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

Kurzfassung

Transport user benefits are of great importance in cost benefit analysis when appraising transport projects. Generally they have the greatest impact 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, consumer surplus measure takes a closed form (the “logsum”) that is easy to calculate. Furthermore, in cost-benefit analyses the change in consumer surplus between an initial and final state is needed, which 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 we describe a mathematical approach for a transport user benefits measure that corresponds to the consumer surplus and is universal for all travel demand models with constraints. We exemplarily derive the measure for a doubly constrained trip distribution. The applicability of the derived approach is shown by a small example.