Policy and Technical Guidance for State and Local Transportation

This page contains policy guidance issued by EPA and/or the U.S. Department of Transportation to assist state and local transportation and air quality agencies implement the transportation conformity program. Policy guidance can be found on the following topics:

On this page:

Transportation Planning and Conformity Requirements

  • Baseline Year for the Baseline Year Test (in 40 CFR 93.119)
    This page provides a table showing the baseline year to be used for each NAAQS and the regulatory provision where it is found.
  • Transportation Conformity Guidance for the South Coast II Court Decision (pdf) (480 KB, November 2018, EPA-420-B-18-050)
    This document provides guidance to address how transportation conformity can be implemented in ozone nonattainment and maintenance areas affected by the February 16, 2018 decision from the U.S. Court of Appeals for the D.C. Circuit in South Coast Air Quality Management District v. EPA (“South Coast II,” 882 F.3d 1138). Specifically, this guidance describes how transportation conformity applies in areas that were nonattainment or maintenance for the 1997 ozone NAAQS (at the time that NAAQS was revoked) as well as designated attainment for the 2008 ozone NAAQS.
  • Transportation Conformity Guidance for 2015 Ozone NAAQS Nonattainment Areas (pdf) (201 KB, June 2018, EPA-420-B-18-023)
    This document provides guidance for meeting transportation conformity requirements in areas designated nonattainment for the 2015 ozone national ambient air quality standards.
  • Transportation Conformity Guidance for 2008 Ozone NAAQS Nonattainment Areas (pdf) (180 KB, July 2012, EPA-420-B-12-045)
    This document provides guidance for meeting transportation conformity requirements in areas designated nonattainment for the 2008 ozone national ambient air quality standards (“2008 ozone NAAQS”). Note: On December 23, 2014, the DC Circuit Court vacated EPA’s revocation of the 1997 ozone NAAQS for transportation conformity purposes only in NRDC v. EPA (12/23/2014). On March 6, 2015 EPA published a final rule revoking the 1997 ozone NAAQS for all purposes. (80 FR 12264) The final rule became effective on April 6, 2015. This final rule also established new attainment dates for the 2008 ozone NAAQS (in accordance with the 2014 DC Circuit Court decision) based on an area’s classification. The maximum attainment dates are: marginal areas - July 20, 2015, moderate areas – July 20, 2018, serious areas – July 20, 2021, severe areas – July 20, 2027, and extreme areas – July 20, 2032.
  • Transportation Conformity Guidance for 2012 PM2.5 Nonattainment Areas (pdf) (193 KB, November 2015, EPA-420-B-15-091)
    This document provides guidance for meeting transportation conformity requirements in areas designated nonattainment for the 2012 PM2.5 national ambient air quality standards.
    • Transportation Conformity Guidance on the Revocation of the 1997 Primary Annual PM2.5 NAAQS (pdf) (127 KB, October 2016, EPA-420-B-16-072)
      This guidance describes how the revocation of the 1997 primary annual PM2.5 national ambient air quality standard (NAAQS) will be implemented and when transportation conformity stops applying for this NAAQS. EPA committed to provide this guidance in its November 2015 guidance concerning conformity guidance for the 2012 PM2.5 NAAQS. Questions 2.2 and 2.3 in that guidance are now outdated and no longer apply; however, the remainder of the November 2015 guidance continues to apply.
  • Guidance for Transportation Conformity Implementation in Multi-Jurisdictional Nonattainment and Maintenance Areas (pdf) (589 KB, July 2012, EPA-420-B-12-046)
    This guidance describes how transportation conformity determinations are done in areas where multiple metropolitan planning organizations (MPOs), states and/or other agencies have jurisdiction in a nonattainment or maintenance area. This guidance updates and supersedes the July 2004 “Companion Guidance for the July 1, 2004 Final Transportation Conformity Rule: Conformity Implementation in Multi-Jurisdictional Nonattainment and Maintenance Areas for Existing and New Air Quality Standards”. This guidance has been updated to apply to existing and future NAAQS, consistent with the Transportation Conformity Rule Restructuring Amendments.
  • Updated guidance on substituting and adding transportation control measures to approved state air quality plans (pdf) (447 KB, January 2009, EPA420-B-09-002)
    This document provides updated guidance to state and local air quality and transportation agencies on implementing the Clean Air Act provision that establishes a streamlined process for substituting or adding transportation control measures to an approved state air quality plan. Transportation control measures, such as public transit and carpooling, can provide alternatives to the public that reduce motor vehicle emissions. This document supersedes the section of the February 14, 2006 interim guidance on implementing the Clean Air Act amendments made by SAFETEA-LU that addressed this provision.
  • EPA and DOT Joint Guidance for the Use of Latest Planning Assumptions in Transportation Conformity Determinations (pdf) (365 KB, December 2008, EPA420-B-08-901)
    This guidance document supersedes the January 18, 2001 guidance memorandum to address the July 1, 2004 transportation conformity final rule revisions on the use of latest planning assumptions in transportation conformity determinations.

