Background on the Clean Air Act (CAA) Mobile Source Programs for Ozone

The Clean Air Act (CAA) has requirements for areas that EPA designates nonattainment for the national ambient air quality standards (NAAQS). The mobile source requirements for ozone NAAQS nonattainment areas are summarized below and include links for more information, including national regulations, guidance, and technical assistance on many of these programs.

On this page:

Overview of CAA Mobile Source SIP Requirements
Vehicle Inspection and Maintenance (I/M) Programs
Fuels Policies and Programs
Clean Fuel Fleets Programs (CFFP)
Vehicle Miles Travelled (VMT) Reporting
Vehicle Miles Travelled (VMT) Offsets
Transportation Conformity

Overview of CAA Mobile Source SIP Requirements

CAA mobile source control requirements depend upon an area's ozone nonattainment classification. The mobile source control requirements are shown in green in the following illustration. Reformulated gasoline (RFG) is required for Severe areas, as noted on the illustration.

(Click image for explanation)

This is a bar chart with the general description of ozone classifications and related mobile source requirements.

Also note that transportation conformity is also a CAA requirement (although different from the mobile source requirements covered on this SIP page); conformity applies in ozone, PM_2.5, PM₁₀, CO, and NO₂ nonattainment and maintenance areas.

Vehicle Inspection and Maintenance (I/M) Programs

I/M programs involve the periodic inspection of the emissions control systems of motor vehicles. The goal of I/M programs across the country is the same: identify and repair high-emitting vehicles to improve air quality. CAA sections 182 and 184 require Enhanced or Basic I/M programs for certain ozone nonattainment areas and the Ozone Transport Region based upon various criteria, such as air quality status/nonattainment classification, population, and/or geographic location. For more information, see the Vehicle Emissions Inspection and Maintenance page.

Fuels Policies and Programs

The CAA requires EPA to regulate fuels and fuel additives for use in motor vehicles, motor vehicle engines, nonroad engines or nonroad vehicles if such fuel, fuel additive or any emission product causes or contributes to air or water pollution that may endanger the public health or welfare. For example, as shown above, the CAA section 211(k)(10)(D) requires reformulated gasoline (RFG) in areas that are reclassified as Severe ozone nonattainment areas. For more information on RFG and other federal and state fuel programs, see the Gasoline Standards page.

Clean Fuel Fleets Programs (CFFP)

CAA section 182(c)(4) requires states with Serious or above ozone nonattainment areas with 1980 populations greater than 250,000 submit a SIP revision to either “include such measures as may be necessary to ensure the effectiveness of the applicable provisions of the clean-fuel vehicle program prescribed under part C of subchapter II of this chapter” or to provide “a substitute for all or a portion of the clean-fuel vehicle program prescribed under part C of subchapter II of this chapter”. These requirements continue to apply to ozone nonattainment areas classified as Serious or above for the 2008 and 2015 ozone NAAQS including areas that are reclassified as Serious for these ozone NAAQS. Areas that are originally classified as Serious must either submit a SIP revision for a Clean Fuel Fleets program within 42 months after the effective date of the designation or submit a SIP revision for a substitute program within 24 months after the effective date of the designation. For areas that are reclassified as Serious, the due date for the SIP revision that addresses the Clean Fuel Fleets requirement will be established in the rule that reclassifies the area as Serious. For more information, see the Clean Fuel Fleets Guidance web page.

Vehicle Miles Travelled (VMT) Reporting

CAA section 182(c)(5) requires a state with a Serious ozone nonattainment area to submit a demonstration as to whether current aggregate vehicle mileage, aggregate vehicle emissions, congestion levels, and other relevant parameters are consistent with those used for the area's demonstration of attainment. Where such parameters and emissions levels exceed the levels projected for purposes of the area's attainment demonstration, within 18 months the State shall develop and submit a SIP revision that includes a transportation control measures program consisting of measures from, but not limited to, CAA section 108(f) that will reduce emissions to levels that are consistent with emission levels projected in the attainment demonstration. For an area that is initially classified as a Serious ozone nonattainment area, the first demonstration is due six years after the effective date of the area’s designation. Subsequent demonstrations are due every three years. If an area is reclassified as a Serious ozone nonattainment area, the due date for the first demonstration will be established in the rulemaking that reclassifies the area as Serious. Subsequent demonstrations are due every three years.

Vehicle Miles Travelled (VMT) Offsets

CAA section 182(d)(1)(A) requires a state with a Severe or Extreme ozone nonattainment area to submit a SIP revision that identifies and adopts specific enforceable transportation control strategies and transportation control measures (TCMs) to offset any growth in emissions from growth in vehicle miles travelled or numbers of vehicle trips in such area. EPA has provided guidance Implementing Clean Air Act Section 182(d)(1)(A): Transportation Control Measures and Transportation Control Strategies To Offset Growth in Emissions Due to Growth in Vehicle Miles Travelled (PDF) (7 pp, 830 K, 420-B-12-053, August 2012). The guidance describes how to determine if there has been any growth in emissions. For an area that is initially classified as a Severe or Extreme ozone nonattainment area the VMT offset demonstration must be submitted as a SIP revision two years after the effective date of the area’s designation. If an area is reclassified as a Severe or Extreme ozone nonattainment area, the due date for the SIP revision that includes the VMT offset demonstration will be established in the rulemaking that reclassifies the area as Severe or Extreme. For more information, see the VMT Offset Demonstration Guidance web page.

CAA section 182(d)(1)(A) contains two additional requirements that pertain to transportation control strategies and TCMs:

States with Severe and Extreme ozone areas must submit a SIP revision that identifies and adopts specific enforceable transportation control strategies and TCMs to reduce motor vehicle emissions as necessary, in combination with other emission reduction requirements to comply with reasonable further progress requirements.
States must consider measures specified in CAA section 108(f) and choose from among those measures and implement such measures as necessary to demonstrate attainment with the relevant ozone NAAQS.

In selecting any measures to implement the State should ensure adequate access to downtown, other commercial, and residential areas and should avoid measures that increase or relocate emissions and congestion rather than reduce them.

Transportation Conformity

Transportation conformity is required under CAA section 176(c). Conformity to the purpose of the SIP means that transportation activities will not cause or contribute to new air quality violations, worsen existing violations, or delay timely attainment of the relevant NAAQS or any interim milestones. OTAQ’s transportation conformity rule (40 CFR Parts 51 and 93) establishes the criteria and procedures for determining whether metropolitan transportation plans, transportation improvement programs (TIPs), and federally supported highway and transit projects conform to the SIP. Transportation conformity is required in ozone, PM_2.5, PM₁₀, NO₂ and CO nonattainment and maintenance areas. For more information, see the Transportation Conformity page.