The conceptual relationship between distance and urban density varies between European and North American cities. While density in North American cities has generally decreased through sub-urbanization and the creation of peripheral centers, in Europe, there was an extension away from the urban center. Furthermore, density in central areas of North American cities has generally declined given land-use changes such as industrial relocation and the reconversion of land use into parking lots. The radius of a North American city (r(NA)) is typically larger than the radius of a European city of a similar population (r(E)). Densities are higher in Europe than in North America.