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Urban spatial traffic patterns
0191-2607190 13.OO+.M) 0 1990 Rrgamon Press plc
7bmspn. Res..A, Vol. 24A. No. 5. pp. 397-398. 1994 Printed in Great Britain.
BOOK REVIEW
Urban Spatial Traffic Patterns. Rodney Vaughan. Pion Ltd., 201 Brondesbury Park, London NW2 5JN, England, 1987. 334 pp. ISBN 0-85086-122-S. This book describes an approach to urban transport modelling developed in the 1960s and 1970s largely in Great Britain and largely by the late Reuben Smeed and his colleagues at University College London, and earlier at the Road Research Laboratory. The central theme is the development of simple models of urban areas that express the pattern of origins and destinations of trips and of the routes used between them. Sadly, Rodney Vaughan died shortly before this book was published, but his distinctive approach to the subject comes through clearly, and the book provides a fitting memorial. The traditional approach to urban transport modelling typically involves the collection of data for large numbers of traffic zones (or their estimation from the socio-economic characteristics of the zones), and for each link in the transport networks; these are then used to provide estimates of travel on each link of the networks, not only under current conditions, but more importantly, under hypothesized changes to the networks or to the traffic generators, and projected into the future. Even today, the mechanics of this process of collecting and processing large numbers of statistics limit the usefulness of the approach. By contrast, the approach in this book asserts that for descriptive purposes, and for some kinds of projections, much of this sophistication is unnecessary, and obscures fundamental simplicities. Thus, most towns have higher densities of activity nearer the center than further from it, and this can be represented by bell-shaped curves; origins of trips can be linked to destinations either at random (for small areas), or by simple assumptions about correlation; routes chosen can be represented, for example, by assuming shortest routes on hypothetical networks such as rectangular grids or rings and radials. The discussion is presented almost entirely in terms of the journey to work, but it is suggested that other kinds of travel could be dealt with similarly. Chapter 2 discusses the joint distribution of homes and workplaces. First, various models are considered for the distribution of homes; then, workplaces are discussed similarly; finally, consideration is given to how homes and workplaces are linked. The author makes no secret of his preferred model: the quadrivariate normal model. But he discusses other approaches, perhaps to such a degree as to detract
from the coherence of the discussion. The quadrivariate normal model is both mathematically elegant and tractable, and it represents the broad character of many towns in a plausible way. Essentially, it says that over the population of commuters, the x and y coordinates of their homes and workplaces follow a multivariate normal distribution; in the standard case, all means are set to zero, as are certain covariances. In this standard case, three parameters describe the whole process: the standard deviations of distances of homes and of jobs from the center, and the correlation between positions of homes and of jobs. This case is consistent with the observation that densities of homes and jobs can each be represented by an exponential falling-off with the square of distance from the center; it can be interpreted as incorporating a gravity model in which the deterrence function falls off exponentially withthe square of the distance travelled. Chapter 3 discusses the structure of transport networks and uses the concept of a ‘routing system’ to describe how a traveller gets from A to B on a given network. Again, much of the material is drawn from the literature, and while useful for reference has little direct bearing on the main argument. There is much here to interest the theoretician concerned with geometrical probability, and considerable insight into how road systems operate on a broad scale, but the material includes much that will be unfamiliar to someone accustomed only to conventional techniques. Matters discussed include route factors (travel distance divided by straight line distance), number of intersections, and optimum network geometry (grids, ring/radial, and so on). Chapter 4 applies the network and routing systems of Chapter 3 to the trip patterns of Chapter 2 to derive travel intensities (distance travelled per unit area), and traffic flows on links of the network, both primarily as functions of distance from the center. It then introduces the fascinating concept of ‘route crossings’: the number, proportion, or density of pairs of trips whose paths cross, for a given routing system on a given network. Chapter 5 calculates various averages or aggregates over the city as a whole, with particular reference to average distance travelled and to the number of crossings of pairs of commuters’ paths. For convenience, most of the algebra is confined to the ‘Smeed City,’ in which homes and workplaces are independently and randomly distributed in a circular area.
