Road capacity change and its impact on traffic in congested networks: evidence and implications

This article reviews explanations of, and international empirical evidence for, 'induced' traffic as a result of increased road capacity and 'suppressed' traffic as a result of decreased road capacity. In essence, the former refers to new traffic appearing as a result of new road construction, while the latter refers to traffic disappearing as a result of road closure. Despite problems with the available data and their measurement, it is concluded that - with the caveats of either pre-existing congestion in the case of capacity increases or no spare capacity in the case of capacity decreases - the weight of evidence indicates that induced and suppressed traffic are indeed real phenomena. It is argued that the link between traffic and road capacity is therefore far more complex than previously understood. The implications this has for both urban passenger transport planning practice and policy formulation are discussed.