Jam in the Tunnel: On Urban Freight Tunnels, Their Operational Scheduling, and Unused Transport Capacity

Underground passageways for the transport of goods and people have fascinated mankind throughout the centuries. Recent initiatives in different parts of the world aim at rekindling the freight tunnel concept for urban logistics. According to this concept, freight is lifted into a tunnel, moved via electrically powered vehicles, rail cars, or maglev shuttles toward inner-city microhubs, and finally delivered with environmentally friendly vehicles to urban recipients. The main justification for this concept to outweigh the huge tunnel boring cost is a substantial reduction of road-based urban freight traffic. To warn about unfulfilled expectations, however, we show that the combination of time-critical goods, small inner-city hubs with restricted storage capacity, and the wrong booking procedure can hinder a loading of tunnel vehicles to capacity. To do so, we consider a polynomially solvable scheduling problem where due date-restricted shipments are to be assigned to tunnel vehicles and delivered to a given set of microhubs with limited storage capacity. Scarce hub capacities, tight due dates, and the wrong booking policy may not only lead to unused tunnel capacity but also to shipments that cannot be feasibly transported through the tunnel. If they are handed over to trucks as a fallback option, we show that a Braess-like paradox can occur: The time-critical, small-lot transports of the leftover shipments can produce even more surface traffic than trucking all shipments right from the start without the tunnel.