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Transportation and Climate Change Clearinghouse

Federal, State, and Local Policy Developments

Clean Air Act


Since the 1950's, the scientific community has agreed that vehicle exhaust fumes are a major contributor to air pollution in urban areas. This knowledge led to widespread State and Federal regulatory activity, which eventually resulted in the passage of the modern Clean Air Act (CAA) in 1970. There are currently National Ambient Air Quality Standards (NAAQS) for six pollutants. Those for which transportation sources are significant include carbon monoxide, particulate matter, and ozone. The most persistent pollution problem is ground level ozone which is not emitted directly but is produced in the air during a complex photochemical reaction involving volatile organic compounds (VOCs) and oxides of nitrogen (NOx) contained in automobile exhaust emissions and other similar types of gases.

Relationship to Climate Change

Transportation-related emissions adversely affect air quality and the quality of life in communities. However, there are many transportation strategies currently employed to reduce emissions from motor vehicles including transportation system management programs to reduce congestion (i.e. traffic flow improvements), which reduce emissions by promoting efficient travel movement, and various travel demand management programs (i.e. ride sharing, transit and pedestrian and bicycle programs) that minimize the aggregate number of single occupancy trips and miles traveled.

These transportation strategies are targeted to reduce the amount of criteria air pollutants, but also have ancillary benefits related to the reduction of greenhouse gas emissions. Despite substantial progress in reducing emissions, the impact of mobile source air pollution continues to be large and emissions of carbon dioxide (CO2), the major greenhouse gas, remain unregulated. Transportation accounts for about a third of CO2 emissions in the U.S., and transportation emissions are among the fastest-rising among all emitting sectors. It is estimated that atmospheric concentrations of CO2 will almost double by 2100.

DOT's Role

Clean air is an important part of a healthy environment. Unfortunately, many industrial and transportation activities that sustain our economy also produce air pollutant emissions as by-products. Safeguarding our air from such contamination is an important priority of the U.S. Department of Transportation (U.S. DOT).

The Clean Air Act's conformity process establishes the major connection between the transportation and air quality planning processes. Conformity is a way to ensure that Federal funding and approval are given to those transportation activities that are consistent with air quality goals. Although the conformity process is intended to ensure that transportation plans and programs do not delay timely attainment of the standards for criteria pollutants, it also provides, as many transportation strategies do, ancillary benefits related to the reduction of greenhouse gas emissions.

Additional Information