James F. Campbell

Hub Location and the p-Hub Median Problem

Hub facilities serve as switching and transshipment points in transportation and communication networks. Hub networks concentrate flows on the hub-to-hub links and benefit from economies of scale in interhub transportation. Most hub location research has focused on problems where each origin/destination is allocated to a single hub. However, multiple allocation to more than one hub is necessary to minimize total transportation costs. This paper defines a p-hub median, analogous to a p-median, and presents integer programming formulations for the multiple and single allocation p-hub median problems. Two new heuristics for the single allocation p-hub median problem are evaluated. These heuristics derive a solution to the single allocation p-hub median problem from the solution to the multiple allocation p-hub median problem. Computational results are presented for problems with 10-40 origins/destinations and up to eight hubs. The new heuristics generally perform well in comparison with other heuristics.

Twenty-Five Years of Hub Location Research

Last year was the 25th anniversary of two seminal transportation hub location publications, which appeared in 1986 in Transportation Science and Geographical Analysis. Though there are related hub location and network design articles that predate these works, the 1986 publications provided a key impetus for the growth of hub location as a distinct research area. This paper is not intended as a comprehensive review of hub location literature; rather, our goal is to reflect on the origins of hub location research, especially in transportation, and provide some commentary on the present status of the field. We provide insight into early motivations for analyzing hub location problems and describe linkages to problems in location analysis and network design. We also highlight some of the most recent research, discuss some shortcomings of hub location research and suggest promising directions for future effort.

Continuous approximation models in freight distribution: An overview

This paper presents an overview of continuous approximation models that have been developed for freight distribution problems. A taxonomy of six classes is used to differentiate the problems and a brief review of each paper is provided. The paper stresses important principles and key results from the models and highlights gaps in the literature and areas for future work.

Hub Arc Location Problems: Part I-Introduction and Results

Hub networks play an important role in many transportation and telecommunications systems. This paper introduces a new model called the hub arc location model. Rather than locate discrete hub facilities, this model locates hub arcs, which have reduced unit flow costs. Four special cases of the general hub arc location model are examined in detail. We provide motivation for the new models, and present examples and optimal solutions, using data for U.S. air passenger traffic. Results are used to compare optimal costs, hub locations, and hub arc locations with corresponding hub median optimal solutions. The results reveal interesting spatial patterns and help identify promising cities and regions for hubs. A companion paper (Campbell et al. 2005) presents integer programming formulations and solution algorithms for the new hub arc problems. It also provides details and computation times for these solution algorithms.

Hub Arc Location Problems: Part II-Formulations and Optimal Algorithms

A companion paper (Campbell et al. 2005) introduces new hub arc location models and analyzes optimal solutions, with special attention to spatial pattens and relationships. This paper provides integer programming formulations and optimal solution algorithms for these problems. We describe two optimal solution approaches in detail and compare their performance, using standard hub location data sets. We present implementation details and show how algorithms can be fine tuned based on characteristics of the data.

Location and allocation for distribution systems with transshipments and transportation economies of scale

Locating transshipment facilities and allocating origins and destinations to transshipment facilities are important decisions for many distribution and logistic systems. Models that treat demand as a continuous density over the service region often assume certain facility locations or a certain allocation of demand. It may be assumed that facility locations lie on a rectangular grid or that demand is allocated to the nearest facility or allocated such that each facility serves an equal amount of demand. These assumptions result in suboptimal distribution systems. This paper compares the transportation cost for suboptimal location and allocation schemes to the optimal cost to determine if suboptimal location and allocation schemes can produce nearly optimal transportation costs. Analytical results for distribution to a continuous demand show that nearly optimal costs can be achieved with suboptimal locations. An example of distribution to discrete demand points indicates the difficulties in applying these results to discrete demand problems.

Hyperbaric Oxygen Therapy in Calciphylaxis-Induced Skin Necrosis in a Peritoneal Dialysis Patient

A 58-year-old white woman on continuous ambulatory peritoneal dialysis for 2 years developed calciphylaxis-induced necrotic skin lesions over both lower extremities. Despite subtotal parathyroidectomy and other conventional measures, skin lesions continued to worsen. Mapping of transcutaneous oxygen pressure showed markedly low values in involved areas. Skin ulcers completely healed after 38 sessions of hyperbaric oxygen therapy. The results in our case indicate that hyperbaric oxygen therapy may be useful in the treatment of skin ulcers secondary to calciphylaxis.

A survey of models and algorithms for winter road maintenance. Part IV: Vehicle routing and fleet sizing for plowing and snow disposal

This is the last part of a four-part survey of optimization models and solution algorithms for winter road maintenance planning. The two first parts of the survey address system design problems for winter road maintenance. The third part concentrates mainly on vehicle routing problems for spreading operations. The aim of this paper is to provide a comprehensive survey of optimization models and solution methodologies for the routing of vehicles for plowing and snow disposal operations. We also review models for the fleet sizing and fleet replacement problems.

Hub location for time definite transportation

Time definite motor carriers provide very reliable scheduled truck transportation service between specified terminals. They provide service competitive with airfreight carriers over continental-scale distances at a much lower cost. This paper provides time definite models for multiple allocation p-hub median problems and hub arc location problems. Service levels are imposed by limiting the maximum travel distance via the hub network for each origin-destination pair. Computational results are presented to demonstrate the effects of the time definite service levels on practical network design for truck transportation in North America.

A survey of models and algorithms for winter road maintenance. Part III: Vehicle routing and depot location for spreading

Winter road maintenance planning involves a variety of decision-making problems related to the routing of vehicles for spreading chemicals and abrasives, for plowing roadways and sidewalks, for loading snow into trucks, and for transporting snow to disposal sites. These problems are very difficult and site specific because of the diversity of operating conditions influencing the conduct of winter road maintenance operations and the wide variety of operational constraints. As the third of a four-part survey, this paper reviews optimization models and solution algorithms for the routing of vehicles for spreading operations. We also review models for the location of vehicle and materials depots and for the assignment of crews to vehicle depots. The two first parts of the survey address system design problems for winter road maintenance. The fourth part of the survey covers vehicle routing problems for plowing and snow disposal operations.