The CAAA require close scrutiny of metropolitan area transportation improvement programs (TIPs), particularly projects, such as highway capacity additions, that could stimulate new motor vehicle travel and thus increase vehicle emission levels. The conformity process required by regulation is the primary instrument by which transportation agencies in nonattainment areas and maintenance areas1 must demonstrate the compatibility of TIPs with state implementation plans (SIPs) for meeting air quality standards by attainment deadlines.
The conformity process as it is currently interpreted in Environmental Protection Agency (EPA) regulations places heavy demands on the modeling and analytic capabilities of metropolitan planning organizations (MPOs). Until SIPs are revised and approved by EPA,2 areas that have not attained national standards must demonstrate that (a) projects in regional plans and TIPs3 will not result in motor vehicle emission levels higher than those in a 1990 baseline year and (b) by building these projects, emissions will be lower in future years 4 than if the projects are not built (i.e., the build–no-build test). Once new SIPs are approved, with new budgets established for motor vehicle emissions, the conformity test changes.5 The build–no-build test is no longer required, but nonattainment areas must demonstrate through regional emissions analyses using network-based transportation demand models that the TIP will not produce aggregate emission levels in excess of the motor vehicle emissions budget in the approved SIP (Federal Register 1993, 62,193–62,194, 62,249).6
The tests required by the conformity regulations, particularly the build–no-build analysis mandated by the interim conformity process, require a precise assessment of whether specific investments in highway capacity at specific locations in metropolitan areas will result in a net gain or a net loss in regional air quality. This assessment would be difficult to make under the best of circumstances. Not only does it involve estimating initial changes in vehicle emissions from changes in traffic flow characteristics as a result of the new capacity, it also requires the long-term responses by transportation system users to be forecast. Travel time savings and the improved access provided by new capacity will influence travel demand.