Network models in traffic management and control

Abstract

The context for network modelling in traffic management and control is described in terms of the current area‐wide nature of traffic management and the range of objectives to which it can contribute. Representation of a road system and traffic management measures in terms of nodes and links and parameters associated with them is described. It is shown that the pattern of traffic has to be represented not only in terms of flows on links of the network but also in terms of numbers of movements per unit time between points of entry to and points of exit from the area being modelled. In modelling so far, these numbers of movements are regarded as given, but the routes taken are estimated by traffic assignment. Models can so far be used for comparison of a range of given schemes and for optimization of traffic control within a scheme. Variation over time is a central feature of the modelling, and this requires the use of time‐dependent queueing theory, and the specification of numbers of movements for a succession of periods of between 10 and 30 minutes. Theoretical approaches to the resulting problems of modelling and optimization are discussed, and the ways in which these are supplemented by heuristic methods in currently available models is described. Some requirements‐for further research are outlined.     

The potential for British coastal shipping in a multimodal chain

A strategy to revitalize coastal shipping within Great Britain is investigated. An examination of the regulatory environment shows government, at both the EU and British levels, aware of the environmental benefits of coastal shipping compared to road freight transport, if only mildly supportive in tangible terms. The geography of Great Britain and the modern preference for just-in-time deliveries have severely restricted the ability of coastal shipping to compete effectively with road freight transport. The primary objective of the paper is to investigate whether coastal shipping could be integrated into a multimodal door-to-door supply chain, where it is currently hampered by high costs of transhipment, slow transport speeds, and the incongruity of load sizes between land and sea modes. Integration requires the co-operation of all organizations within the multi-modal supply chain. However, coastal shipping companies have been known to be individualistic and mistrusting of alliances. Therefore, a Delphi study is undertaken to investigate the standpoint of leading managers in such companies towards multimodal integration. The results of the Delphi study indicate that managers are in favour of multimodal developments, in particular cooperation between coastal shipping and road haulage. However, there was agreement that the business is highly competitive, and this may prevent the forms of collaboration required for multimodal systems. The secretive nature of the coastal shipping sector could impede joint marketing to promote a mode of transport that is often 'invisible' to shippers. There was little consensus on the relationship that should exist between coastal shipping and ports, a topic worthy of further investigation.     

Problems with certification and legislation

This paper discusses the vast growth of requirements directed toward mobile offshore drilling units attempting to be internationally certified. As a simplistic example, the regulations regarding heliports are examined with the pitfalls and benefits of an international standard discussed from the viewpoint of owner and operator. Recommendations contained in an International Maritime Organization resolution are suggested for use by the various regulatory bodies when dealing with new or amended rules. It is suggested that a more universal standard would evolve if the guidance of the resolution were heeded.     

Aspects of family and kinship in a North Carolina coastal community: a comparative study

The almost total absence of social-science data on North Carolina coastal fishing communities has provoked this preliminary study of the role played by family and kinship ties among fishermen and boat builders in such a community, with a view to developing new fisheries management strategies from an assessment of the social and economic impacts of those which already exist.     

Factors contributing to the severity of intersection crashes

Road crashes are a leading cause of death and serious injuries both developed and developing countries. Intersections are recognized as being among the most hazardous locations on the roads. Although crashes at intersections form about 35 % of the reported accidents account for about 32% of traffic‐related serious injuries and fatalities in Singapore, there is no known study that examines the factors contributing to the severity of these crashes. In this study, the ordinal probit model was applied to crash data from 1992 to 2002 to investigate the role a variety of factors play in determining the severity of intersection crashes. Our study shows that vehicle type, road type, collision type, driver's characteristics and time of day are important determinants of the severity of crashes at intersections in Singapore.     

Route swapping in dynamic traffic networks

A dynamic traffic assignment (DTA) model typically consists of a traffic performance model and a route choice model. The traffic performance model describes how traffic propagates (over time) along routes connecting origin-destination (OD) pairs, examples being the cell transmission model, the vertical queueing model and the travel time model. This is implemented in a dynamic network loading (DNL) algorithm, which uses the given route inflows to compute the link inflows (and hence link costs), which are then used to compute the route travel times (and hence route costs). A route swap process specifies the route inflows for tomorrow (at the next iteration) based on the route inflows today (at the current iteration). A dynamic user equilibrium (DUE), where each traveller on the network cannot reduce his or her cost of travel by switching to another route, can be sought by iterating between the DNL algorithm and the route swap process. The route swap process itself takes up very little computational time (although route set generation can be very computationally intensive for large networks). However, the choice of route swap process dramatically affects convergence and the speed of convergence. The paper details several route swap processes and considers whether they lead to a convergent system, assuming that the route cost vector is a monotone function of the route inflow vector.     

The corridor problem: Preliminary results on the no-toll equilibrium

Consider a traffic corridor that connects a continuum of residential locations to a point central business district, and that is subject to flow congestion. The population density function along the corridor is exogenous, and except for location vehicles are identical. All vehicles travel along the corridor from home to work in the morning rush hour, and have the same work start-time but may arrive early. The two components of costs are travel time costs and schedule delay (time early) costs. Determining equilibrium and optimum traffic flow patterns for this continuous model, and possible extensions, is termed “The Corridor Problem”. Equilibria must satisfy the trip-timing condition, that at each location no vehicle can experience a lower trip price by departing at a different time. This paper investigates the no-toll equilibrium of the basic Corridor Problem.     

A splitting rate model of traffic re-routeing and traffic control

The paper presents an idealised dynamical model of day-to-day or within-day re-routeing using splitting rates at nodes, or node-exit flows, rather than route-flows. It is shown that under certain conditions the dynamical model gives rise to a sequence of link flow vectors which converges to a set of approximate Wardrop equilibria. A special dynamical signal green-time re-allocation model is added; the combination is also shown (in outline) to converge to the set of approximate consistent equilibria under certain conditions. Finally the paper uses model network results to illustrate a method of designing fixed time signal timings to meet different scenarios.     

美国铁路车钩缓冲装置的发展(待续)

介绍美国铁路车钩缓冲装置的发展过程、结构特点及运用情况。     

Travel time reliability: a review of late time valuations,elasticities and demand impacts in the passenger rail market in Great Britain

This paper provides an extensive review and reconciliation of British and European evidence relating to the value of, and demand responses to, rail reliability. In particular, we compare the elasticities implied by stated preference valuations of late time with directly estimated lateness elasticities. We find that the implied lateness elasticities are substantially greater than those directly estimated. A possible explanation for this is that lateness has been over-valued, but more sobering explanations would be to suggest that, whilst rail travellers dislike unreliability, they may be unwilling or unable to reduce their rail travel in response to experiences of poor performance, or else conventional economic approaches to deducing elasticities are not appropriate. The findings have been used to update the recommendations of the UK rail industry’s Passenger Demand Forecasting Handbook.