EFFECT OF ROUNDABOUT GEOMETRY ON OPERATIONAL PERFORMANCE OF URBAN TRAFFIC FLOW IN ILORIN, NIGERIA

Abstract

This study investigates the impact of roundabout geometry on operational traffic flow within the Ilorin metropolis, Kwara State, Nigeria, focusing on three major junctions: Sango-Maraba-Fate, Post Office, and Kwara State University Teaching Hospital (KWASUTH). Urban traffic in developing economies is characterized by heterogeneity and increasing volumes that often exceed the design capacity of existing rotary systems. The methodology integrated physical tape surveys for geometric documentation with manual and drone-assisted traffic counts. These data were processed using SIDRA INTERSECTION 9.0 to simulate performance metrics, including Degree of Saturation (v/c), control delay, and Level of Service (LOS). Results indicate a significant disparity in performance across the sites. The Sango-Maraba-Fate roundabout reached absolute capacity (v/c = 1.000), with the Sango approach experiencing an LOS F and an average delay of 89.1 seconds. Conversely, the Post Office and KWASUTH roundabouts maintained stable LOS B conditions, with average delays of 10.0 and 16.5 seconds, respectively. Statistical analysis via a One-Way ANOVA showed that while intersection-wide delays varied, the differences were not statistically significant (p = 0.13) due to high intra-site variance. However, linear regression revealed a robust positive correlation (R² = 0.97) between inscribed circle diameter and delay, suggesting that larger rotary designs in this context are underperforming. The study concludes that the Sango-Maraba-Fate junction requires immediate geometric intervention, such as entry flaring or slip-lane integration, to mitigate systemic failure. These findings provide essential guidance for urban traffic management and future infrastructure planning in sub-Saharan African cities.