Multilevel Variable Neighborhood Search for Periodic Routing Problems

In this work we present the extension of a variable neighborhood search (VNS) with the multilevel refinement strategy for periodic routing problems. The underlying VNS was recently proposed and performs already well on these problems. We apply a path based coarsening scheme by building fixed (route) segments of customers accounting for the periodicity. Starting at the coarsest level the problem is iteratively refined until the original problem is reached again. This refinement is smoothly integrated into the VNS. Further a suitable solution-based recoarsening is proposed. Results on available benchmark test data as well as on newly generated larger instances show the advantage of the multilevel VNS compared to the standard VNS, yielding better results in usually less CPU time. This new approach is especially appealing for large instances.