Project-Level Conformity

Emission Models and Conformity

The information here pertains to emissions models for SIP development and regional emissions analyses for transportation conformity determinations. Information on emissions models for PM and CO hot-spot conformity analyses can be found at Project-Level Conformity and Hot-Spot Analyses Emissions Models and Methods.

MOVES4 Model

MOVES3 Model

Use of EMFAC model in California

  • EPA’s approval of Heavy-Duty I/M Program Interim Adjustment Factors for EMFAC2021 and EMFAC2017 (pdf) (206.77 KB, May 25, 2023)
    On May 26, 2023, EPA approved interim off-model adjustment factors for use in California that account for the emission benefits of California’s Heavy-Duty Vehicle Inspection and Maintenance Program for use in transportation conformity determinations in California. These adjustment factors are multipliers that would be applied to vehicle emissions modeled by EMFAC2021 or EMFAC2017.

    The HD I/M adjustment factors for EMFAC2021 and EMFAC2017 are approved for regional emissions analyses in transportation plan and TIP conformity determinations where the SIP motor vehicle budgets are based on EMFAC2021 or EMFAC2017. These adjustment factors are not approved for CO, PM10, or PM2.5 hot-spot analysis for project-level conformity determinations. The EMFAC2017 adjustment factors also account for benefits of additional regulations included in EMFAC2021, similar to the adjustment factors approved with EMFAC2021 (more information below). Therefore, MPOs should apply only one set of EMFAC2017 adjustment factors. In addition, the EMFAC2017 adjustment factors can no longer be used for new regional emissions analyses after the end of the EMFAC2021 transportation conformity grace period.

    The California Air Resources Board (CARB) requested EPA’s approval of these adjustment factors in the following letters:
  • Federal Register Notice of Availability: Official Release of EMFAC2021 Motor Vehicle Emission Factor Model for Use in the State of California (published November 15, 2022)
    This notice describes EPA's approval of the EMFAC2021 model for SIP and transportation conformity purposes in California. It also announces a two-year conformity grace period for regional emissions analyses, which ends on November 15, 2024, and a one-year conformity grace period for CO, PM10 and PM2.5 hot-spot analyses in California, which ends November 15, 2023. Once the grace periods end, EMFAC2021 must be used for new regional emissions analyses and hot-spot analyses in California. EPA's approval of the model does not affect MOVES model users in other states.

    This notice also describes EPA’s approval of the EMFAC2017 Off-Model Adjustment Factors for use in California SIP development and in regional conformity emissions analyses where adequate SIP motor vehicle emissions budgets are based on EMFAC2017 with adjustment factors. EMFAC2017 adjustment factors are not approved for CO, PM10, or PM2.5 hot-spot analyses for transportation conformity. The California Air Resources Board (CARB) requested EPA’s approval of the EMFAC2021 model as well as approval of adjustment factors to their previous model, EMFAC2017, in the following letters:

Guidance on other models

State Implementation Plans (SIPs) and Conformity

Quantifying Benefits of Control Measures in SIPs and Conformity

Land Use and Conformity

March 2, 1999 Conformity Court Decision

(subsequently incorporated into July 2004 rulemaking)

State and Local Transportation

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Last updated on May 23, 2024