GEOMETRIC CHARACTERISTICS OF SELECTED ROUNDABOUTS AND THEIR EFFECT ON TRAFFIC FLOW IN ILORIN, KWARA STATE, NIGERIA

Abstract

This study examined how geometric characteristics influence the operational performance of selected roundabouts in Ilorin metropolis. Field traffic surveys, drone-based video recording, SIDRA Intersection simulation, analysis of variance (ANOVA), and regression analysis were used to assess three busy roundabouts: KWASUTH, Post-Office, and Fate. The geometric variables considered included inscribed circle diameter, circulating width, entry radius, entry angle, number of lanes, and entry lane width, while the operational indicators included traffic volume, queue length, delay, degree of saturation, peak hour factor, and level of service (LOS). Results showed that Post-Office Roundabout experienced the highest weekly traffic volume (92,140 vehicles) and the poorest operational performance, with LOS E and degree of saturation of 0.92, indicating near-capacity operation. KWASUTH Roundabout, despite having the largest geometric dimensions, operated at LOS D because of heavy motorcycle and tricycle interaction and unstable lane discipline. Fate Roundabout recorded the most stable operation at LOS C, supported by a more balanced traffic pattern and lower demand. ANOVA results indicated a statistically significant relationship between LOS and traffic flow (p < 0.05). Regression analysis showed that entry width had a positive and significant influence on traffic flow, while larger entry angles reduced operational efficiency. A 20% geometric modification simulated in SIDRA produced substantial improvements, reducing delay, queue length, and saturation while improving LOS at the most congested sites. The findings confirm that geometric redesign, combined with traffic management measures, can significantly improve roundabout performance in mixed-traffic urban environments.