A suburb is a residential area, either existing as part of a city or urban area (as in Australia, New Zealand and the United Kingdom), or as a separate residential community within commuting distance of a city (as in the United States and Canada). Some suburbs have a degree of administrative autonomy, and most have lower population densities than inner city neighborhoods. 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. Suburbs tend to proliferate around cities that have an abundance of adjacent flat land. Any particular suburban area is referred to as a suburb, while suburban areas on the whole are referred to as the suburbs or suburbia, with the demonym for a suburb-dweller being suburbanite. Colloquial usage sometimes shortens the term to burb.