California Department of Transportation

Last Updated: Wednesday, March 30, 2016 12:50 PM

Federal Clean Air Act Transportation Conformity

NOTE: links to U.S. Code, Federal Regulations, and federal agency guidance lead to web sites external to Caltrans Exit Caltrans website.

"Conformity" is a requirement of the Federal Clean Air Act (Section 176(c), at 42 U.S.C. 7506(c)), to ensure that federal actions are consistent with the State Implementation Plan ("SIP") to achieve and maintain Federal air quality standards. "Transportation Conformity" requirements for highway and transit projects are defined by U.S. Environmental Protection Agency (EPA) regulations in 40 CFR 93 Subpart A, US EPA and US DOT (FHWA) guidance, and local consultation procedures set up by Metropolitan Planning Organizations and Air Pollution Control Districts. The map and table below illustrate the areas that are subject to Transportation Conformity requirements in California in 2014. For other types of federal actions, not related directly to highways and transit, the conformity process is governed by 40 CFR 93 Subpart B (General Conformity).

Conformity requirements apply only in areas that are designated nonattainment, or attainment with a "Maintenance SIP" as defined in 42 U.S.C. 7505a, by the US EPA. It applies only for the pollutants that trigger those designations, and analysis requirements only apply to those pollutants. It is therefore possible for some conformity procedures (for instance, project-level hot spot requirements) to be inapplicable if the area is attainment/unclassifiable for the pollutants triggering them, even though the area is subject to conformity requirements in general. Conformity requirements do not apply at all if an area is attainment/unclassifiable for all federal air quality standards.

Conformity Areas map



Click on the graphic image at left for an enlarged image, or use one of the following links to download the map image. Save the file using the method appropriate for your browser:

GIF format (96 dpi, 114 KB file size)

PDF format (363 KB file size)

Or view a Table of Areas Subject to Transportation Conformity Requirements

Table of Conformity Areas with Regional Conformity Status (PDF file)

For statewide interagency consultation information and records of Statewide Conformity Working Group meetings, see the Statewide Conformity Working Group page at this site.

Transportation Conformity Overview

Transportation Conformity applies in areas that are "nonattainment" or "attainment-maintenance" for National Ambient Air Quality Standards, and only for the standards that actually are or previously were violated. The number of standards violated varies by area, from only one (some ozone, CO2, PM2.5, or PM10-only areas) to nearly the entire possible range (South Coast air basin); conformity analysis must be done for all of the standards that are or (in maintenance areas) were violated. The Table of Areas Subject to Transportation Conformity Requirements lists the pollutants that trigger conformity applicability in the various areas.

Conformity requirements do not apply in areas that are attainment or unclassifiable, without a Maintenance SIP as defined in 42 U.S.C. 7505a, for all federal air quality standards. There is no conformity or conformity-like process for California's state-level air quality standards. Do not, therefore, confuse conformity with the environmental analysis (CEQA and NEPA) process in general — CEQA and NEPA apply everywhere and normally consider in some way all state and federal air quality standards.

This summary is necessarily abbreviated. For more details see the Caltrans Standard Environmental Reference (Chapter 11 - Air Quality and air quality forms and templates), the EPA Transportation Conformity web site Exit Caltrans website, and the FHWA Conformity web site Exit Caltrans website. Nonattainment and "maintenance" areas in California can be identified through the EPA Region 9 Air Quality Maps web site Exit Caltrans website, the EPA "Green Book" Exit Caltrans website, and the Conformity Areas Table at this web site.

Conformity analysis and determinations may be done at statewide, regional (Metropolitan Planning Organization - MPO - or isolated rural area), and project scales.

Statewide Analysis

California's State Transportation Plan is a policy-only product that does not identify and analyze specific projects. It is not, therefore, subject to conformity requirements. A statewide interagency consultation process is maintained; see the Statewide Conformity Working Group web site.

Regional Analysis

Regional analysis addresses the effect of all regionally significant projects in a nonattainment area, and normally addresses emission budgets from a EPA-approved State Implementation Plan. The regional analysis considers the Design Concept (what type of project it is) and scope (how long, capacity, etc.) of all projects to be implemented by various analysis years. Regional analysis also documents implementation of Transportation Control Measures contained in the EPA-approved State Implementation Plan. Regional conformity must be determined not less often than every 4 years by a MPO with a nonattainment or maintenance area, and usually is analyzed more often as Regional Transportation Plan (RTP) and Transportation Improvement Program (TIP) amendments happen. Regional analysis in areas that are "isolated rural" - have no MPO within the nonattainment area - is done as part of the project-level conformity process for regionally significant projects as they occur.

Project-Level Analysis

Project-level conformity analysis is done while a NEPA environmental document is being prepared. A project-level conformity determination must be completed before the NEPA document is approved, and is reaffirmed or re-done at each federal action point (usually funding approvals) after that. Full-scale project-level conformity analysis is normally required only for projects that are not exempt from conformity (40 CFR 93.126, 128, and in ozone-only areas 127) and are regionally significant (see definition at 40 CFR 93.101). References link to the Laws and Regulatons page at this web site, then to the "eCFR" web site are at the U.S. Government Printing Office web site Exit Caltrans website.

A project-level analysis usually includes the following:

    • Regional Conformity: document that the project's Design Concept, Scope, and open-to-traffic delivery date match those assumed for regional analysis purposes. If something changes, project-level conformity cannot be determined until the RTP and TIP are amended with revised conformity determinations.
    • Hot Spot Analysis: if a project is in a carbon monoxide (CO2) or particulate matter (PM10 and/or PM2.5) nonattainment or maintenance area, analysis must confirm that the project will not cause or worsen a localized violation of the standard. This is done in two ways:
      • CO2 areas: a project is analyzed using the Caltrans/UCD CO2 Protocol, and modeled if necessary using the CALINE4 model. This process documents whether or not the project will cause a new CO2 violation in the project vicinity. Since all CO2 areas in California are attainment-maintenance, there currently are no violations of the CO2 standard, so it is not possible to worsen an existing violation and the requirement of 40 CFR 93.116 to reduce the severity/number or eliminate existing nearby violations does not apply..
      • Particulate Matter (PM10 and/or PM2.5) areas: a project is initially reviewed with a regional interagency consultation process to determine whether it's a "project of concern" or "POAQC". If it is not, public notice may be required of that decision and the results of that process (and the finding that the project is not a POAQC) will be documented as part of a conformity analysis report and section of the NEPA document. If a project is determined to be a POAQC, detailed quantitative analysis is required based on EPA Guidance, using EPA's approved dispersion models (CAL3QHCR or AERMOD). To meet conformity requirements in this case, the project must not cause or worsen nearby violations of the PM standard(s) at issue.
    • Documentation of the project's effect (if any) on Transportation Control Measures specified in an applicable SIP, and of interagency consultation and public notice procedures in general if not specifically discussed in another section.

More details regarding project-level analysis methods used in California and related information can be found in the Analysis Tools section of this web site, and in the Caltrans Standard Environmental Reference sections noted above.




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