Expert Report

Each report is produced by a committee of experts selected by the Academy to address a particular statement of task and is subject to a rigorous, independent peer review; while the reports represent views of the committee, they also are endorsed by the Academy. Learn more on our expert consensus reports.

Emissions from mobile sources contribute significantly to air pollution in the United States. Such sources include cars and light- and heavy-duty trucks; diesel-powered cranes, bulldozers, and tractors; and equipment such as lawnmowers that run on small gasoline engines. The role of state versus federal government in establishing mobile-source emissions standards is an important environmental management issue. With this in mind, Congress called on EPA to arrange an independent study of the practices and procedures by which California develops separate emissions standards from the federal government and other states choose to adopt the California standards. The report provides an assessment of the scientific and technical procedures used by states to develop or adopt different emissions standards and a comparison of those policies and practices with those used by EPA. It also considers the impacts of state emissions standards on various factors including compliance costs and emissions. The report concludes that, despite the substantial progress in reducing emissions from mobile sources nationwide, more needs to be done to attain federal air-quality standards in many parts of the country. Additionally, California should continue its pioneering role in setting emissions standards for cars, trucks, and off-road equipment.

Key Messages

  • CARB and EPA have essentially the same starting point and motivation for setting new or stricter standards: attainment of the National Ambient Air Quality Standards (NAAQS). Each agency follows a series of procedural steps leading to a finalized regulation. These steps include identification of the need for new emissions standards, evaluation of potential control strategies, publication of proposed regulations, and solicitation of public comments on proposals before promulgating the regulations.
  • CARB has demonstrated some flexibility in setting emissions standards for small engines to deal with some of the difficulties inherent in the non-integrated industry.
  • Manufacturers of mobile sources have raised objections to the adoption of California standards by other states. Up to this point, adopting states and manufacturers have resorted to the courts to resolve their technical and legal disputes when direct negotiations have failed. Although EPA is an appropriate entity to comment on some of these disputes, it has no authority over states' adoption decisions.
  • Recognizing the needs of some states to adopt more stringent mobile-source emissions standards to help meet air quality goals, a desirable objective is harmonization of CARB�s and EPA�s certification procedures.
  • The California program has been beneficial overall for air quality by improving mobile-source emissions control.
  • The committee finds a need for a comprehensive study of the costs of state standards. This study should include the difference in costs for the states that adopt California standards compared with costs for California, the distribution of those costs, and their cost-effectiveness.
  • The committee finds that it is difficult to determine what parties bear what fraction of the costs of emissions standards.
  • The waiver review process usually takes several years to complete, and waivers are often granted shortly before the vehicles and engines that meet the standards are in the market.