Path finding under uncertainty

Path finding problems have many real‐world applications in various fields, such as operations research, computer science, telecommunication, transportation, etc. In this paper, we examine three definitions of optimality for finding the optimal path under an uncertain environment. These three stochastic path finding models are formulated as the expected value model, dependent‐chance model, and chance‐constrained model using different criteria to hedge against the travel time uncertainty. A simulation‐based genetic algorithm procedure is developed to solve these path finding models under uncertainties. Numerical results are also presented to demonstrate the features of these stochastic path finding models.     

A reliability‐based network design problem

This paper presents a reliability‐based network design problem. A network reliability concept is embedded into the continuous network design problem in which travelers' route choice behavior follows the stochastic user equilibrium assumption. A new capacity‐reliability index is introduced to measure the probability that all of the network links are operated below their capacities when serving different traffic patterns deviating from the average condition. The reliability‐based network design problem is formulated as a bi‐level program in which the lower level sub‐program is the probit‐based stochastic user equilibrium problem and the upper level sub‐program is the maximization of the new capacity reliability index. The lower level sub‐program is solved by a variant of the method of successive averages using the exponential average to represent the learning process of network users on a daily basis that results in the daily variation of traffic‐flow pattern, and Monte Carlo stochastic loading. The upper level sub‐program is tackled by means of genetic algorithms. A numerical example is used to demonstrate the concept of the proposed framework.     

An actuary method for the estimation and comparison of urban bus transit cost of services

This paper presents a new cost allocation method developed for estimating the fully-allocated costs of the excess peak hour bus transit service provided by two public transit systems. The estimates were produced for the explicit purpose of carrying out a realistic comparison of these costs and the costs of the potential provision of the same service by private operators. The method utilizes the same service data for the analysis. The pragmatic estimation of the fully allocated costs of service by the public and private sectors enables a more accurate estimation of potential cost savings. Sensitivity analysis was also performed using the same costing procedure to determine the range of cost savings that are feasible in a competitive contract arrangement of private sector providers. The new method separates costs of the direct provision of service from other indirect costs and thus facilitates the identification of each cost item and its significance in comparative cost estimates.     

Household trip generation choice—Alternative empirical approaches

Trip generation models have generally received less attention than other aspects of travel decision making. This article presents some explorations into the structure of trip decisions for shopping, using data from weekly shopping diaries. The paper compares alternative formulations of the naturally-ordered choice model used by Sheffi (1979) to avoid the problems inherent in multinomial logit models. Firstly, imposing cross-alternative restrictions on some of the coefficient values is termed the constrained model. Secondly, when no such restrictions are imposed, the model decomposes into a sequence of binary models, and this is termed the unconstrained model, which can be used to test the validity of the restrictions. The variables used include both shopping expenditures and locational factors, both of which are found to play a key role in shopping travel decisions, as well as more conventional socio-economic variables. A clearer understanding of the role of car ownership in travel decisions is obtained.     

Economic policies and seaports: The diversity of port policies

This paper contrasts the economic policies adopted towards seaports by different countries in terms of their approaches to national, regional or local control. Various examples are presented and described, together with some of the difficulties that have been encountered. Examples are cited where national, regional and local governments have become involved, even though constitutional provisions appeared to present this. It is argued that national, regional and local governments all have interests in the efficiency of ports from various points of view; hence all should be involved. The arguments for and difficulties experienced in centralized planning and co-ordination are also discussed.

The variety of activities directly undertaken by port authorities is discussed, with the extreme positions labelled 'landlord' and 'comprehensive'. The remarkable variety of intermediate positions is also described. Whilst oversimplification is dangerous, the opportunities for expanding the private sector are analysed, even to the extent of wondering whether public sector port authorities are needed at all. This, therefore is the subject of the next paper.     

Monitoring urban spatial dynamics

As urban systems increase in size and diversity, the factors which influence spatial change increase in number and their interactions become more complex. Increasing emphasis is thus being placed by some governments and agencies on regular reviews of policies and programs, to ensure that they are consistent with and facilitate the evolving urban form. Consequently, the monitoring of urban transport and activity systems is of increasing importance, to aid the review processes. This paper considers the progress which has been made in and use of procedures for monitoring urban, transport-related spatial changes. The specification, estimation and presentation of appropriate indicators is briefly discussed. Experience with the use of spatial monitoring procedures on data for two Australian cities has revealed several possible implications for transport investment practice and the development of monitoring and modelling procedures. These are discussed and the paper then indicates the approach which is currently being taken at ARRB towards developing a monitoring system.     

Cost benefit analysis of urban minibus operations

Earlier work by the Transport Studies Group of the Polytechnic of Central London on minibus development in Britain for the Transport and Road Research Laboratory provided a financial analysis of intensive urban minibus operation. This is taken as the basis for the application of cost-benefit analysis. Unit operating cost savings and passenger benefits are taken into account, using typical demand elasticities derived from earlier work. Particular attention is paid to the problem of evaluating passenger benefits for which waiting time at the roadside is not necessarily an adequate proxy. A direct survey of passenger waiting times indicates that these do not necessarily decrease when a higher-frequency minibus service replaces a conventional bus service but the improved convenience produces an increase in ridership which may be assessed in terms of a demand curve shift to estimate benefits obtained.