A D Pearman

SCENARIO CONSTRUCTION FOR TRANSPORT PLANNING

This paper argues that the types of uncertainty having the most serious consequences for transport planning are not captured by existing modelling procedures, which typically rely on a limited amount of sensitivity testing of plans based on extrapolative forecasts. It recommends as an alternative scenario‐based planning, an approach which is finding increasing application in private sector policy formation. The paper summarises the origins of scenario planning and the gradual broadening in its range of application, before going on to suggest methods of scenario construction suitable for transport planning applications. It considers the arguments for and against associating probabilities with scenarios and also addresses the problems of formulating transport strategies and evaluating them within the context of a scenario‐based approach to strategy choice.

Disaggregate and aggregate car ownership forecasting in Great Britain

Abstract The first part of this paper argues that the variable ‘cars per household’, as well as the proportion of households owning more than a given number of cars, should be modelled by sigmoid functions with finite saturation levels. Next, a summary and critique of the work of the Regional Highway Traffic Model, group is presented. Problems relating to choice of functional form, interaction of effects, and the usefulness of the explanatory variable ‘Car Purchasing Income’ are discussed. This leads to an analysis of the concept of a saturation level, with a caution against using external data to determine it when the true functional form of the car ownership growth path is unknown. Following this, Transport and Road Research Laboratory work is discussed and its limitations highlighted. Data for the period 1953–1974 is reworked yielding the conclusion that there is no evidence that growth in this period has been non-logistic. This implies that previous overpredictions by TRRL were due to incorrect parameter estimates, rather than the choice of the wrong model form.

ROAD PRICING - SOME OF THE MORE NEGLECTED THEORETICAL AND POLICY IMPLICATIONS

The conventional argument on urban traffic congestion is examined; some doubts are expressed about its theoretical basis and its value as a guide to policy making. Little is known of the extent to which society should tolerate congestion on urban highways, and less is known of the balance between the expenditure of money on the road system and the expenditure of time in transit. However, despite some doubts, it appears that the basic theory, applied to a very simple situation, is technically correct. There appears to be some foundation in the argument that the main impact of road pricing is to reduce the welfare of low income motorists and increase that of high income road users. The empirical basis for the conventional view that the elimination of traffic congestion is worthwhile is questioned. An argument is put forward to support the view that congestion is a positively helpful way of dealing with a resource allocation problem. Quite often people would prefer to queue rather than pay higher prices for expanded services which would remove waiting. (TRRL)

Some outstanding problems in the causal modelling of car ownership at the local level

The authors wish to acknowledge the support of the Social Science Research Council in financing their ongoing research into car ownership forecasting and to thank John Bates, Mick Roberts and Hugh Gunn for providing the Maximum Likelihood computer programme and additional help.