Comparative Analysis of 85th Percentile Operating Speeds for Different Vehicle Categories Under Daytime and Nighttime Conditions: A preliminary Investigation

Abstract

Understanding spatiotemporal variations in vehicle operating speeds is essential for improving road safety on rural highways, particularly under low visibility nighttime conditions. This study investigates 85th percentile operating speeds (V85) of three vehicle categories, motorized two wheelers (MTWs), passenger cars (CARs), and heavy commercial vehicles (HCVs) at five key points along 30 horizontal curves on a two-lane rural highway in Karnataka, India. Spot speed data, collected using laser speed cameras under free flow conditions, were statistically validated and analysed across both daytime and nighttime. Results reveal consistent speed reductions at night, with CARs showing the highest nighttime deceleration, particularly at curve midpoints, while MTWs maintained moderate speeds across lighting conditions. Paired t-tests, Wilcoxon signed rank tests, ANOVA, and post hoc Tukey analyses confirmed significant inter vehicle and inter segment speed variability. The segment from point of curvature (PC) to midpoint (MC) emerged as the most critical for all vehicle types and conditions based on composite variability rankings. These findings underscore the need for targeted countermeasures such as enhanced mid curve delineation, vehicle specific entry warnings, and improved nighttime visibility, tailored to segment and vehicle class. The study offers a replicable framework for prioritizing design and safety interventions and highlights the value of integrating statistical diagnostics and composite ranking approaches in rural highway safety assessments. © 2025 NIPES Pub. All rights reserved.