Sustainable last-mile delivery with drones and on-demand outsourcing

This study examines two intriguing aspects of last-mile delivery: on-demand outsourcing and drones. Logistics providers may sometimes rely on on-demand outsourcing, in addition to private fleets, to achieve quicker turnaround times, reduce carbon emissions, or address insufficient fleet capacity. The paper explores the delivery via a private fleet comprising trucks and drones that operate independently from a central depot along with the option of on-demand outsourcing. For on-demand outsourcing, an estimate of the total delivery time together with the associated carbon emissions is known in advance. The problem is formulated as a bi-objective Mixed Integer Linear Programme (MILP) with delivery makespan and carbon emissions as the two objectives. A two-step ϵ-constraint bi-objective method is provided to generate Pareto optimal solutions for small to medium-sized problems. For larger instances, an iterated local search heuristic is developed, which effectively solves all relevant instances from the standard CVRPLIB library, balancing delivery time and carbon emissions.