Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

Transportation Conformity: A Basic Guide for State and Local Officials

Key Elements of a Metropolitan Transportation Plan/TIP Conformity Determination

One way to understand transportation conformity is to know the key requirements and how they interact. The major components of a conformity determination include:

Interagency Consultation

Experience has shown that ongoing coordination and communication between Federal, State and local transportation and air quality agencies is vital to a smoothly running conformity process. In addition, a clear understanding of roles and responsibilities of participating agencies is essential.

The conformity rule requires that Federal, State and local transportation and air quality agencies establish formal procedures to ensure interagency coordination on critical issues. Typical participants in interagency consultation include FHWA, FTA, EPA, State doTs, MPOs and other local transportation agencies, and State and regional air quality agencies. In addition, public transportation operators are often active participants in interagency consultation. Interagency consultation is a forum for discussing key assumptions to be used in conformity analyses, strategies to reduce mobile source emissions, specific impacts of major projects, issues associated with travel demand and emissions modeling, and the development of motor vehicle emissions budgets. The specific process that will be followed in each area must be adopted as part of the SIP and must be used to develop metropolitan transportation plans, TIPs and the SIP. These adopted interagency consultation procedures are included in the "conformity SIP."

Public Involvement

Good public involvement processes are proactive, easily accessible to the public and keep the public informed on an ongoing basis.

MPOs are required to make metropolitan transportation plans, TIPs and conformity determinations available for public review. MPOs must also respond to public comment and provide adequate notice of relevant meetings. The public involvement requirements for transportation planning must be met; there are no additional public involvement requirements for conformity.

Latest Planning Assumptions and Emissions Model

Conformity requires that latest planning assumptions and latest EPA-approved emissions estimation model be used each time a regional emissions analysis is conducted. This requirement ensures that the latest planning, travel, vehicle age and fleet mix, demographic, and economic assumptions are reflected in conformity determinations.

Transportation agencies must make assumptions about the future in their metropolitan transportation plans. These assumptions include population, employment, travel needs, vehicle fleet composition (proportions of types of vehicles), land use, and economic development. The conformity rule requires that when metropolitan transportation plans and TIPs are developed or updated, the assumptions used to forecast future conditions must be based on the latest available information. Current motor vehicle fleet information is essential to good planning and forecasting and is one of the key planning assumptions in conformity. Likewise, the latest EPA-approved emissions estimation model must be used that reflect the latest science and policies regarding motor vehicle emissions and the emissions benefits of cleaner engine and fuel standards.

Regional Emissions Analysis

Regional emissions analysis is the key analytical component of a conformity determination. The analysis supports the demonstration that transportation investments are consistent with air quality goals.

Estimating regional emissions from on-road mobile sources traveling on the planned transportation system is essential to a conformity determination. The conformity rule requires that future emissions estimates include the entire horizon of the metropolitan transportation plan (at least 20 years) for the region. Note that MPOs have the option to shorten the time horizon for the conformity demonstration if certain requirements are met. The regional emissions that are forecast through models are compared to the motor vehicle emissions budget ("budget") from the SIP that sets a limit on emissions from on-road sources. This budget cannot be exceeded in order for an area to make a conformity determination. In the absence of an approved or adequate budget, areas must pass interim tests that basically compare emissions associated with the proposed transportation network ("build" scenario) with emissions from either a "no-build" scenario or baseline year, or both. The regional emissions analysis is based on motor vehicle travel across the entire current and planned transportation system and reflects the investments detailed in the metropolitan transportation plan and TIP.

Motor Vehicle Emissions Budget

The SIP accounts for emissions of each pollutant for each source type. There are four types of sources: on-road mobile, non-road mobile, stationary (i.e., refineries), and area (i.e., dry cleaners). The State air quality agency is responsible for the development of the entire SIP. The air quality agency identifies how pollution from all sources will be reduced sufficiently to achieve the purpose of the SIP. Required emission reductions are calculated and control measures are adopted to achieve needed reductions.

A motor vehicle emissions budget is that portion of the total allowable emissions in the SIP that is allocated to on-road mobile sources, such as cars, trucks, and buses. It is the level of on-road emissions that the area can have and still meet the SIP's goals. Budgets are established in the applicable SIP as part of the air quality planning process by State air quality or environmental agencies, and approved by EPA. Transportation agencies participate in this process in accordance with required interagency consultation procedures.

For transportation conformity, projected emissions from highway and public transportation use must be less than or equal to the budgets. In other words, the budget acts as a ceiling on emissions from the on-road mobile sector.

Timely Implementation of Transportation Control Measures (TCMs)

When an EPA-approved SIP includes TCMs, each time a conformity determination is made the MPO must demonstrate that these measures are being implemented on schedule as called for in the SIP.

TCMs are measures included in an approved SIP to help reduce emissions from on-road mobile sources. Section 93.101 of the conformity regulations contains a definition of TCMs for conformity purposes. Some of these measures are specifically listed in the CAA5 and transportation and air quality agencies often consider whether such measures are beneficial and needed to meet air quality requirements. TCMs are designed to reduce emissions from motor vehicles by reducing vehicle use, changing traffic flow, or changing congestion conditions. Examples include high-occupancy vehicle (HOV) lanes, improving public transportation, and vanpooling programs. If an EPA-approved SIP includes any of these measures, the MPO must show, as part of the conformity determination, that the measures are being implemented on schedule and given priority for Federal funding. Not all areas have these measures in their approved SIPs; nevertheless, these types of measures are often routinely implemented through the metropolitan transportation plan and TIP (i.e., public transportation services, telecommuting programs). If not included in an approved SIP, such measures are not TCMs for the purpose of conformity, and the MPO does not have to demonstrate their timely implementation.

Photo of a multi-lane highway and traffic.If an MPO finds that a TCM has become delayed, the MPO may decide to replace the delayed TCM with a new TCM through a process called TCM substitution6 in order to meet its timely implementation requirement. Through this process, an MPO does not need to go through a full SIP revision in order to substitute the delayed TCM for a new TCM.

Fiscal Constraint

Metropolitan Transportation Plans and TIPs in nonattainment or maintenance areas must be shown to meet the FHWA/FTA fiscal constraint requirements.

The FHWA/FTA transportation planning regulations7 require that metropolitan transportation plans and TIPs be based upon reasonable estimates about future revenues. In addition, in the first two years of the TIP, projects must be limited to those for which funds are known to be available or committed. This is known as fiscal constraint. The conformity rule requires that the fiscal constraint requirements of the planning regulations be met prior to determining conformity on a metropolitan transportation plan or TIP.

5 CAA Section 108(f)(1)(A)

6 Guidance for Implementing the Clean Air Act Section 176(c)(8) Transportation Control Measure Substitution and Addition Provision - EPA-420-B-09-002 January 2009

7 23 CFR Part 450

Updated: 03/14/2012
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000