Transportation conformity ensures that transportation plans do not degrade air quality and that they conform to air quality State Implementation Plans (SIPs).
For areas that are not -- or previously were not -- in attainment of Federal air quality standards, regional councils of government must project future population and vehicle travel growth resulting in motor vehicle emissions. The Air Division then uses an EPA transportation/air quality model to project future motor vehicle emissions, and compares these emissions to motor vehicle emission budget allowances contained in the applicable SIP(s). Emissions resulting from transportation plans must not exceed the emission budgets in the applicable SIP.
The process is detailed in Colorado Air Quality Control Commission Regulation Number 10, Criteria for Analysis of Transportation Conformity: