Cars have changed the way cities are organised. This column uses a sample of 123 cities in 57 countries to show that car ownership reduces the density of people and employment by allowing low-density expansion into the urban periphery. The findings have implications for cities in developing countries, where strong car ownership increases are expected in the near future. If these countries follow a similar path, their cities will be more spread out. This is likely to cause more traffic congestion, as lower-density areas tend to be more car-dependent.