<P> A suburb is a mixed - use or residential area, existing either as part of a city or urban area or as a separate residential community within commuting distance of a city . In most English - speaking countries, suburban areas are defined in contrast to central or inner - city areas, but in Australian English and South African English, suburb has become largely synonymous with what is called a "neighborhood" in other countries and the term extends to inner - city areas . In some areas, such as Australia, China, New Zealand, the United Kingdom, and a few U.S. states, new suburbs are routinely annexed by adjacent cities . In others, such as Saudi Arabia, Canada, France, and much of the United States, many suburbs remain separate municipalities or are governed as part of a larger local government area such as a county . </P> <P> Suburbs first emerged on a large scale in the 19th and 20th centuries as a result of improved rail and road transport, which led to an increase in commuting . In general, they have lower population densities than inner city neighborhoods within a metropolitan area, and most residents commute to central cities or other business districts; however, there are many exceptions, including industrial suburbs, planned communities, and satellite cities . Suburbs tend to proliferate around cities that have an abundance of adjacent flat land . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table>

The growth of suburbs and urban sprawl were primarily facilitated by