arXiv subjects

Piotr Matl

Publications and source records attributed to Piotr Matl.

4 recordsLinked to original sources

arXiv2019

A concise guide to existing and emerging vehicle routing problem variants

Vehicle routing problems have been the focus of extensive research over the past sixty years, driven by their economic importance and their theoretical interest. The diversity of applications has motivated the study of a myriad of problem variants with different attributes. In this article, we provide a concise overview of existing and emerging problem variants. Models are typically refined along three lines: considering more relevant objectives and performance metrics, integrating vehicle routing evaluations with other tactical decisions, and capturing fine-grained yet essential aspects of modern supply chains. We organize the main problem attributes within this structured framework. We discuss recent research directions and pinpoint current shortcomings, recent successes, and emerging challenges.

cs.DM↗

arXiv2018

Workload Equity in Vehicle Routing: The Impact of Alternative Workload Resources

In practical vehicle routing problems (VRPs), important non-monetary benefits can be achieved with more balanced operational plans which explicitly consider workload equity. This has motivated practitioners to include a wide variety of balancing criteria in decision support systems, and researchers to examine the properties of these criteria and the trade-offs made when optimizing them. As a result, previous studies have provided a much-needed understanding of how different equity functions affect the resulting VRP solutions. However, by focusing exclusively on models which balance tour lengths, a critical aspect has thus far remained unexplored -- namely the impact of the workload resource subject to the balancing. In this work, we generalize previous studies to different workload resources, extend the scope of those analyses to additional aspects of managerial and methodological significance, and reevaluate accordingly previous conclusions and guidelines for formulating a balance criterion. We propose a classification of workload resources and equity functions, and establish which general types of balanced VRP models can lead to unintended optimization outcomes. To explore in greater detail the differences between models satisfying the given guidelines, we conduct an extensive numerical study of 18 alternative balance criteria. We base our observations not only on smaller instances solved to optimality, but also on larger instances solved heuristically, to gauge the extent to which our conclusions hold also for larger VRPs. Overall, our study counter-balances the focus of previous works and reveals the importance of selecting the right workload resource.

math.OC↗

arXiv2018

Workload Equity in Vehicle Routing Problems: A Survey and Analysis

Over the past two decades, equity aspects have been considered in a growing number of models and methods for vehicle routing problems (VRPs). Equity concerns most often relate to fairly allocating workloads and to balancing the utilization of resources, and many practical applications have been reported in the literature. However, there has been only limited discussion about how workload equity should be modeled in VRPs, and various measures for optimizing such objectives have been proposed and implemented without a critical evaluation of their respective merits and consequences. This article addresses this gap with an analysis of classical and alternative equity functions for biobjective VRP models. In our survey, we review and categorize the existing literature on equitable VRPs. In the analysis, we identify a set of axiomatic properties that an ideal equity measure should satisfy, collect six common measures, and point out important connections between their properties and those of the resulting Pareto-optimal solutions. To gauge the extent of these implications, we also conduct a numerical study on small biobjective VRP instances solvable to optimality. Our study reveals two undesirable consequences when optimizing equity with nonmonotonic functions: Pareto-optimal solutions can consist of non-TSP-optimal tours, and even if all tours are TSP optimal, Pareto-optimal solutions can be workload inconsistent, i.e. composed of tours whose workloads are all equal to or longer than those of other Pareto-optimal solutions. We show that the extent of these phenomena should not be underestimated. The results of our biobjective analysis are valid also for weighted sum, constraint-based, or single-objective models. Based on this analysis, we conclude that monotonic equity functions are more appropriate for certain types of VRP models, and suggest promising avenues for further research.

math.OC↗

arXiv2017

Heuristic Rectangle Splitting: Leveraging Single-Objective Heuristics to Efficiently Solve Multi-Objective Problems

Real-life problems are often characterized by conflicting optimization objectives. Consequently, there has been a growing interest not only in multi-objective models, but also in specialized multi-objective metaheuristics for solving those models. A wide variety of methods, e.g. NSGA-II, SPEA, IBEA, scatter search, Pareto local search, and many others, have thus been proposed over the years. Yet in principle, multi-objective problems can be efficiently solved with existing tailored single-objective solvers -- this is the central idea behind the well-known epsilon-constraint method (ECM). Despite its theoretical properties and conceptual simplicity, the epsilon-constraint method has been largely ignored in the domain of heuristics and remains associated mostly with exact algorithms. In this article we dispel these preconceptions and demonstrate that the epsilon-constraint framework can be a highly effective way to directly leverage the existing research on single-objective optimization for solving multi-objective problems. We propose an improved version of the classical ECM adapted to the challenges and requirements specific to heuristic search. The resulting framework is implemented with an existing state-of-the-art single-objective solver for the Capacitated Vehicle Routing Problem (CVRP) and tested on the VRP with Route Balancing (VRPRB). Based on an extensive computational study, we show the added value of our adaptations compared to the classical ECM, and demonstrate that our simple epsilon-constraint algorithm significantly outperforms the current state-of-the-art multi-objective metaheuristics with respect to multiple quality metrics. We conclude with a discussion of relevant success factors and promising directions for further research.

math.OC↗