CAPACITY EVALUATION OF ROUNDABOUT JUNCTIONS ON ARTERIAL ROADS IN ILORIN. (A Case Study of Pakata, Adeta and Mandate Roundabout Junctions)

Abstract

ABSTRACT: Rapid urbanization and increasing traffic demand have resulted in recurring congestion and reduced operational efficiency at many urban roundabouts in Nigeria. This study evaluated the operational performance of selected roundabouts in Ilorin Metropolis and assessed the effectiveness of geometric improvements in enhancing traffic flow. Peak-hour traffic volume data were collected at Pakata, Adeta, and Mandate roundabouts during the evening peak period (4:00 to 6:00pm). Roundabout entry capacities and degrees of saturation (DS) were estimated using an ARCADY-style empirical capacity model, while PTV VISSIM microscopic traffic simulation was employed to evaluate vehicular delay and Level of Service (LOS) under existing and proposed geometric conditions. The results showed that the existing single-lane approaches operated under oversaturated conditions, with DS values ranging from 1.30 to 6.28, indicating that traffic demand exceeded available entry capacities. These conditions resulted in excessive vehicular delays, long queues, and poor Levels of Service. A proposed redesign involving the widening of entry approaches from one lane to three lanes significantly increased entry capacities and reduced DS values to between 0.14 and 0.94 across the evaluated approaches. Simulation results further demonstrated substantial reductions in average vehicular delays and marked improvements in traffic operations and Levels of Service following the proposed geometric modifications. The combined application of the empirical capacity model and PTV VISSIM proved effective for evaluating roundabout performance and assessing improvement alternatives. The study concludes that widening roundabout entry approaches is a practical and sustainable strategy for mitigating congestion, improving traffic efficiency, and enhancing the operational performance of urban roundabouts in rapidly growing cities. The findings provide valuable guidance for transportation engineers and urban planners involved in the planning, design, and management of roundabout intersections. Keywords: Roundabout performance; Entry capacity; Degree of saturation; PTV VISSIM; Level of service; Traffic simulation; Geometric improvement.