The federal Clean Air Act (CAA) establishes the framework for controlling mobile-source emissions in the United States. During the development of the CAA in 1967, Congress recognized that the imposition of many different state standards could result in inefficiencies in vehicle markets. Therefore, state-established emissions standards were preempted by federal emissions standards in what is now section 209 of the CAA. A special exemption to this federal preemption was made in section 209 for California because of the state’s special air quality problems and pioneering efforts in the control of air pollutants. This exemption, still in existence, gives the state of California the authority to set on-road vehicle standards that differ from the federal standards as long as they are as protective in the aggregate as federal standards. Later amendments to section 209 granted California the authority to set emissions standards and regulations for some nonroad engines, and section 177 was added to allow other states to adopt California standards.

The National Research Council (NRC) convened the Committee on State Practices in Setting Mobile Source Emissions Standards in response to a request from Congress in its fiscal 2003 appropriations report for the U.S. Environmental Protection Agency (EPA) to arrange for an independent study of the practices and procedures by which states develop separate emission standards. For this report, the committee was asked to assess the scientific and technical procedures used by states to develop or adopt different emissions standards and to compare those policies and practices with those used by EPA. The committee was also asked to consider the factors that caused states to move toward more stringent emissions standards and to consider the impacts of state emissions standards on various factors, including emissions, compliance costs, energy consumption, air quality, and human health. As part of its work plan, the committee was directed to consider the effects of California’s experience in setting separate emissions standards and the effects of California’s standards on federal emissions standards. The full Statement of Task for the committee is provided in Chapter 1.


Role of New Mobile-Source Emissions Standards

Despite the substantial progress made over the past few decades in reducing air pollutant emissions from many sources, including mobile

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