We examine the bias involved in adopting inefficient reference states as the basis for appraising road investments. User benefits from highway schemes are estimated with respect to both unpriced networks and ones subject to marginal cost (congestion) pricing on all links. We show from the analysis of several policies defined on a Cardiff network how the difference between these measures depends on: the parameters of the demand and user cost functions and the highway policy under test. While the value of an investment under free use will typically exceed that under congestion pricing, under certain conditions, the reverse may occur. We have used "single link", "binary link", and equilibrium network models to interpret this behaviour in terms of the quantity and composition of generated traffic associated with capacity expansion policies and, in particular, the contributions from route substitution and induced traffic. (Author/publisher).
Abstract