A suburb is a residential area or a mixed use area, either existing as part of a city or urban area or as a separate residential community within commuting distance of a city. In most English-speaking regions, suburban areas are defined in contrast to central or inner city areas, but in Australian English, "suburb" has become largely synonymous with what is called a "neighborhood" in other countries. 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 France, Arabia, most of the United States, and Canada, many suburbs remain separate municipalities or are governed as part of a larger local government area such as a county.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 an 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.