Impacts of Motorcycle Dedicated Lanes on Traffic Flow and Safety of Unsignalized Intersections–A Microsimulation Case Study in Kampala, Uganda

Abstract

Traffic congestion and undeveloped public transport define the modus operandi in Kampala, the capital city of Uganda. Most people see motorcycles as a solution to escape the growing traffic congestion and poor public transport. This study thus aimed to determine the traffic efficiency and safety effectiveness of introducing motorcycle dedicated lanes in non–lane based and mixed traffic. These effects were studied by microsimulation of unsignalized intersections without priority markings in urban areas of developing countries. The study centered on an intersection commonly known as “Spear Motors” in Kampala. In addition, off-peak traffic was simulated since traffic police controls the intersection during peak time and microsimulation model cannot simulate traffic while a policeman (or two) is on duty.
Video images of the intersection were officially obtained from Uganda Police and processed to traffic data such as traffic volume, trajectories, composition, and conflicts. In addition, geometric data (lane width, slope, intersection dimensions, coordinates, and aerial pictures) about the intersection was physically collected during a daytime site visit.
A representation of the current traffic conditions at the intersection, referred to as the base model, was developed in VISSIM 21. The base model was calibrated using Particle Swarm Optimization (PSO). Thereafter, the model was microscopically and macroscopically validated.
Three scenarios were considered in this study, which are: scenario 1 with straight crossings of motorcycles in the intersection, scenario 2 with deflected crossings and scenario 3 with a roundabout separated for the main road traffic and motorcycles.
Simulations with driver behavior parameters set to optimum values were performed to replicate the actual behavior. Consequently, traffic conflicts were generated from vehicle trajectories per scenario. Post Encroachment Time (PET) and Time to Collision (TTC) were calculated for the conflicts. The use of conflicts is proactive compared to reactive approaches that use crashes.
The use of deflected crossings for motorcycles and then a roundabout separated for main traffic and motorcycles increased the flow of the intersection by 31% and 20%, respectively. Additionally, the implementation of deflected motorcycle crossings and roundabout reduced critical density by 38.6% and 63.7%, respectively.
Regarding safety, more benefits are expected by using either deflected crossings or roundabout than straight crossings. The number of severe critical conflicts reduced by 87.9% with the application of a roundabout separated for main road traffic and motorcycles. This is higher than the decrease of 48.5% after the application of dedicated motorcycle lanes with deflected crossings. To determine the conflict rates per scenario, the number of severe critical conflicts was divided by total traffic simulated. Deflected crossings for motorcycles reduced severe critical conflict rate by 40% whilst it decreased by 75% with intersection improvement to a roundabout separated for main traffic and motorcycles.
With the high motorcycle demand of over 50% of traffic, implementation of dedicated motorcycle lanes at unsignalized intersections as well as urban roads in general will result into better flow of traffic. In addition, the motorcycle lanes will contribute to the general road safety in Kampala.