The authors in this issue

This paper considers the main road-traffic parameters that determine air pollution, i.e. the total volume of traffic, road speeds and the composition of the vehicle fleet. Changes in the amounts of pollutants emitted, and the importance of each of the three parameters, have been computed by using a traffic assignment model, which also represents emission factors. The types of policies that may be implemented to reduce the environmental impact of transport are then considered. The study demonstrates, for example, that the impact of a deterioration in traffic conditions is limited in comparison with the effect of forecast increases in traffic and improvements in the environmental performance of vehicles. As a consequence, if cities and urban transport are to achieve sustainable development, urban expansion must take place in a controlled way.