Evaluating Transport User Benefits: Adjustment of Logsum Difference for Constrained Travel Demand Models

Transport user benefits are of great importance in cost–benefit analysis when transport projects are appraised. Generally, these benefits have the greatest effect on the results of cost–benefit analyses. It is common to adopt the consumer surplus for calculating transport user benefits. The consumer surplus measure is based on the underlying demand model and follows from the integration of the demand curve. If the popular logit model is used for forecasting travel demand, a consumer surplus measure takes a closed form that is easy to calculate. Furthermore, in cost–benefit analyses the change in consumer surplus between an initial and final state is needed; the change can be easily derived by the difference of the logsums of the two states. This logsum approach is proven and correct for travel demand models based on the logit model without multiple constraints. However, for travel demand models dealing with two or more sets of constraints, the logsum approach fails. In this paper, a mathematical approach is described for a transport user benefits measure that corresponds to the consumer surplus and is universal for all travel demand models with constraints. The measure for a doubly constrained trip distribution is derived. The applicability of the derived approach is shown by a simple example.