Showing papers in "Transportation Science in 1997"

A Tabu Search Heuristic for the Vehicle Routing Problem with Soft Time Windows

Abstract: This paper describes a tabu search heuristic for the vehicle routing problem with soft time windows. In this problem, lateness at customer locations is allowed although a penalty is incurred and added to the objective value. By adding large penalty values, the vehicle routing problem with hard time windows can be addressed as well. In the tabu search, a neighborhood of the current solution is created through an exchange procedure that swaps sequences of consecutive customers (or segments) between two routes. The tabu search also exploits an adaptive memory that contains the routes of the best previously visited solutions. New starting points for the tabu search are produced through a combination of routes taken from different solutions found in this memory. Many best-known solutions are reported on classical test problems.

An Endogenous Segmentation Mode Choice Model with an Application to Intercity Travel

Abstract: This article uses an endogenous segmentation approach to model mode choice. This approach jointly determines the number of market segments in the travel population, assigns individuals probabilistically to each segment, and develops a distinct mode choice model for each segment group. The author proposes a stable and effective hybrid estimation approach for the endogenous segmentation model that combines an Expectation-Maximization algorithm with standard likelihood maximization routines. If access to general maximum-likelihood software is not available, the multinomial-logit based Expectation-Maximization algorithm can be used in isolation. The endogenous segmentation model, and other commonly used models in the travel demand field to capture systematic heterogeneity, are estimated using a Canadian intercity mode choice dataset. The results show that the endogenous segmentation model fits the data best and provides intuitively more reasonable results compared to the other approaches.

Controlled Optimization of Phases at an Intersection

Abstract: This paper presents a general purpose algorithm for real-time traffic control at an intersection. Our methodology, based on dynamic programming, allows optimization of a variety of performance indices such as delay, stops and queue lengths. Furthermore, optimal phase sequencing is a direct by-product of this new approach. These features make the new methodology a powerful tool for intersection control. We demonstrate the usefulness of the approach by a simulation experiment in which our intersection control algorithm is interfaced with a well established simulation package called TRAF-NETSIM. Our study compares the controlled optimization of phases methodology with fully-actuated as well as semi-actuated control. We show that consistent reductions in delay may be possible by adopting the new algorithm.

A General Fixed-Point Approach to MultiMode Multi-User Equilibrium Assignment with Elastic Demand

Abstract: This paper presents a fixed-point formulation of multi-mode multi-user equilibrium assignment with elastic demand. Users of different classes may have different behavioral characteristics as well as sets of available routes and modes. They may also behave according to different deterministic and/or probabilistic choice models with different utility specifications. Demand elasticity is dealt with without using the inverse of demand function; in addition, the mode choice can be explicitly dealt with. Conditions for existence and uniqueness of solutions are stated, which generalize and extend those in the literature. A general framework for solution algorithms is also developed, and a simple new algorithm is proposed to solve asymmetric (stochastic) multi-mode multi-user equilibrium with elastic demand.

Heuristic Algorithms for the Handicapped Persons Transportation Problem

Abstract: We examine the problem of determining an optimal schedule for a fleet of vehicles used to transport handicapped persons in an urban area. The problem is a generalization of the well-known advance-request Pickup and Delivery Problem with Time Windows. Due to the high level of service required by this kind of transport, several additional operational constraints must be considered. The problem is NP-hard in the strong sense, and exact approaches for the solution of real-life problems (typically with hundreds of users to be transported) are not practicable. We describe a fast and effective parallel insertion heuristic algorithm which is able to determine good solutions for real-world instances of the problem in a few seconds on a personal computer. We also present a Tabu Thresholding procedure which can be used to improve the starting solution obtained by the insertion algorithm. The application of the proposed procedures to a set of real-life instances for the city of Bologna, involving about 300 trips each...

