Adaptive memory programming for the vehicle routing problem with multiple trips

https://doi.org/10.1016/j.cor.2005.02.044Get rights and content

Abstract

The Vehicle Routing Problem with Multiple Trips is an extension of the classical Vehicle Routing Problem in which each vehicle may perform several routes in the same planning period. In this paper, an adaptive memory algorithm to solve this problem is proposed. Computational experience is reported over a set of benchmark problem instances.

Introduction

The problem of distributing goods from depots to final consumers plays an important role in the management of many distribution systems, and its adequate programming may produce significant savings. The Vehicle Routing Problem (VRP) and its many variations have been subject of research during the last four decades. Some well studied characteristics include the existence of demands, time windows and heterogeneous vehicles [1].

However, some aspects that arise in real applications have not received much attention in the Operations Research literature. For instance, it usually assumed that each vehicle may perform at most one route in the same planning period, and in many cases the number of available vehicles is supposed to be unlimited. In many practical applications, this assumptions are unrealistic. When the vehicle capacity is small or when the planning period is large, performing more than one route per vehicle may be the only practical solution. In urban areas, where travel times are rather small, it is often the case that after performing short tours vehicles are reloaded and used again.

The Vehicle Routing Problem with Multiple Trips (VRPMT) overcomes the mentioned limitations, besides considering the classic VRP constraints. Solving the VRPMT not only implies the design of a set of routes, but the assignment of those routes to the available vehicles. This makes the VRPMT a very practical problem, specially at an operational level, in which daily driver schedules must be designed for a fixed vehicle fleet.

In this paper we describe a heuristic to solve the VRPMT, which is based on the Adaptive Memory Procedure (AMP) proposed by Rochat and Taillard [2]. A definition of the VRPMT is given in Section 2, as well as a literature review. The proposed algorithm is described in Section 3. Computational behavior of the algorithm is reported and analyzed in Section 4, while conclusions and future work are considered in Section 5.

Section snippets

The VRP with multiple trips

Let G=(V,E) be a graph where V={0,1,,n} is the set of nodes and EV×V is the set of arcs. If (i,j)E, then it is possible to travel from i to j, incurring in a cost cij and a travel time tij. Node 0 represents a depot where a fleet K={1,,m} of identical vehicles is based. Each vehicle has a limited capacity Q. The nodes in V{0} represent customers, each one having a demand qi. Finally, there exists a time horizon, denoted by T, which establishes the duration of a working day. It is assumed

Adaptive memory algorithm

The AMP was first proposed by Rochat and Taillard [2] as an enhancement of Tabu Search (TS) to solve the VRP. It was motivated by the work of Glover regarding surrogate constraints [15]. An important principle behind AMP is that good solutions may be constructed by combining different components of other good solutions. A memory containing components of visited solutions is kept. Periodically, a new solution is constructed using the data in the memory and improved by a local search procedure.

Benchmark problems

The algorithm was tested over the 104 benchmark problems proposed by Taillard et al. [7]. These were constructed using the same graphs, demands and vehicle capacities than the problems 1–5 and 11, 12 proposed by Christofides et al. [14] and problems 11 and 12 proposed by Fisher [28] for the VRP. For each VRP instance, several values of m are used. For each m, two time horizons T1=[1.05z*/m] and T2=[1.10z*/m] are proposed, where z* is the best known solution value for the VRP instance and [a]

Conclusion and future work

An algorithm based on the Adaptive Memory Programming principle has been proposed to solve the Vehicle Routing Problem (VRP) with Multiple Trips, an important extension of the classic VRP.

The algorithm was ran over a set of benchmark problems and the solutions obtained were compared with those reported for three previously proposed algorithms [7], [9], [11]. Our algorithm obtained more feasible solutions than the previous approaches. Further analysis shows that a good compromise between routing

Acknowledgements

The authors are grateful to the anonymous referees, whose comments helped to improve the presentation of this paper and the quality of the reported results.

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