Source: adapted from Muller, P.O. (1995) "Transportation and Urban Form: Stages in the Spatial Evolution of the American Metropolis", in S. Hanson (ed.) The Geography of Urban Transportation, 2nd Edition, New York: Guilford, p. 29.
- I - Walking-horsecar era (1800-1890). This era was characterized by the absence of urban mass transportation. People had access to locations that could be reached (walked) in less than 45 minutes (4 to 6 km). Urban densities were very high as the available space was strongly constrained by accessibility. All economic activities were concentrated in a central node along with residential areas. Reduced mobility (pedestrian area) accounted for this imposed concentration. The horsecar made its debut, which allowed the development of the first urban transit corridors. From the 1850s, railways enabled radial development adjacent to railway stations, especially in Europe and in older American cities (e.g. New York).
- II - Streetcar era (1890-1920). The development of the first forms of urban mass transportation improved accessibility and enabled cities to expand along main tramway (streetcar) lines, creating full fledged transit corridors. In tripling the speed of urban transport, electric streetcars expanded the spatial structure of cities. This permitted the emergence of a specialized downtown area with commercial and service activities. In Europe, tramway lines tended to expand towards long established adjacent towns, which were swallowed and integrated in the expanding city. This allowed the development of urban activities beyond city limits. The emergence of commercial centers along transit corridors became apparent because a growing share of the population having access to trolleys, decided to relocate outside the city limits. This reinforced social stratification and favored the emergence of neighborhoods differentiated by socioeconomic status. Less fortunate people, limited in their mobility, tended to remain in central areas while the wealthier class relocated in the early suburbs. The first suburban railroads entered in service and specialized industrial districts started to take shape.
- III - Automobile era (1920-1945). Motorized transportation, mainly buses and cars, radially expanded cities, further improving accessibility. This technical innovation had a substantial impact on spatial organization. Initially, only wealthy classes could afford their own automobiles which were used mainly for recreational purposes. The private car was linked with the emergence of the firsts low density suburbs with increased ethnic and economic segregation. This went on par with the decentralization of commercial and industrial activities. It is during this phase that European and North American urban development started to diverge. In order to facilitate the diffusion of the road as a mode of urban transportation in the United States, several oil and car companies bought and dismantled tramway systems. For instance, in 1938 General Motors and Standard oil bought the Pacific Electric Railway of Los Angeles, dismantled it and replaced tramways with buses. Consequently, the influence of streetcars in the urban development in North America was being removed, while it endured in many European cities. This was a contributing factor in the divergence of the spatial structure.
- IV - The highway era (1945-2000). The post World War Two era saw the large diffusion of the automobile with a growth of individual mobility. Highways favored the extension of full fledged suburbs, especially in North America. This process also took place in Europe, but to a lower extent and involved higher densities along existing transit corridors. No significant new urban transit technologies emerged during this era, but improvements in transport infrastructures significantly increased accessibility. Residential and employment decentralization was thus accentuated. Also, several sub-centers emerged to serve suburbs, a process favored by the construction of ring roads around metropolitan areas. The development of new highways which circled urban perimeters promoted the agglomeration of commercial, distribution and manufacturing activities around high accessibility clusters in suburban areas.