An Exact Algorithm for the Vehicle Routing Problem with Backhauls

Abstract: The Vehicle Routing Problem with Backhauls is an extension of the capacitated Vehicle Routing Problem where the customers' set is partitioned into two subsets. The first is the set of Linehaul, or Delivery, customers, while the second is the set of Backhaul, or Pickup, customers. The problem is known to be NP-hard in the strong sense and finds many practical applications in distribution planning. In this paper we consider, in a unified framework, both the symmetric and the asymmetric versions of the vehicle routing problem with backhauls, for which we present a new integer linear programming model and a Lagrangian lower bound which is strengthened in a cutting plane fashion. The Lagrangian lower bound is then combined, according to-the additive approach, with a lower bound obtained by dropping the capacity constraints, thus obtaining an effective overall bounding procedure. A branch-and-bound algorithm, reduction procedures and dominance criteria are also described. Computational tests on symmetric and as...

Projected dynamical systems in the formulation, stability analysis, and computation of fixed-demand traffic network equilibria

Abstract: This paper proposes, for a fixed demand traffic network problem, a route travel choice adjustment process formulated as a projected dynamical system, whose stationary points correspond to the traffic equilibria. Stability analysis is then conducted in order to investigate conditions under which the route travel choice adjustment process approaches equilibria. We also propose a discrete time algorithm, the Euler method, for the computation of the traffic equilibrium and provide convergence results. The notable feature of the algorithm is that it decomposes the traffic problem into network subproblems of special structure, each of which can then be solved simultaneously and in closed form using exact equilibration. Finally, we illustrate the computational performance of the Euler method through various numerical examples.

The Traffic Equilibrium Problem with Nonadditive Path Costs

Abstract: In this paper we present a version of the (static) traffic equilibrium problem in which the cost incurred on each path is not simply the sum of the costs on the arcs that constitute that path We motivate this nonadditive version of the problem by describing several situations in which the classic additivity assumption fails We describe existence and uniqueness conditions for this problem, and we also present convergence theory for ageneric algorithm for solving nonadditive problems

Modeling Vehicular Traffic Flow using M/G/C/C State Dependent Queueing Models

Abstract: In this paper, M/G/C/C state dependent queueing models are proposed for modeling and analyzing vehicular traffic flows. Congestion aspects of traffic flow are represented by introducing state dependent service rates as a function of number of vehicles on each road link. Analytical models for unidirectional and multisource flows are presented. Finally, queueing models to analytically determine the optimal capacity and performance measures of the road links are incorporated into a series of software programs available from the authors.

Advances in the optimization of airline fleet assignment

Abstract: We present an advanced model for the formulation and solution of large scale fleet assignment problems that arise in the scheduling of air transportation. Fleet assignment determines the type of aircraft to operate each flight in a given schedule, subject to a variety of side constraints, due to marketing, operational, maintenance and crew restrictions. We model the problem as mixed-integer multicommodity flow on networks encoding activities linking flight departures. We focus on fully representing flight connection possibilities, while accurately capturing complex operational rules. We also provide a unified framework for the expression of resource constraints via piecewise linear penalties, which permits a profitability-based tradeoff between operational goals and revenue. Computational results on actual schedules show that high quality assignments for one-day problems can be obtained within an hour of computation. The use of the model at USAir results in an annual benefit of at least $15 million.

A tabu search heuristic for the vehicle routing problem with backhauls and time windows

Abstract: This article describes a tabu search heuristic for the vehicle routing problem with backhauls and time windows. In this problem, the set of customers is partitioned into two subsets: linehaul customers where a given quantity of goods is delivered from a central depot, and backhaul customers where a given quantity of goods is collected and transported to the depot. Mixed routes with both linehaul and backhaul customers are constructed. A variant of this problem is examined where linehauls must precede bachhauls on each route. Furthermore, the time of beginning of service at each customer location must occur within a particular time interval. Computational results are reported on a standard set of test problems, and comparisons are made with other heuristic approaches and with optimal solutions.