397
398
Book Review
Chapter 6 applies the results of earlier chapters to various issues. These include the possible trade-off between networks and routing systems which reduce distances travelled, and those that reduce the number of journeys whose paths cross each other; the optimum placing of ring roads; and the influence of home and job locations on amount and location of travel. Throughout the book, numerous references are quoted: 379 in all. Of these, about one-half are from the 1960s or earlier; few are from the 1980s. I think it is fair to say that most use is made of the earlier references; certainly I felt that in respect to my own work some early papers were given too much emphasis, but that the author had not really absorbed the lessons to be learned from some later papers. It seems to me that this book, and the approach that it exemplifies, is most valuable in introducing transport planners and engineering of the future to some of the broader aspects of their subject. I would be unhappy if practitioners, despite being well versed in the latest computerized modelling techniques, had no feel for how traffic behaves on the large scale, or of ideas such as routing systems and route crossings. Not that these ideas can necessarily be applied directly- the real world is usually too complicated for that- but they may encourage us to think about problems in a simplified way, and not fail to see the forest for the trees. Having said that, the mathematics in this book may deter some: they are not particularly difficult, but they are rather pervasive. I would also encourage the more academic sort of practitioner to dip into the book, not so much to find specific techniques, but rather to find new approaches to problems. At a detailed level, I found a few trivial printing or editing errors, but no errors of substance. A good deal of the writing is obscure or long-winded, but one can usually follow the argument without too much difficulty. Perhaps I may be permitted to use my personal involvement in the CRISTAL modelt [somewhat misdescribed by Vaughan on page 3061 to illustrate the difficulties of using models of this kind to illuminate real issues. The objective was to develop a model that TJ. C. Tanner, L. Gyenes, D. A. Lynam, S. V. Magee, and A. H. Tulpule, Development and Culibmtion of the CRISTAL Transport Plunning Model. Report LR574. (Crowthorne. Berkshire, England: Transport and Road Research Laboratory, 1973.)
was capable of evaluating a range of strategic transport policy options for the Greater London area. These included various pricing options on both roads and public transport networks, in particular, motorway schemes that tended to take the form of a system of rings and radials. It was quickly apparent that the model could not be simple enough to be evaluated in purely mathematical terms; a computational approach was needed throughout. It was then clear that rather than deal with a numerical approximation to a continuous model, of the kind in the present book, it would be better to define its elements in a discrete form, with a finite number of ring and radial transport routes. The key simplification was to assume radial symmetry: not only did this reduce the computational problems, but it clarified some of the conceptual issues. Important aspects of the model were the demand side (how numbers of trips and their split between modes would vary with transport costs and over time) and the supply side (how traffic volumes would influence travel times and costs); discussion of both of these groups of relationships was explicitly excluded from the present book. Apart from various issues that would have been equally difficult however the networks had been formulated, there were many problems specific to the assumed ring-radial network structure. For example, great care was needed in relating observed traffic speeds on actual routes to assumed speeds on the simplified networks; similar difficulties applied to walking, waiting, and travel times on public transport. Although the model was used to provide evaluations of a very wide range of policies, in a way that would have been very difficult with a full network model, it was not the easy option that it might have appeared at the outset. Nevertheless, despite the fact that the work received a mixed reception at the time, I believe that it showed the feasibility of using simplified networks for providing answers about real systems. So to sum up, one should not go to this book for ready-made answers, but it is nevertheless a stimulating one, with insights into many aspects of real systems. JOHN TANNER
Half Way Tree Wellingtonia Avenue Berkshire, RGII 6AE England
7bmspn. Res..A, Vol. 24A. No. 5. pp. 397-398. 1994 Printed in Great Britain.
BOOK REVIEW
Urban Spatial Traffic Patterns. Rodney Vaughan. Pion Ltd., 201 Brondesbury Park, London NW2 5JN, England, 1987. 334 pp. ISBN 0-85086-122-S. This book describes an approach to urban transport modelling developed in the 1960s and 1970s largely in Great Britain and largely by the late Reuben Smeed and his colleagues at University College London, and earlier at the Road Research Laboratory. The central theme is the development of simple models of urban areas that express the pattern of origins and destinations of trips and of the routes used between them. Sadly, Rodney Vaughan died shortly before this book was published, but his distinctive approach to the subject comes through clearly, and the book provides a fitting memorial. The traditional approach to urban transport modelling typically involves the collection of data for large numbers of traffic zones (or their estimation from the socio-economic characteristics of the zones), and for each link in the transport networks; these are then used to provide estimates of travel on each link of the networks, not only under current conditions, but more importantly, under hypothesized changes to the networks or to the traffic generators, and projected into the future. Even today, the mechanics of this process of collecting and processing large numbers of statistics limit the usefulness of the approach. By contrast, the approach in this book asserts that for descriptive purposes, and for some kinds of projections, much of this sophistication is unnecessary, and obscures fundamental simplicities. Thus, most towns have higher densities of activity nearer the center than further from it, and this can be represented by bell-shaped curves; origins of trips can be linked to destinations either at random (for small areas), or by simple assumptions about correlation; routes chosen can be represented, for example, by assuming shortest routes on hypothetical networks such as rectangular grids or rings and radials. The discussion is presented almost entirely in terms of the journey to work, but it is suggested that other kinds of travel could be dealt with similarly. Chapter 2 discusses the joint distribution of homes and workplaces. First, various models are considered for the distribution of homes; then, workplaces are discussed similarly; finally, consideration is given to how homes and workplaces are linked. The author makes no secret of his preferred model: the quadrivariate normal model. But he discusses other approaches, perhaps to such a degree as to detract
from the coherence of the discussion. The quadrivariate normal model is both mathematically elegant and tractable, and it represents the broad character of many towns in a plausible way. Essentially, it says that over the population of commuters, the x and y coordinates of their homes and workplaces follow a multivariate normal distribution; in the standard case, all means are set to zero, as are certain covariances. In this standard case, three parameters describe the whole process: the standard deviations of distances of homes and of jobs from the center, and the correlation between positions of homes and of jobs. This case is consistent with the observation that densities of homes and jobs can each be represented by an exponential falling-off with the square of distance from the center; it can be interpreted as incorporating a gravity model in which the deterrence function falls off exponentially withthe square of the distance travelled. Chapter 3 discusses the structure of transport networks and uses the concept of a ‘routing system’ to describe how a traveller gets from A to B on a given network. Again, much of the material is drawn from the literature, and while useful for reference has little direct bearing on the main argument. There is much here to interest the theoretician concerned with geometrical probability, and considerable insight into how road systems operate on a broad scale, but the material includes much that will be unfamiliar to someone accustomed only to conventional techniques. Matters discussed include route factors (travel distance divided by straight line distance), number of intersections, and optimum network geometry (grids, ring/radial, and so on). Chapter 4 applies the network and routing systems of Chapter 3 to the trip patterns of Chapter 2 to derive travel intensities (distance travelled per unit area), and traffic flows on links of the network, both primarily as functions of distance from the center. It then introduces the fascinating concept of ‘route crossings’: the number, proportion, or density of pairs of trips whose paths cross, for a given routing system on a given network. Chapter 5 calculates various averages or aggregates over the city as a whole, with particular reference to average distance travelled and to the number of crossings of pairs of commuters’ paths. For convenience, most of the algebra is confined to the ‘Smeed City,’ in which homes and workplaces are independently and randomly distributed in a circular area.
