The freight multimodal transport problem with buses and drones: An integrated approach for last-mile delivery

This paper introduces a novel approach to traditional last-mile delivery by proposing a freight multimodal transport problem that integrates buses and drones. More precisely, buses are responsible for transporting parcels to lockers at bus stops for storage, while drones are used to deliver each parcel from the locker to the corresponding customer. The integrated bus-drone system synergistically expands drone service coverage using the bus network to ensure efficient final delivery. Minimizing the total operational costs while satisfying customer demands necessitates the joint optimization of parcel assignments and drone flights. We model the problem into a compact mixed-integer linear programming formulation and propose an integer programming formulation with exponentially many variables. To address real-world scale instances, we propose a Branch-Price-and-Benders-Cut algorithm for this non-deterministic polynomial-time (NP)-hard problem. This algorithm, integrating column generation and Benders decomposition within a Branch-and-Bound framework, is developed to obtain optimal or near-optimal solutions. Additionally, we introduce algorithmic enhancements aimed at accelerating the convergence of the algorithm. Computational experiments on instances generated from real-world bus data demonstrate that the proposed algorithms outperform CPLEX regarding both efficiency and solution quality. Moreover, our approaches can lead to over 6 % cost savings compared to situations where we determine parcel assignments and drone flights sequentially. Finally, we evaluate the environmental advantages of integrating buses and drones, study the impact of different cost parameters in the system, and investigate the impact of the parcel locker configuration and drone service range on performance. All of these provide valuable managerial insights regarding the potential of the integrated bus-drone system for urban logistics management.