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Air Quality Program

Transportation Conformity

The federal Clean Air Act (CAA) establishes Transportation Conformity to ensure that air quality planning and transportation planning in nonattainment and maintenance areas are linked. Attainment and maintenance State Implementation Plans (SIPs) establish Motor Vehicle Emissions Budgets (MVEBs) to limit emissions from motor vehicles. A transportation conformity determination ensures that emissions from an area’s transportation system meet the limits in the SIP and thus achieve air quality goals.


Transportation Conformity ensures that federal transportation funds support highway and transit activities that are consistent with the SIP.  Under the CAA, transportation activities must not

  • Produce new air quality violations
  • Increase the severity and frequency of existing air quality violations
  • Delay timely attainment

Two prominent features of Transportation Conformity are Transportation Conformity Determinations and Interagency Consultation.

Transportation Conformity Determinations:

A Transportation Conformity determination demonstrates that emissions from the regional transportation system in a long range Transportation Plan or a shorter-term Transportation Improvement Programs (TIP) meet the MVEBs established by an attainment or maintenance plan and thus conforms to the SIP. The responsibility for a conformity determination depends on the location of the nonattainment or maintenance area.

Organizations responsible for Conformity Determination:



Metropolitan Planning Organization (MPO)

Federally designated Metropolitan Planning Areas

Regional Transportation Planning Organization (RTPO)

Urban and rural areas other than Metropolitan Planning Areas

Interagency Consultation

State and Federal regulations require formal interagency consultation for developing SIPs, TPs, TIPs, and transportation conformity determinations. The participating agencies in Washington State include:

  • U.S. Environmental Protection Agency
  • Federal Highway Administration
  • Federal Transit Administration
  • Metropolitan Planning Organizations
  • Washington State Department of Transportation
  • Washington State Department of Ecology

Transportation Conformity Regulations:

Transportation Conformity Guidance, Policies, and Resources:

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