397
398
Book Review
Chapter 6 applies the results of earlier chapters to various issues. These include the possible trade-off between networks and routing systems which reduce distances travelled, and those that reduce the number of journeys whose paths cross each other; the optimum placing of ring roads; and the influence of home and job locations on amount and location of travel. Throughout the book, numerous references are quoted: 379 in all. Of these, about one-half are from the 1960s or earlier; few are from the 1980s. I think it is fair to say that most use is made of the earlier references; certainly I felt that in respect to my own work some early papers were given too much emphasis, but that the author had not really absorbed the lessons to be learned from some later papers. It seems to me that this book, and the approach that it exemplifies, is most valuable in introducing transport planners and engineering of the future to some of the broader aspects of their subject. I would be unhappy if practitioners, despite being well versed in the latest computerized modelling techniques, had no feel for how traffic behaves on the large scale, or of ideas such as routing systems and route crossings. Not that these ideas can necessarily be applied directly- the real world is usually too complicated for that- but they may encourage us to think about problems in a simplified way, and not fail to see the forest for the trees. Having said that, the mathematics in this book may deter some: they are not particularly difficult, but they are rather pervasive. I would also encourage the more academic sort of practitioner to dip into the book, not so much to find specific techniques, but rather to find new approaches to problems. At a detailed level, I found a few trivial printing or editing errors, but no errors of substance. A good deal of the writing is obscure or long-winded, but one can usually follow the argument without too much difficulty. Perhaps I may be permitted to use my personal involvement in the CRISTAL modelt [somewhat misdescribed by Vaughan on page 3061 to illustrate the difficulties of using models of this kind to illuminate real issues. The objective was to develop a model that TJ. C. Tanner, L. Gyenes, D. A. Lynam, S. V. Magee, and A. H. Tulpule, Development and Culibmtion of the CRISTAL Transport Plunning Model. Report LR574. (Crowthorne. Berkshire, England: Transport and Road Research Laboratory, 1973.)
was capable of evaluating a range of strategic transport policy options for the Greater London area. These included various pricing options on both roads and public transport networks, in particular, motorway schemes that tended to take the form of a system of rings and radials. It was quickly apparent that the model could not be simple enough to be evaluated in purely mathematical terms; a computational approach was needed throughout. It was then clear that rather than deal with a numerical approximation to a continuous model, of the kind in the present book, it would be better to define its elements in a discrete form, with a finite number of ring and radial transport routes. The key simplification was to assume radial symmetry: not only did this reduce the computational problems, but it clarified some of the conceptual issues. Important aspects of the model were the demand side (how numbers of trips and their split between modes would vary with transport costs and over time) and the supply side (how traffic volumes would influence travel times and costs); discussion of both of these groups of relationships was explicitly excluded from the present book. Apart from various issues that would have been equally difficult however the networks had been formulated, there were many problems specific to the assumed ring-radial network structure. For example, great care was needed in relating observed traffic speeds on actual routes to assumed speeds on the simplified networks; similar difficulties applied to walking, waiting, and travel times on public transport. Although the model was used to provide evaluations of a very wide range of policies, in a way that would have been very difficult with a full network model, it was not the easy option that it might have appeared at the outset. Nevertheless, despite the fact that the work received a mixed reception at the time, I believe that it showed the feasibility of using simplified networks for providing answers about real systems. So to sum up, one should not go to this book for ready-made answers, but it is nevertheless a stimulating one, with insights into many aspects of real systems. JOHN TANNER
Half Way Tree Wellingtonia Avenue Berkshire, RGII 6AE England