Car lights. Time exposure photograph of light trails from cars on a motorway at dusk; in the background are the tower blocks of a city. Photographed on the San Bernardino Highway in California, USA, with the city of Los Angeles in the background. Los Angeles has a low population density, making public transport unviable and encouraging people to use their cars. The cars produce airborne pollutants which are trapped over Los Angeles by the hills around the city. These pollutants react with sunlight to form smog; the deep red colour of the sky in this image is partly due to this smog.