Transportation is also the major contributor to urban air pollution such as photochemical smog.
In North America, and increasingly in other parts of the world, the automobile is the most popular form of urban transportation and the largest source of these emissions. Combined with the fact that many automobile trips are made with a single occupant in the vehicle, the automobile is a very inefficient means of urban transportation. Public transit in the form of buses, trains, streetcars, or subways is a far more efficient means of transportation than the single occupant vehicle, using less energy and producing fewer emissions of greenhouse gases and other pollutants per passenger mile. Car pools are another way to reduce transportation energy use and CO2 emissions. The most fuel efficient forms of transportation with respect to energy use and CO2 emissions are non-mechanized modes such as walking and bicycling.