On the formation of travel demand models and economic evaluation measures of user benefit
This paper examines a variety of issues within the context of two main themes: the formation of travel demand and economic evaluation measures which are mutually consistent within a theory of rational choice; and a consideration of the structure of models which are representations of the trip decision process over several dimensions: location, mode, and route. Random utility theory is invoked to explore both the role and properties of composite costs or index prices in the 'recursive' approach to the structuring of travel choice models, and their significance in the economic evaluation problem. It is shown that the specification of these costs must be made very precisely, with respect to the demand model form chosen, in order to retain the underlying assumption that the traveller is an optimal decisionmaker. It is argued that the structure of 'simultaneous' models currently in use is inconsistent with the form of utility function assumed to generate these models. Furthermore, it is shown that the 'simultaneous' and 'recursive' forms are special cases of a more general choice model structure which takes specific account of correlation or 'commonality' of trip attributes. A number of applications are discussed in which consistent demand models and perceived user benefit measures are constructed. These include the formation of strategic transport planning models and of models for mixed-mode, multimode, and multiroute systems. The formalism allows definitive answers to be given to a number of problems of current interest in transportation planning, which have been incorrectly or incompletely treated.