EPA has usually led California in establishing standards for on-road heavy-duty diesel vehicles and off-road diesel engines. This shared leadership promotes improvements in the efficiency of EPA’s and CARB’s regulatory efforts and allows sharing of expertise.

The mobile-source emissions standards developed by CARB, like those developed by EPA, have typically been “technology forcing.”² In forcing technology development, California has been a laboratory for emissions-control innovations. An advantage of having a state laboratory for innovation is that the risk of failure to develop the required technologies is restricted to a limited geographic area. CARB’s regulatory process is supportive of this laboratory role in that California’s standards can be amended rapidly in the face of changing market and technological conditions in contrast to EPA’s regulatory process.

The original reasons for which Congress authorized California to have a separate set of standards remain valid. California still has some of the worst air quality conditions in the country, and certain emission-reduction needs are greater in California than in the rest of the country. California has used its authority as Congress envisioned: to implement more aggressive measures than the rest of the country and to serve as a laboratory for technological innovation. These have resulted in successes, such as CARB’s early recognition of the need to couple fuel composition with emissions control, and failures, such as the promotion of widespread use of electric vehicles under the original zero-emissions vehicle mandate.

California’s authority to set its own mobile-source emissions standards inevitably imposes additional risks and costs, such as design, production, and distribution costs, although the costs and benefits are difficult to quantify. However, experience to date indicates that the California program has been beneficial overall for air quality by improving mobile-source emissions control.

California should continue its pioneering role in setting mobile-source emissions standards. The role will aid the state’s efforts to achieve