Traffic growth gives rise to increased highway congestion which, in turn, can influence traffic growth, particularly in or near urban areas. This mutual dependency is examined using equilibrium models of interacting travel markets. As a special case of the general model the author examines interactions in which public transport costs are subject to external economies of scale. He also illustrates the conditions under which highway investments may be counterproductive and lead to an increase in the cost of both public and private transport.
Huw C.W.L. Williams (Fri,) studied this question.