Skip to content
Facebook iconYouTube iconTwitter iconFlickr iconLinkedInInstagram
Office of Planning, Environment, & Realty (HEP)
HEP Events Guidance Publications Glossary Awards Contacts

Transportation Conformity Brochure

HOV Lane - Atlanta

HOV Lane - Atlanta

Transportation conformity is...

A process required by the Clean Air Act (CAA) Section 176(c) which establishes the framework for improving air quality to protect public health and the environment. The goal of transportation conformity is to ensure that Federal Highway Administration (FHWA) and Federal Transit Administration (FTA) funding and approvals are given to highway and transit activities that are consistent with air quality goals.

The CAA requires that metropolitan transportation plans, metropolitan transportation improvement programs (TIPs) and Federal projects conform to the purpose of the State Implementation Plan (SIP). Conformity to a SIP means that such activities will not cause or contribute to any new violations of the National Ambient Air Quality Standards (NAAQS); increase the frequency or severity of NAAQS violations; or delay timely attainment of the NAAQS or any required interim milestone.

What is a National Ambient Air Quality Standard (NAAQS)?

A Federal standard that establishes an air quality concentration to protect public health and welfare. The NAAQS are set for six principal pollutants, also called criteria pollutants.

Six Criteria Pollutants

  • Ozone (O3)
  • Nitrogen Dioxide (NO2)
  • Carbon Monoxide (CO)
  • Lead (Pb)
  • Particulate Matter (PM2.5 and PM10)
  • Sulfur Dioxide (S02)

Transportation Conformity:

The link between air quality and transportation planning.

The graphic depicts how conformity is the link between air quality planning (the State Implementation Plan or SIP) and transportation planning (metropolitan transportation plan and transportation improvement program or TIP)

What pollution does transportation conformity address?

Air pollution comes from a variety of sources. Transportation conformity only addresses air pollution from on-road mobile sources which include emissions created by cars, trucks, buses, commuter rail, and motorcycles.

The graphic depicts the major sources of air pollution (non-road mobile depicted by an airplane, area depicted by a paint can, stationary depicted by an industry, and on-road mobile depicted by a road with traffic).  Transportation conformity applies to on-road mobile sources.

Transportation conformity applies in...

All nonattainment and maintenance areas for ozone (O3), particulate matter (PM10 and PM2.5), nitrogen dioxide (NO2), and carbon monoxide (CO), and their appropriate precursors (precursor pollutants are those pollutants which contribute to the formation of other pollutants).

Precursor Emissions*
  Direct Emissions NOx VOC Ammonia (NH3) Sulfur Dioxide (SO2)
Ozone (O3)   X X    
PM10 X X X    
PM2.5 X X X X X
NO2   X      
CO X        

Precursor Emissions

* Not all precursors are required to be analyzed in every area.

What transportation activities are subject to transportation conformity?

What does a conformity determination show?

For metropolitan plan or TIP conformity, the determination shows that the total emissions projected for that metropolitan plan or TIP are within the on-road mobile source emissions limits ("budgets") established by the SIP to protect public health for the NAAQS. Transportation control measures (TCMs) must be implemented in a timely fashion and State and local agencies must be consulted on data, modeling, and other issues related to the determination. For project-level conformity, the determination shows that the project is consistent with the regional conformity determination and that potential localized emissions impacts on health-based pollutant standards are addressed.

What are Transportation Control Measures (TCMs)?

TCMs are specific programs designed to reduce emissions from transportation sources by reducing vehicle use or changing traffic flow or congestion conditions and are included in the approved SIP. Examples include programs for improving public transit, developing high occupancy vehicle (HOV) lanes, and employing ordinances to promote non-motor vehicle travel.

Who makes a conformity determination?

Conformity determinations are made by FHWA/FTA. Metropolitan Planning Organization (MPO) policy boards make initial conformity determinations for plans and programs in metropolitan areas, while State Departments of Transportation (DOTs) usually do so in areas without MPOs and typically conduct the analyses associated with project-level conformity.

SEPTA Commuter Train - Philadelphia
SEPTA Commuter Train - Philadelphia

What are the major transportation conformity requirements?

Conformity must be determined...

What happens if a conformity determination is not made?

When a conformity determination is not made according to schedule, areas have a 1-year grace period to make the determination before there is a conformity lapse and the use of Federal-aid funds is restricted. During a lapse, only certain types of projects can proceed: (1) projects that are exempt from conformity; (2) TCMs in approved SIPs; and (3) projects or project phases that are already authorized. The 1-year conformity lapse grace period does not apply to new nonattainment areas that must make a determination on their plans and TIPs within 12 months of final designation.

What projects are exempt from conformity?

There are three groups of exempt projects in the conformity rule:

  1. Projects exempt from all conformity requirements include specific projects under the categories of safety, mass transit, air quality, and other (§93.126)
  2. Projects exempt from regional emissions analysis include intersection channelization, and interchange reconfiguration (§93.127)
  3. Traffic Signal Synchronization projects (§93.128)

What are the major project-level conformity requirements?

Project-level conformity must be redetermined if...

What is a hot-spot analysis?

A hot-spot analysis is an assessment of the emissions impact of a project on local air quality concentrations. It helps demonstrate that the project meets Clean Air Act requirements in CO nonattainment and maintenance areas. In PM areas, hot-spot analyses are required for projects of air quality concern only.

What are the interagency consultation requirements?

Where can I get more information?

FHWA Conformity Website:

EPA Conformity Website:

Subway - Chicago
Subway - Chicago

* Please refer to the Transportation Conformity Rule and associated guidance at all times to ensure compliance with appropriate requirements.*


Updated: 6/28/2017
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000