The problem

Motor vehicles generate three major pollutants: hydrocarbons, nitrogen oxides (NO_x), and carbon monoxide (CO). Hydrocarbons react with NO_x in the presence of sunlight to form ground-level ozone (smog). NO_x contribute to the formation of acid rain, although sulfur dioxide (SO₂) is the primary contributor. Acid rain increases the level of pollutants in lakes, rivers, bays, and other waterways. CO, a colorless, odorless gas, can impair mental functions and visual perception and is deadly in high concentrations. Motor vehicles also emit large quantities of carbon dioxide (CO₂), which has the potential to trap Earth's heat and contribute to global warming.

Motor vehicle emission control devices have dramatically reduced pollutant emissions per vehicle during the past 20 years. But the number of cars and trucks on the road, and the number of miles they are driven, have doubled. Vehicles are now driven more than a trillion miles each year in the United States. This growth in vehicle travel is offsetting the progress achieved through improved vehicle emission controls. Cars and trucks release millions of tons of pollutants into the air each year.