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 private automobile is the most popular form of urban transportation and the largest source of these emissions. The freedom and flexibility of car travel, as well as low fuel prices in many jurisdictions have combined to make the automobile a popular choice for urban travel. Unfort unately, automobile use today tends to be a very inefficient means of transportation. Combined with the fact that many automobile trips are made with a single occupan t in the vehicle, the automobile is a very inefficient means of urban transportation, using far more energy, and emitting far more CO2 and other pollutants per passenger mile traveled than other alternatives. Car pools are another way to reduce transportation energy use and CO2 emissions.