"Nonattainment" areas (NAA) are geographic areas that do not meet the federal air quality standards.  Maintenance areas are areas that formerly violated, but currently meet the federal air quality standards. If no violations of air quality standards have been found, the area is considered to be in compliance, or in attainment, with federal air quality standards.

An area can be designated "nonattainment" for one pollutant and in attainment for another. Transportation conformity is required for all ozone, carbon monoxide, nitrogen dioxide, and particulate matter nonattainment and maintenance areas.

The Clean Air Act (CAA) identifies the actions states and MPOs must take to reduce emissions from on-road mobile sources in nonattainment and maintenance areas. 

The challenge for MPOs in nonattainment or maintenance areas is to decide on a mix of transit and highway investments that, combined with measures such as Inspection and Maintenance (I/M) programs or reformulated gasoline, will keep emissions within the allowable limits for emissions from motor vehicles. 

According to the CAA, transportation plans, TIPs, and projects cannot:

  • Create new violations of the National Ambient Air Quality Standards (NAAQS)
  • Increase the frequency or severity of existing violations of the standards
  • Delay attainment of the standards.

MPOs are encouraged to participate in air quality planning and to identify transportation strategies that will help reduce emissions from on-road mobile sources of pollution. 

One important provision in federal transportation legislation allows for the use of certain federal-aid highway program and federal transit program funds for either highway or transit projects. This is referred to as flexible funding. "Flexible funding" provisions were a radical departure from traditional transportation policy; federal transit, highway, and safety programs formerly had very strict eligibility requirements, and funds could not be transferred between the programs. The ability to transfer funds (with certain restrictions) between highway and transit programs was introduced so metropolitan areas could apply federal transportation funds to their highest priority transportation projects.

If an MPO cannot make a conformity determination according to applicable deadlines, it will have a grace period of 12 months after the deadline is missed before conformity will lapse. During the grace period, transportation projects from the previously conforming plan and TIP may continue to be eligible for funding. However, no changes may be made to those documents without re-establishing conformity. If conformity has not been re-established after the 12 month grace period, the transportation conformity status for the area goes into "lapse." During a conformity lapse, FTA and FHWA funds can only be spent on exempt projects, such as safety projects and certain public transportation projects, TCMs from an approved SIP, and project phases that were authorized by FHWA and FTA prior to the lapse. 

Motor Vehicle Emission Simulator or MOVES: MOVES2010 is a computer model designed to estimate emissions factors and emissions inventories of VOCs, NOx, CO, PM10, PM2.5 and other pollutants and precursors for cars, trucks, buses, and motorcycles. MOVES2010 was designed to replace the previous emissions model, MOBILE6.2, which was released in 2004 (69 FR 28830). MOVES2010 replaces MOBILE6.2 as the emissions model that EPA will maintain and support. 

EPA has performed a preliminary comparison of MOVES2010 to MOBILE6.2 using local data for several different urban counties, varying the local data used by fleet age distribution, fraction of light- and heavy-duty vehicle miles travelled (VMT), local fuel specifications, meteorology, and other input factors. Actual results will vary based on local inputs in a given area, with local variations in fleet age distribution and composition having a significant influence on the final results. In general, VOC emissions are lower when using MOVES2010 when compared to MOBILE6.2, while both NOx and PM emissions are higher. 

MOVES2010 includes the capability to estimate vehicle exhaust and evaporative emissions as well as brake wear and tire wear emissions for criteria pollutants and precursors. However, MOVES2010 does not include the capability to estimate emissions of re-entrained road dust. To estimate emissions from re-entrained road dust, practitioners should continue to use the latest approved methodologies.