...since the traffic problem is to be found largely within the cities, towns and suburbs, and the problem is, therefore, one of social welfare in general, the transport economist, as it happens, is not the man to consult. For the transport economist addresses himself in the main to the mutual frustration experienced by the increasing traffic; that is, only to those external diseconomies that additional traffic impose on existing traffic, and not with costs imposed by traffic as a whole on the rest of society.