Modular vehicle-based transit system for passenger and freight co-modal transportation

The emergence of vehicle modularity technologies provides an opportunity to develop flexible public transport services that allow en-route vehicle capacity adjustments to meet fluctuating demands. Given the largely imbalanced demands for passenger and freight transport in urban areas, the use of modular vehicles in collaborative passenger and freight transport (co-modality) is expected to increase overall transport efficiency. This study explores the potential of integrating modular vehicles into a co-modal transit system that serves both passengers and freight. Specifically, we consider a bi-directional transit corridor where modular vehicles are scheduled to meet time-dependent passenger and freight demands. Each modular vehicle comprises identical modules that can be assigned to either passengers or freight individually. Moreover, a modular vehicle can adjust (i) the number of composed modules by docking and undocking modules, and (ii) module allocation between passengers and freight at intermediate stations. To minimize passenger waiting costs and vehicle operating costs, a mixed integer programming formulation is established that optimizes vehicle dispatch times, (un)docking operations, module allocation, and freight allocation. Given the solution complexity, we propose a two-stage algorithm to solve this problem efficiently. In the first stage, we address vehicle dispatch times, and in the second stage, vehicle formation and freight allocation are optimized. Numerical experiments verify that the proposed two-stage algorithm outperforms state-of-the-art solvers and can obtain near-optimal solutions in an acceptable time for large-scale problems. A case study based on a transit line in Hong Kong validates the effectiveness of co-modal modular transit systems, where station-wise (un)docking and co-modality lead to a cost reduction of 8.2% and 11.2%, respectively. Finally, sensitivity analysis examines the impact of key parameters on the performance of co-modal modular transit systems.