Historically, Americans mainly lived rural lives, with a few important cities of moderate size. Following World War II, however, Americans began living in increasing numbers in the suburbs, belts around major cities with higher density than rural areas, but much lower than urban areas. This move has been attributed to many factors such as the automobile, the availability of large tracts of land, the convenience of more and longer paved roads, increasing violence in urban centers (see white flight), and the lower expense of housing.