Optimizing consolidation processes in hubs: The hub-arrival-departure problem

To realize economies of scale in transportation, hubs are important entities in today’s distribution networks. Hubs, such as cross docks for trucks, central ports for container vessels, transshipment yards for freight trains, and air hubs for airplanes, allow to consolidate less-than-vehicle-load shipments and to improve the utilization of transport capacities. In this context, we consider a generic optimization problem to streamline hub processes. We assign vehicles consolidating their shipments at a central hub each an arrival time slot where a vehicle is unloaded and a later departure time slot where it is loaded with the goods it takes back to its designated territory. Since the processing of goods inside a hub terminal takes some time, we want to synchronize the exchange of goods among vehicles such that three different consolidation-oriented objectives are optimized. For instance, we aim to minimize the number of goods missing their dedicated departures after a fixed consolidation time for goods inside the hub. We prove important structural properties, provide a comprehensive analysis of computational complexity, and derive suited solution procedures. Our problem occurs in the real world at the air hub of postal service provider DHL in Leipzig, where freight airplanes from all over the world exchange goods and need to be assigned landing and departure slots on the runway. Our problem, however, is generic, and we sketch some extensions in order to adapt it to further hubs.