Air pollution comes from many different sources: stationary sources such as factories and power plants; smaller, dispersed sources (known as area sources) such as dry cleaners and painting operations; mobile sources such as cars, buses, planes, trucks, and trains; and naturally occurring sources such as windblown dust, and volcanic eruptions. Mobile sources can be further subdivided into on-road sources such as automobiles, trucks and buses, and non-road sources such as construction and farm equipment, airplanes, boats, and trains. The CAA provides the principal framework for National, State, and local efforts to protect air quality from all man-made pollution sources.
The CAA requires that, in areas experiencing air quality problems, transportation planning must be consistent with air quality goals. This is determined through the transportation conformity process. In some areas, this process has forced State and local transportation officials to make tough decisions in order to meet both air quality and mobility goals. Where CAA goals were not being met, some State and local transportation officials have been challenged to find ways to reduce vehicle emissions by developing transportation plans, TIPs, and projects that will alter travel patterns, reduce the number of single-occupant vehicles, and make alternative modes of transportation (such as transit and bicycles) an increasingly important part of the transportation network.