and van pools. The DFW SIP also includes controls on small-engine equipment, such as gas-fired water heaters, small boilers, and process heaters, and relies on regulating smaller equipment to contribute to total NOX emissions reductions. Table 3-11 includes a proposal for speed limit reductions, which were not adopted by the state legislature.


Regulation of mobile-source emissions through standards on new vehicles and engines is an important component of air quality management. The CAA gave the state of California the authority to set mobile-source emissions standards that differ from the federal standards as long as they are as protective in the aggregate as federal standards. Later amendments to the CAA allowed other states to adopt California standards. The history of California mobile-source emissions standards is told through a succession of scientific findings, legislative actions, and administrative and judicial interpretations that are summarized in this chapter. Several conclusions come from this examination.

  • Over the history of mobile-source regulation to date, California has typically led EPA in tightening emissions standards on light-duty vehicles and become a laboratory for emission-control innovations. CARB’s regulatory process is supportive of this laboratory role in that California emissions standards can be amended rapidly in the face of changing market and technological conditions.

  • Each time CARB sets or substantially revises a California mobile-source emission standard, it must seek a waiver from EPA. The waiver review process can take several years and significant EPA resources to complete, even for relatively straightforward and uncontroversial waivers. In some cases, waivers have been approved after vehicles and engines that meet the standards are in the market. This process creates uncertainty for California, other states considering adopting California standards, and manufacturers.

  • States first began using their authority under section 177 of the CAA in the early 1990s when New York and Massachusetts adopted California emissions standards for new light-duty vehicles. The primary reason that these states adopted California emissions standards was to obtain additional emissions reductions to help attain and maintain the NAAQS.

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