Decomposition of path choice entropy in general transport networks

Abstract: This paper shows that the LOGIT type stochastic assignment/stochastic user equilibrium assignment can be represented as an optimization problem with only link variables. The conventional entropy function defined by path flows in the objective can be decomposed into a function consisting only of link flows. The idea of the decomposed formulation is derived from a consideration of the most likely link flow patterns over a network. Then the equivalence of the decomposed formulation to LOGIT assignment is proved by using the Markov properties that underlie Dial's algorithm. Through the analyses, some useful properties of the entropy function and its conjugate dual function (expected minimum cost function) have been derived. Finally, it is discussed that the derived results have a potential impact on the development of efficient algorithms for the stochastic user equilibrium assignment.

Airline Scheduling for the Temporary Closure of Airports

Abstract: The poor scheduling of flights or a fleet resulting from temporary closure of airports may cause a substantial loss of profit and decreased levels of service for airline carriers. This research develops a framework in order to help carriers handle schedule perturbations resulting from the temporary closure of airports. The framework is based on a basic model constructed as a time-space network from which several strategic network models are developed for scheduling. These network models are formulated as pure network flow problems or network flow problems with side constraints. The former are solved using the network simplex method whereas the latter are solved using a Lagrangian relaxation-based algorithm. To test how well the model may be applied in the real world, a case study regarding the international operations of a major Taiwan airline was performed. The results show that the framework could be useful in actual operations.

Integrated routing and scheduling in hazardous materials transportation

Abstract: Recognition of time-varying patterns of accident rates and exposure parameters can be used to improve routing and scheduling decisions for hazardous materials shipments. An integrated routing/scheduling approach can be used to reduce overall risk, and find preferred solutions in a multiobjective context. We describe a method for performing an integrated routing/scheduling analysis, and illustrate its use in a case study. We also construct estimates of the “value of information” for incorporation of this time-dependent data.

Low Probability—High Consequence Considerations in Routing Hazardous Material Shipments

Abstract: This paper is concerned with the development and analysis of a mathematical model for determining a route that attempts to reduce the risk of low probability—high consequence accidents related with the transportation of hazardous materials. The approach adopted considers trade-offs between the conditional expectation of a catastrophic outcome given that an accident has occurred, and more traditional measures of risk dealing with the expected value of the consequence and the accident probability on a selected path. More specifically, the problem we address involves finding a path that minimizes the conditional expectation objective value, subject to the expected value of the consequence being lesser than or equal to a specified value v, and the probability of an accident on the path being also constrained to be no more than some value η. The values v and η are user-prescribed and could be prompted by the solution to the shortest path problems that minimize the respective corresponding linear risk functions...

Transit service and path choice models in stochastic and time-dependent networks

Abstract: This paper develops a new path choice model that incorporates both time-dependent and stochastic transit service characteristics, and allows passengers to update path choice decisions while waiting. To develop this model, a new transit service model is proposed that represents route segments using a shuttle model. Such a model balances requirements for stochastic and time-dependent service modeling with the ability to aggregate to a larger transit corridor or network. This service model leads to a dynamic model of transit path choice, in which the passenger may wait until a vehicle is about to depart before making a boarding decision. A formal definition of this dynamic path choice model is given, and its differences with previous path choice models are noted. Based on this definition, two mathematical formulations of the dynamic model are developed. The first formulation assumes that the passenger will use the dynamic model for all possible vehicle departure times in the future, and is formulated as an o...

On the airline schedule perturbation problem caused by the ground delay program

Abstract: The ground delay program is one of several programs that the Federal Aviation Administration is currently administering for efficient and equitable use of scarce airspace and airport capacity. In this paper, we study airline schedule perturbation problem caused by the ground delay program with the goal of improving airline dependability statistics defined by Department of Transportation as percentage of flights delayed more than 15 minutes. The problem is modeled as an integer program. To solve the model, we have derived valid inequalities for the integer programming formulation for strengthening the LP relaxation bound. The number of integer variables was reduced dramatically based on analysis of the model and its impact on the problem solubility was shown to be of significant importance. A heuristic procedure based on solving a restricted version of the model has been designed for finding good feasible solutions. Computational results indicate the effectiveness of the model reduction and the valid inequ...

