Mass transit projects proposed for federal funding must meet the requirements of the Clean Air Act in addition to the National Environmental Policy Act. The Clean Air Act contains detailed transportation “conformity” requirements the purpose of which is to ensure that federally-funded highway and transit projects conform to the applicable State (air quality) Implementation Plan (SIP). The SIP documents the measures that will be taken in a specific nonattainment area in order to attain the national air quality standards. The conformity requirements apply only in nonattainment and maintenance areas.
The conformity process is governed by a regulation issued by the U.S. Environmental Protection Agency (40 CFR Part 93). The EPA’s Office of Transportation and Air Quality has issued numerous guidance documents and materials related to the conformity process which can be found in the Conformity section of the following website: Environmental Protection Agency - Transportation Conformity.
EPA's Office of Air Quality Planning and Standards has responsibility for setting and revising national air quality standards and for designating areas that fail to meet those standards. Information on the National Ambient Air Quality Standards, including the status of the revised ozone and particulate matter standards, can be found at: Environmental Protection Agency - Air Quality Planning and Standards.
The Federal Highway Administration (FHWA) also has information on the conformity process on its Web site: Air Quality Transportation Conformity and has related information on its Web site regarding the Congestion Mitigation and Air Quality Improvement (CMAQ) Program. Many transit agencies have a special interest in the CMAQ program because over the years the program has been an important source of funding for transit projects in nonattainment and maintenance areas, supplementing FTA’s traditional funding programs. The current CMAQ program guidance can be found at this Web site along with related materials, such as tables showing annual CMAQ apportionments to the states and reports summarizing projects and activities funded with CMAQ funds in prior years.