The form of our cities to a large extent reflects the transport technologies which were dominant at different stages of their development. The traditional dense European city can be viewed as a product of the external economies of agglomeration in an era of slow and costly transportation. The coming of the railways permitted increased residential decentralisation-in particular to areas within walking distance of the new stations-and marked the start of the trend towards lower residential densities in central areas of cities such as London. Bus transport brought greater flexibility to the suburbanisation process. The era of the motor car added a further dimension, producing fundamental changes in the character of many European cities and allowing the creation of the low density, weak-centred cities of North America.
Increasing concern about the implications-especially for environmental quality-of the seemingly inexorable growth in car usage has generated particular interest in how planning policies might utilise the apparently greater transport efficiency of traditional urban forms to at least moderate the pressures for growth in travel demand. This interest was given particular impetus by the work of Kenworthy and Newman (1989) and it has been taken up in particular in the national land use policy frameworks of the Netherlands (Huut, 1991) and, more recently, the United Kingdom (Departments of the Environment and Transport, 1994). The low prospective rates of population growth in western Europe mean however that here at least the prime interest has to be in the implications of alternative options for accommodating incremental growth, rather than in the possible form of entirely new major settlements.
A range of cross-sectional evidence is available on how density and urban size influence travel behaviour, some of which is reviewed later in this chapter. However, the economic and social interactions, and potential market responses, to new policies made possible by modern transport technology mean that land use policies and their implications need to be considered at the level of the city