The Contribution of Suppressed and Induced Traffic in Highway Appraisal, Part 1: Reference States

In this paper and its sequel we investigate the contribution of suppressed and induced traffic to emissions and user benefits derived from new roads and capacity changes under different pricing regimes. Equilibrium transport models are used to appraise selected highway and pricing policies over the period 2000–20. Here we examine the suppression of demand through increasing congestion in two reference states against which capacity changes will be appraised in the second paper. The first state is the consequence of ‘doing nothing’ over the twenty-year period; the second corresponds to the imposition of marginal cost (congestion) pricing on network links. Taking both a network model of Cardiff, and a single representative link (SRL) model, we estimate the effect of suppressed demand on the emission of: carbon monoxide, hydrocarbons, nitrogen oxides, particulates, and carbon dioxide. We show both analytically and numerically that, by reducing effective growth and moderating speeds, the suppression of traffic can have significant implications for vehicle emissions in the peak period, the effect varying in proportion to the elapsed time from the base year. At a travel time elasticity of −0.25, congestion pricing is shown to make an important contribution to containing all emissions, reducing carbon monoxide and carbon dioxide peak volumes by a quarter to a third of their unpriced level by the year 2010.