A Trade-off between User-Equilibrium and System Optimal Traffic Assignments for Congestion Mitigation in City Networks

Abstract

Major cities worldwide are plagued by severe traffic congestion during peak periods. Traffic routing to utilise unused link capacities is a good strategy to reduce congestion and increase network performance. A multi-objective optimization model that seeks a via-media solution between System Optimal (SO) and User Equilibrium (UE) network flows while ensuring better utilization of the excess capacities available in the network is proposed. The number of paths between the O-D pairs is restricted to eliminate paths with long travel times and a constraint on Total System Travel Time (TSTT) is introduced. The proposed problem is solved using the weighted average method to determine Pareto optimal solutions. It is found that restricting the flow to only four paths resulted in a TSTT value that is only marginally higher (1.4%) than the SO model. When evaluated against the UE and SO models, the proposed model significantly improved the link capacity utilization (up to 73%) on some of the links. Additionally, the travel times improved by up to 30% on some paths, compared to the UE model. Although certain paths have longer travel times, the TSTT of Pareto front points is less than or equivalent to the UE model. Thus, the model yields a better distribution of flow in the network and is found to be superior to the existing models. Such a model can potentially contribute to sustainability, safety, and better management of transportation infrastructure.