Transportation and the Urban Structure

Urbanization involves an increased number of trips in urban areas. Cities have traditionally responded to the growth in mobility by expanding the transportation supply by building new highways and transit lines. This has mainly meant building more roads to accommodate an ever-growing number of vehicles. Several urban spatial structures have accordingly emerged, with the reliance on the automobile being the most important discriminatory factor. Four major types can be identified at the metropolitan scale:

·         Type I – Completely Motorized Network. Representing an automobile-dependent city with limited centrality and dispersed activities.

·         Type II – Weak Center. Representing the spatial structure where many activities are located in the periphery.

·         Type III – Strong Center. Representing high-density urban centers with well developed public transit systems.

·         Type IV – Traffic Limitation. Representing urban areas that have implemented traffic control and modal preference in their spatial structure. Commonly, the central area is dominated by public transit.

·         There are different scales where transportation systems influence the structure of communities, districts, and the whole metropolitan area. For instance, one of the most significant impacts of transportation on the urban structure has been the clustering of activities near areas of high accessibility.

·         The impact of transport on the spatial structure is particularly evident in the emergence of suburbia. Although many other factors are important in the development of suburbia, including low land costs, available land (large lots), environmental considerations (clean and quiet), safety, and car-oriented services (shopping malls), the spatial imprint of the automobile is dominant. Suburban developments have occurred in many cities worldwide, although no other places have achieved such a low density and automobile dependency than in North America. The automobile is also linked with changes in street layouts. While older parts of cities tend to have a conventional grid layout, from the 1930s, new suburbs started to be designed in a curvilinear fashion, which included some cul-de-sacs (dead ends). By the 1950s, the prevailing design for new suburbs was privileging cul-de-sacs. Although the aim was to create a more private and safe environment, particularly in cul-de-sac sections, the outcome was also a growing sense of isolation and car use.

·         With the expansion of urban areas, congestion problems, and the increasing importance of inter-urban movements, the existing structure of urban roads was judged to be inadequate. Several ring roads have been built around major cities and became an important attribute of the spatial structures of cities. Highway interchanges in suburban areas are notable examples of clusters of urban development that have shaped the multicentric character of many cities. The extension (and the over-extension) of urban areas have created what may be called peri-urban areas. They are located well outside the urban core and the suburbs, but are within reasonable commuting distances; the term “edge cities” has been used to label a cluster of urban development taking place in suburban settings.