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Air Quality

Transportation Conformity

General Information

What is Transportation Conformity?

Transportation conformity is a process that links a state's air quality efforts with its transportation planning activities. It requires multiple agencies and groups to work together to ensure that federal transportation dollars go to projects that are consistent with a state's air quality goals. The federal government requires transportation conformity in all geographic areas that fail to meet one or more National Ambient Air Quality Standards.

As part of transportation conformity, emissions analyses must be conducted for every proposed transportation plan, program or project in a nonattainment area. An emission analysis estimates the mobile source emissions that will likely result if a project is carried out. Analyses use the latest planning assumptions, traffic modeling, and federally approved emissions models.

The process is meant to ensure that new projects will not produce new air quality violations, worsen existing violations, or delay the timely attainment of air quality standards. For a project to receive federal funds, it must be shown that emissions resulting from it will not exceed the area's emissions budget. The budget is part of the state's air quality plan, also known as the State Implementation Plan (SIP). The federal government cannot fund or approve transportation plans, programs or projects that do not conform to the SIP.

As the state's air quality planning agency, DHEC developed and is responsible for updating South Carolina's Transportation Conformity SIP in consultation with transportation agencies and air quality partners.

  • The federal Clean Air Act (CAA) requires that air quality in every state meet health-based National Ambient Air Quality Standards. States that are designated as nonattainment areas by the EPA for Ozone (O3), carbon monoxide (CO), particulate matter (PM10 and PM2.5), or nitrogen dioxide (NO2) are required to revise their state implementation plan (SIP) to ensure that the standards are attained and maintained. Nonattainment states are also required to implement transportation conformity for future projects. As part of this requirement, they must adopt interagency consultation procedures either by regulation or as a memorandum of agreement (MOA) between parties and incorporated into the Transportation Conformity SIP.
  • The revised Attainment SIP includes the necessary control measures to ensure that the air quality standards are attained. It requires the development of a "motor vehicles emissions budget" (MVEB). The budget establishes a ceiling for on-road mobile source emissions. Transportation conformity enforces the MVEB.
  • The CAA requires interagency consultation between DHEC, the U.S. Environmental Protection Agency, the U.S. Department of Transportation, the S.C. Department of Transportation, metropolitan planning organizations (MPOs), and applicable transportation planning agencies.

Areas designated as nonattainment or maintenance areas must have approved interagency consultation procedures in place to determine conformity of transportation plans, programs, and projects. This is to avoid a lapse and the possible imposition of federal highway funding sanctions.

Section 175 of the CAA sets forth the elements of a maintenance plan for areas seeking redesignation from nonattainment to attainment:

  • The plan must demonstrate continued attainment of the applicable national ambient air quality standards for at least 10 years after the attainment redesignation has been approved.
  • Eight years after the redesignation, the area must submit a revised maintenance plan demonstrating that attainment standards will continue to be met for the decade following the initial 10-year period.
  • Each plan must contain the contingency measures necessary to assure that the area will promptly correct any violation of the standard that occurs after the area is redesignated as an attainment area.
  • At a minimum, all SIP measures relating to control of the air pollutant in question must be implemented.

South Carolina's Status

Currently, the only nonattainment area subject to transportation conformity in South Carolina is the Rock Hill-Fort Mill Area Transportation Study (RFATS) Metropolitan Planning Organization (MPO) in York County. This is due to the area's inclusion in the Charlotte Metropolitan Statistical Area (MSA). The Charlotte MSA violated the 1997 8-hour ozone standard. (For more information, visit the Rock Hill-Fort Mill Area Transportation Study website.)

However, in the future, EPA continues to review and in some instances strengthen the National Ambient Air Quality Standards. To make the transportation conformity process more efficient in the future, all parties involved in the interagency consultation process should familiarize themselves with the state's transportation conformity SIP.


For more information please contact the Bureau of Air Quality at (803) 898-4123 or by email.