Description

This paper develops updated free-speed predictive equations for distinct vehicle categories operating on multilane highways in India. Free speed—defined as the speed at which drivers choose to travel when unimpeded by other vehicles—is a critical fundamental parameter in traffic engineering, highway geometric design, speed management, and economic evaluation models like HDM-4. Given the rapid modernization of India's highway network and vehicle fleet, existing speed relationships often fail to reflect contemporary driver behavior and vehicle capabilities. The study gathered high-precision speed and geometric data using radar guns and automated sensors across four-lane and six-lane divided highways under low-volume traffic conditions. Multiple regression analysis was performed to establish empirical relationships between free speed and key roadway and environmental characteristics, including lane width, shoulder width, median type, horizontal curvature, longitudinal gradient, and surface roughness (measured via IRI). Separate equations were formulated for passenger cars, light commercial vehicles, buses, heavy commercial vehicles, and two-wheelers. The findings reveal that horizontal alignment and surface smoothness are the primary determinants of free speed, with modern heavy vehicles demonstrating significantly higher desired speeds than historical assumptions. The resulting speed models enhance the accuracy of highway planning tools and traffic simulation models in heterogeneous traffic environments.