Sustainability means capable of being continued. Sustainable transport means finding ways of
meeting transportation needs that are environmentally sound, socially equitable, and
economically viable in the long term.
To be economically sustainable, transport must be cost-effective and continuously responsive to
changing demands. Cost-effective transportation is not easily secured because travellers are not
directly confronted with many of the overall social costs associated with transportation. The
combination of externalities and a lack of pricing of road space often gives rise to traffic
congestion, a significant problem in many of the larger cities of the advanced economies (the net
social costs of traffic congestion are estimated to be between two and three per cent of GDP in
the advanced economies)1 and an acute problem in many third world megacities (notably
Bangkok, where costs could be as high as 8 per cent of regional gross product, and Mexico
City).