Selecting an Optimum Configuration of One-Way and Two-Way Routes

Abstract: This paper considers a network with established transportation flows between nodes which take place along shortest paths. Links on the network may be one-way or two-way. One-way links, although lengthening some shortest paths, are faster, effectively reducing the travel time in the permitted direction. The objective is to select the optimum configuration of one-way and two-way routes to minimize the total flow-weighted transportation time (distance) in the system.

Facility location and reliable route planning in hazardous material transportation

Abstract: This paper considers integrated location and routing models for minimizing the expected number of hazardous material transport accidents. Two different routing policies are considered: (1) most reliable route planning and (2) multiple routing with random selection. Path reliability measurements are used to derive the expected number of accidents over a given planning horizon, where reliability refers to the probability of completing a journey without accident. Corresponding to the two routing policies, two location models are introduced: the reliable 1-median and a location framework for considering multiple routes. Based on realistic assumptions for assigning accident probabilities to segments created when an edge location is considered, vertex optimality results for both problems are obtained. An example to illustrate reliable routing and location in Massachusetts is provided.

Objectives Derived form Viewing Hazmat Shipments as a Sequence of Independent Bernoulli Trials

Abstract: This paper derives six objectives for routing hazardous material by viewing shipments as a sequence of independent Bernoulli trials. An example that compares these objectives is presented. Suitable approximations toward the use of these objectives in an optimization framework are provided. The paper also discusses the objectives from the viewpoint of their use to make actual hazardous material routing decisions.

On The Overspecification of Multinomial and Nested Logit Models Due to Alternative Specific Constants

Abstract: Discrete choice models as demand forecasting techniques have been used for transportation applications for more than thirty years. The multinomial and nested logit models are probably the most widely applied in this context. Alternative specific constants, although playing an important role in these models, have received very little attention in theoretical studies. In this paper, we try to fill this gap by providing an analysis of the overspecification caused by alternative specific constants to the log-likelihood function of multinomial and nested logit models. The theoretical results lead directly to a particular strategy of alternative specific constant specification, called here the orthogonal strategy. The analysis of the relationship between any two arbitrary strategies and the derivation of an interesting geometrical property of the orthogonal strategy provide a motivation to prefer the latter.

A generalized traveling salesman problem approach to the directed clustered rural postman problem.

Abstract: In this paper, we examine the directed Clustered Rural Postman Problem (CRPP). The CRPP is a restricted version of the Rural Postman Problem in which each connected component of arcs to be serviced has to be completely serviced before servicing another component. We present an enumerative solution approach for the CRPP based on transforming the CRPP into a version of a Generalized Traveling Salesman Problem. This work also represents a simple yet elegant unifying view for some classes of arc and node routing problems.

Modeling the increased complexity of New York City's refuse marine transport system

Abstract: The New York City Department of Sanitation operates the world's largest refuse marine transport system Waste trucks unload their cargo at land-based transfer stations where refuse is placed in barges and then towed by tugboats to the Fresh Kills Landfill in Staten Island In the early 1980s, the city commissioned the development of a computer-based model for use in fleet sizing and operations planning As a result of the complexities introduced by environmental regulation and technological innovation, the marine transport system operations changed and the existing model became obsolete Based on the success achieved with the first model in 1993, the city commissioned the development of a new model In this paper, we present a PC-based model developed to meet the increased complexity of the system Analysis performed for validation and calibration of the model demonstrates that it tracks well the operations of the real system We illustrate through a detailed design exercise how to use the model to config

