Leon Y.O. Li

A new tabu search heuristic for the open vehicle routing problem

In this paper, another version of the vehicle routing problem (VRP)—the open vehicle routing problem (OVRP) is studied, in which the vehicles are not required to return to the depot, but if they do, it must be by revisiting the customers assigned to them in the reverse order. By exploiting the special structure of this type of problem, we present a new tabu search heuristic for finding the routes that minimize two objectives while satisfying three constraints. The computational results are provided and compared with two other methods in the literature.

The school bus routing problem: a case study

This paper describes a case study of the school bus routing problem. It is formulated as a multi-objective combinatorial optimisation problem. The objectives considered include minimising the total number of buses required, the total travel time spent by pupils at all pick-up points, which is what the school and parents are concerned with most, and the total bus travel time. It also aims at balancing the loads and travel times between buses. A heuristic algorithm for its solution is proposed. The algorithm has been programmed and run efficiently on a PC. Numerical results are reported using test data from a kindergarten in Hong Kong. It has shown to be effective with a saving of 29% in total travelling times when comparing to current practice.

A Tabu Search based Heuristic for Arc Routing with a Capacity Constraint and Time Deadline

The problem considered is to find the minimum cost set of routes which enable vehicles to service a set of required arcs in a network subject to the service capacity of the vehicles and a time constraint by which each arc must be serviced. The problem is NP-Hard so heuristic methods are required to give good solutions within an acceptable computing time. A new tabu search based heuristic called LOCSAR is introduced and compared with other approaches.

A unified tabu search algorithm for vehicle routing problems with soft time windows

The different ways of allowing time window violations lead to different types of the vehicle routing problems with soft time windows (VRPSTW). In this paper, different types of VRPSTW are analysed. A unified penalty function and a unified tabu search algorithm for the main types of VRPSTW are presented, with which different types of VRPSTW can be solved by simply changing the values of corresponding parameters in the penalty function. Computational results on benchmark problems are provided and compared with other methods in the literature. Some best known solutions for the benchmark problems in the literature have been improved with the proposed algorithm.

Coordinated replenishments with alternative supply sources in two-level supply chains

We consider a two-level supply chain with two warehouses and two retailers. Each facility uses a continuous review (R,Q)-ordering policies for stock replenishments. The retailers are identical and face independent Poisson demands. Each warehouse places orders for replenishments with an outside supplier which has unlimited stocks. Transportation times are constant and all stockouts at both levels are completely backordered. Under normal circumstances, all orders of each retailer are delivered by only one warehouse, which is designated to serve that retailer. But in case that the warehouse is out of stock, there are two alternatives to the retailer: to wait an extra time until the warehouse replenishes its stock or to place an order with the other warehouse. However, the latter decision incurs a higher transportation cost. In this paper, we study two inventory models with alternative supply possibilities and different levels of coordination between warehouses and retailers, and compare the performance of these models with that of a traditional model where ordering from only one of the warehouses is allowed for each retailer.

Erratum: A new tabu search heuristic for the open vehicle routing problem

Correction to:Journal of the Operational Research Society (2005) 56, 267–274. doi:10.1057/palgrave.jors.2601817

n-newsvendor biform game of trading capacity futures

We consider a group of suppliers who have the same facilities and similar capabilities to produce goods with a very short lead-time for n retailers (newsvendors) who sell non-identical products. We treat such short lead-time capacity as a commodity that can be traded as futures to the retailers. In a two-stage inventory model, retailers buy physical goods and capacity futures as inventory portfolios in the first stage to determine their inventory positions in the selling season. After realization of demand is observed in the second stage, retailers make a replenishment decision that is limited to the capacity futures on hand. However, retailers are allowed to form coalitions to transfer the residual capacity futures among themselves. Therefore, retailers have bidirectional adjustments to their inventory positions. We prove that this mechanism is a good tool to induce suppliers to offer their capacities in the season, and the futures market provides a hedge for them. We employ a biform game to analyze the risk and payoff of retailers as players in both non-cooperative and cooperative stages. The Nash equilibrium in the first stage and the core in the second stage have been identified. Our findings suggest retailers can share risk among different supply chains with different products to mitigate inventory risk and improve their payoffs. However, the game discriminates against those retailers that have lower profit margin, lower inventory cost and lower lost sales penalty.