derived from the field data, by minimising the difference between the observed and the ideal flow profiles. A new saturation flow model is then developed using the regression method. The model is validated with the saturation flows collected from different locations. In addition, the flow predicted by the saturation flow model is used to estimate delay using the HCM and Webster's models, and these delays are compared to the observed delays. The proposed model resulted in lower error compared to the conventional flow estimation techniques.