Capacity Evaluation of Roundabout Junctions on Arterial Roads in Ilorin, Nigeria

Abstract

Rapid urbanisation and escalating traffic demand have resulted in persistent congestion and degraded operational efficiency at urban roundabouts across Nigerian cities. This study evaluated the operational performance of three roundabout junctions  Pakata, Adeta, and Mandate located along arterial roads in Ilorin Metropolis, Kwara State. Peak-hour traffic volume data were collected during the evening peak period (4:00–6:00 p.m.), and roundabout entry capacities and degrees of saturation (DS) were estimated using an ARCADY-style empirical linear capacity model. PTV VISSIM microscopic traffic simulation software was subsequently employed to quantify vehicular delay and Level of Service (LOS) under both existing and proposed geometric configurations. Results revealed that all existing single-lane approaches operated under severely oversaturated conditions, with DS values ranging from 1.30 to 6.28, indicating that prevailing traffic demand substantially exceeded available entry capacities. These conditions produced excessive vehicular delays, persistent queue formation, and poor LOS classifications. A geometric redesign involving the widening of entry approaches from one lane to three lanes markedly increased entry capacities and reduced DS values to between 0.14 and 0.94 across all evaluated approaches. PTV VISSIM simulation results corroborated these findings, demonstrating substantial reductions in average vehicular delays and significant improvements in traffic operations and LOS following the proposed geometric modifications. The combined application of the ARCADY-style empirical model and PTV VISSIM proved to be an effective and complementary methodology for roundabout performance evaluation. The study concludes that entry-approach widening constitutes a practical and sustainable intervention strategy for mitigating congestion, improving traffic efficiency, and enhancing the operational performance of urban roundabouts in rapidly growing Nigerian cities.