Network effects and total economic impact in transport appraisal

It is sometimes claimed that transport infrastructure projects have network effects, which are not taken into account in the appraisal of these projects. This paper reviews the concept of network effects, relates this to transport appraisal practice, and links to the concept of 'total economic impact'. The limitations of transport modelling and appraisal in estimating total economic impact are reviewed. Good quality appraisals should be capable of picking up relevant network effects in the transport market, but the state of the art remains limited on the linkages between transport and the wider economy.