Abstract
A roundabout is deemed as an alternative means of traffic management and control at intersections. It offers a considerable safety and operational benefits. This study investigates the traffic state through an empirical evaluation of traffic homogeneity, lane utilisation and composition at some selected urban roundabouts in Nigeria. The study focused on the potential operational challenges posed by mixed traffic conditions. Using unmanned aerial vehicle (UAV) data from Gombe, Jos, Kano, and Maiduguri cities, the research evaluates the vehicle category lane preferences to enter roundabouts using either the inner or outer lane and their implications for roundabout performance. Findings from the study revealed that vehicular traffic at the studied Nigerian urban roundabouts is highly heterogeneous, with homogeneity factor (HF) much lower than the threshold of 80%, comprising of varying proportions of trucks, cars, tricycles, and motorcycles. Significant disparities in lane utilization among the various categories of vehicles were recorded, with the majority of motorcycles and tricycles favouring outer lanes. Cars and trucks exhibited constrained usage of both lanes, with a slight preference for the outer lane. At the give-way line into roundabouts, the mean lane distribution of traffic ranges from 31 to 37% and 63 – 69% on the inner and outer lanes, respectively. An analysis on traffic composition revealed the dominant traffic category for inner and outer lanes in the ratio of 72:67%, 60:42%, 49:49% and 45:81% as motorcycles for Gombe, cars for Jos, tricycles for Kano and tricycles for Maiduguri cities, respectively. The study findings signify the inadequacy of the generic Highway Capacity Manual 2016 (HCM6) models in capacity analysis of urban roundabouts in Nigeria, emphasizing the need for localized calibration.

Key words: Roundabout, Mixed Traffic, Lane Utilization, Traffic Composition, HCM6 Capacity Models