of the two processes can create important questions about regulatory fairness. The procedures of the CAA were designed so that, in setting emission budgets, decision makers would take forecasts of aggregate emissions and then explicitly allocate emission estimates across sectors in ways that created legitimacy and stakeholder commitment to the outcome. If transportation emission forecasts are updated while other sectors’ are not, the validity and perceived fairness of the results can be questioned and stakeholder support for pollution reductions can be undermined.

Some proponents of conformity hoped that linking transportation and land use would encourage broader acceptance of land-use regulations to reduce emissions from mobile sources. However, the impact of conformity on land-use decision-making, which is in the hands of local governments that do not have a direct role in conformity, has been modest (Howitt and Moore 1999a).

• Regulations on LDVs and LDTs have resulted in significant reductions in the emissions per mile traveled. In the case of CO, as shown in a recent NRC study (NRC 2003b), those reductions have resulted in significant reductions in overall population exposure. Further emission reductions are anticipated from the implementation of stricter emission regulations in the coming years.

• Emission regulations on LDVs and LDTs have promoted the development and application of new cleaner technologies for vehicles—technologies that are now used worldwide. Furthermore, the actual costs of these technologies were likely less than anticipated.

• Regulations on fuel properties, including content, have also resulted in air quality benefits, most notably is the phase-out of lead in gasoline that made the use of catalytic converters possible and reduced population exposure to lead. RFG resulted as well in reductions in population exposure to benzene. New regulations on sulfur content in fuels promise to further enhance the effectiveness of catalytic controls and reduce emissions of the on-road fleet.

• Gaps remain in the ability to monitor, predict, and regulate in-use vehicle emissions. The existence of high emitters is a major challenge, and