Urbanization is occurring in accordance with the development of urban transport systems, particularly in terms of their capacity and efficiency. Historically, movements within cities tended to be restricted to walking, which made urban mobility rather inefficient and time-consuming. Thus, activity nodes tended to be agglomerated, and urban forms compact with mixed uses. Many modern cities have inherited an urban form created under such circumstances, even though they are no longer prevailing. The dense urban cores of many European and East Asian cities, for example, enable residents to make between one third and two-thirds of all trips by walking and cycling. At the other end of the spectrum, the dispersed urban forms of most Australian, Canadian, and American cities, which were built more recently, encourages automobile dependency and are linked with high levels of mobility. Still, Chinese cities have experienced a high level of motorization, implying the potential of convergence towards more uniform urban forms. Many cities are also port cities with trade playing an enduring role not only for the economic vitality but also in the urban spatial structure, with the port district being an important node. Airports terminals have also been playing a growing role in the urban spatial structure as they can be considered as cities within cities.
The evolution of transportation has generally led to changes in urban form. The more radical the changes in transport technology, the more the alterations on the urban form. Among the most fundamental changes in the urban form is the emergence of new clusters in peripheral areas expressing new urban activities and new relationships between elements of the urban system. Many cities are assuming a polycentric form a change that is associated with new mobility patterns. The central business district (CBD), once the primary destination of commuters and serviced by public transportation, has been transformed by new manufacturing, retailing, and management practices. Whereas traditional manufacturing depended on centralized workplaces and transportation, technological and transportation developments rendered modern industry more flexible. In many cases, manufacturing relocated in a suburban setting, if not altogether, to entirely new low-cost locations offshore. Retail and office activities are also suburbanizing, producing changes in the urban form. Concomitantly, many important transport terminals, namely port facilities, and railyards, have emerged in suburban areas following new requirements in modern freight distribution brought in part by containerization. The urban spatial structure shifted from a nodal to a multi-nodal character, implying new forms of urban development and new connections to regional and global economic processes.
Initially, suburban growth mainly took place adjacent to major road corridors, leaving plots of vacant or farmland in between. Later, intermediate spaces were gradually filled up, more or less coherently. Highways and ring roads, which circled and radiated from cities, favored the development of suburbs and the emergence of important sub-centers that compete with the central business district for the attraction of economic activities. As a result, many new job opportunities have shifted to the suburbs, and the activity system of cities has been considerably modified. Depending on the economic sector they specialize in, cities and even different parts of a metropolitan area can be experiencing development at entirely different rates (or even decline), leading to a highly heterogeneous urban landscape. These changes have occurred according to a variety of geographical and economic contexts, notably in North America and Europe, as each subsequent phase of urban transportation developments led to different spatial structures. Sometimes, particularly when new modern urban road infrastructures are built, the subsequent changes in the urban form can be significant. Two processes had a substantial impact on contemporary urban forms:
· Urban sprawl has been dominant in North America since the end of World War II, where land is abundant, transportation costs were low, and where the economy became dominated by tertiary and quaternary activities. Under such circumstances, a strong negative relationship between urban density and automobile use emerged. In the context of cities with high automobile dependency, their built-up areas have grown at a faster rate than their populations, resulting in declining densities. In addition, commuting became relatively inexpensive compared with land costs, so households had an incentive to buy lower-priced housing at the urban periphery. Wherever there is motorization, a pattern of sprawl takes shape.
· The decentralization of activities resulted in two opposite effects. First, commuting time has remained relatively stable in duration. Second, commuting increasingly tends to be longer in terms of distance and made by using the automobile rather than by public transit. Most transit and road systems were developed to facilitate suburb-to-city, rather than suburb-to-suburb commuting. As a result, suburban highways are often as congested as urban highways.