The transportation sector is a predominant source of air pollution in the U.S. Particularly in urban areas, mobile sources – including passenger cars, trucks, buses, ships, locomotives and aircraft – and the fuels on which they run, collectively can produce 90 percent or more of local air pollution, including levels of ozone (smog) and particle pollution (soot). Vehicles can also produce a significant amount of toxic pollutants, as well as greenhouse gases and emissions that contribute to the formation of acid rain. While tremendous progress has been made in cleaning up mobile sources and fuels over the past four decades, additional emission reductions can and must still be achieved from this source category.