INFLUENCE OF TRAFFIC MIX ON PCU VALUE OF VEHICLES UNDER HETEROGENEOUS TRAFFIC CONDITIONS

Praveen, Prema Somanathan; Arasan, Venkatachalam Thamizh · 2013 · Crossref

DOI: 10.7708/ijtte.2013.3(3).07

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Summary

This study addresses the challenge of accurately estimating Passenger Car Unit (PCU) values for heterogeneous traffic conditions prevalent in developing countries like India. Unlike homogeneous traffic in developed nations, Indian urban traffic consists of diverse vehicle types—such as buses, trucks, cars, two-wheelers, and bicycles—that share road space without lane discipline. Existing PCU guidelines, such as those from the Indian Roads Congress, rely on constant values derived from limited data, failing to account for the complex interactions and varying impedance caused by different vehicle mixes. The research aims to quantify how traffic volume and composition influence PCU values for various vehicle categories on four-lane divided urban roads. To achieve this, the authors utilized HETEROSIM, an indigenously developed microscopic traffic simulation model capable of replicating heterogeneous traffic flow where vehicles occupy space based on availability rather than lane discipline. The model treats the road as a single unit, representing vehicles as rectangular blocks with specific dimensions and dynamic characteristics. Input data, including free speeds, vehicle dimensions, and lateral clearance requirements, were derived from literature and field observations. Field data were collected via video recording at a mid-block section of Anna-Arch Road in Chennai, India, capturing 3,599 vehicles over one hour. The simulation model was calibrated using statistical tests, confirming that vehicle arrival patterns followed a Poisson distribution and headway distributions followed a negative exponential distribution. The study simulated traffic streams comprising eight vehicle categories under both homogeneous (cars-only) and heterogeneous conditions across a wide range of traffic volumes and compositions. The primary finding is that PCU values are not static; they significantly vary with changes in traffic volume and composition. The simulation results demonstrate that the relative impedance caused by a specific vehicle type is highly dependent on the surrounding traffic context. For instance, the PCU value for a given vehicle category changes as the proportion of other vehicle types in the stream shifts, and as the overall traffic volume increases or decreases. This variability contradicts the assumption of constant PCU factors used in current design standards. The significance of this research lies in its demonstration that traditional constant PCU values are inadequate for heterogeneous traffic environments. By establishing that PCU values are dynamic functions of traffic volume and composition, the study highlights the need for more sophisticated, context-specific methodologies for traffic capacity analysis and roadway design in India. The findings suggest that future guidelines should incorporate variable PCU factors that reflect real-time traffic conditions, thereby improving the accuracy of traffic flow predictions and infrastructure planning in regions with mixed traffic streams.

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• traffic density