urban sprawl

undesigned and uncontrolled spread of a city to the surrounding environs

Urban sprawl, also known as suburban sprawl, is the spreading out of a city and its suburbs over rural land at the fringe of an urban area. Residents of sprawling neighborhoods tend to live in single-family homes and commute by automobile to work. Low population density is an indicator of sprawl. Urban planners emphasize the qualitative aspects of sprawl such as the lack of transportation options and pedestrian friendly neighborhoods. Conservationists tend to focus on the actual amount of land that has been urbanized by sprawl.
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Noun
1. an aggregation or continuous network of urban communities
(synonym) conurbation
(hypernym) urban area, populated area
(hyponym) subtopia