Incorporating Insurance Costs in Hazardous Materials Routing Models

Abstract: The economic loss associated with the involvement of a hazardous materials truck in an accident can be very high. Although the immediate costs are usually borne by an insurer, the carrier would incur an increase in its future insurance costs as a result. Thus, a hazardous materials carrier can benefit from considering its long-term insurance costs in routing the required shipments. In this paper, we introduce the hazardous materials routing problem under insurance costs. We present a solution methodology that facilitates evaluation of alternative routing policies. The proposed methodology involves solution of a manageable number of shortest path problems in determining the most appropriate route(s). We argue that, for each shipment, the route to be taken by the truck should be determined on the basis of the expected increase in insurance costs due to a possible accident as well as the cost of transportation.

Analytical Models for Vehicle/Gap Distribution on Automated Highway Systems

Abstract: Highway congestion has in recent years become a pervasive problem for urban and suburban areas alike. The concept of Automated Highway Systems is based on the belief that integration of sensing, communication, and control technologies into vehicles and highways can lead to a large improvement in capacity and safety without requiring a significant amount of additional highway right-of-way. A fundamental determinant of Automated Highway Systems capacity is the vehicle-following rule, the rule that governs the behavior of vehicles traveling along a common lane (e.g., the spacing between any two longitudinally adjacent vehicles). Vehicle following affects the longitudinal capacity (achievable flow within a lane), the lateral capacity (achievable flow between lanes) and the conflicting relationship between the longitudinal flow and lateral capacity. The issues are investigated by developing probabilistic models for vehicle/platoon and gap distributions, for vehicles that travel in platoons, in slots, or as fre...

Applications of Non-Order-Preserving Path Selection of Hazmat Routing

Abstract: In this paper, we consider the problem of determining a path that maximizes a multi-attribute, non-order-preserving value function. The motivating application is the determination of a most preferred path for transporting hazardous materials based on transportation cost and risk to population. A sub-path of an optimal path may not be optimal for a non-order-preserving value function, implying that a traditional application of dynamic programming may intentionally or unintentionally produce sub-optimal paths. We consider two approximation procedures for two general cases, the q = 0 case and the q > 0 case, where q is the number of required intermediate stops between origin and destination. The first approximation procedure involves applying dynamic programming as if a sub-path of an optimal path were always optimal. The second approximation procedure involves determining a linear order-preserving criterion that approximates the non-order-preserving value function and then applying dynamic programming. We u...

Instantaneous Power Peak Reduction and Train Scheduling Desynchronization in Subway Systems

Abstract: In a subway system, when a train leaves one station to go to another, power consumption increases and, at one point, reaches a peak. When such peaks occur simultaneously, the total power peak of the system can be very large. On the other hand, if departures are totally desynchronized, it could affect the overall operation of the system. This paper deals with the problem of finding a procedure to desynchronize train departures during a typical rush hour operation so that overall power peaks are minimized and the frequency, safety, and operational constraints of the system are respected. A formulation of the problem and a heuristic approach are proposed. Results, using operational data from the Montreal subway system, are presented.

Fully Loaded Direct Shipping Strategy in One Warehouse/N Retailer Systems without Central Inventories

Abstract: In this paper, we consider one warehouse/multiple retailer systems with transportation costs The planning horizon is infinite and the warehouse keeps no central inventory It is shown that the fully loaded direct shipping strategy is optimal among all possible shipping/allocation strategies if the truck capacity is smaller than a certain quantity, and a bound is provided for the general case

In Memoriam—William S. Vickrey, 1914–1996

Abstract: Overview of life and numerous contributions of William S. Vickrey (1914–1996), recipient of the Nobel Price in Economics in 1996.

Tributes to Robert Herman

Abstract: Robert Herman, a past president of ORSA and one of the founders and the first chairman of the Transportation Science Section, passed away in Austin, Texas, on February 13, 1997. Herman is internationally recognized as the originator of traffic science. His contributions span the entire 40 years of scientific activity in this field.