A suburb (or suburban area or suburbia) is a mixed-use or residential area. It can exist either as part of a city/urban area and can often have a large population of employed people. In some metropolitan areas, they exist as separate residential communities within commuting distance of a city. 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 are less densely populated than inner-city neighborhoods within the same metropolitan area, and most residents routinely commute to city centers or business districts via private vehicles or public transits; 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.