From the 1950s, the growth of suburbs was mainly taking place adjacent to major
road corridors, leaving a lot of vacant/farm land in between. Later, intermediate spaces
were gradually filled up, more or less coherently. Highways and ring roads, which circled
and radiated from cities, have 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
(if not to entirely new locations abroad) and the activity system of cities has been
considerably modified. Different parts of a city have a different dynamism depending on
its spatial pattern. These changes have occurred differently according to the variety of
geographical and historical contexts, notably in North America and Europe. In addition,
North American and European cities have seen different changes in urban density.
However, two processes have a substantial impact on contemporary urban form s: