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Transportation Conformity

Section 176 (c) of the federal Clean Air Act (CAA) requires the transportation planning processes for states to be consistent with air quality goals in state implementation plans (SIPs) that United States Environmental Protection (U.S. EPA) approves for areas with “nonattainment” or “maintenance” air quality designations under a National Ambient Air Quality Standard (NAAQS). IDEM works to ensure consistency in Indiana’s transportation planning process according to U.S. EPA guidance. As such, IDEM routinely coordinates with appropriate federal, state and local agencies including but not limited to U.S. EPA, the Indiana Department of Transportation's Transportation Planning Division, and metropolitan planning organizations (MPOs) representing Indiana’s urbanized areas, to ensure transportation activities in our state will not cause new air quality violations, worsen existing violations, or delay timely attainment of the NAAQS.

IDEM’s transportation conformity process requires new or expanded local or state transportation projects to clearly demonstrate they fall below the motor vehicle emission budgets (MVEBs) included as maximum allowable emissions from mobile sources in U.S. EPA-approved SIPs such as requests for redesignation and maintenance plans. If model-projected emissions of all transportation activities included in an area’s transportation plan are below the MVEBs included in the approved SIP, the transportation plan is said to “conform,” and transportation projects included in the plan can proceed towards completion.

  • Federal Register Notice Concerning U.S. EPA Approval and Promulgation of the Transportation Conformity State Implementation for Indiana (August 17, 2010) [ZIP]
    • Transportation Conformity State Implementation Plan Submittal, Transmittal Letter (June 4, 2010) [ZIP]
      • Attachments Detailing Transportation Conformity Process and Procedures [ZIP]
        • Attachments A-G
        • Attachments H-N
        • Attachment O