University of California Transportation Center

On November 24, 1993, the US Environmental Protection Agency adopted the transportation conformity rule, pursuant to Section 176(c)(4) of the Clean Air Act. The conformity rule requires that transportation plans, programs and projects funded or approved by the federal government or their agents under Title 23 (Highways) U.S.C. or the Federal Transit Act conform with state or Federal air quality implementation plans. Federal transportation planning regulations contain reciprocal language (40 CFR 450.312(d)), stipulating that the MPO shall not approve any plan or program that does not conform to the SIP as determined in accordance with the conformity rule.

The final transportation/air quality conformity rule (23 CFR Part 450 and 49 CFR Part 613) constrains the development of transportation plans, improvement programs and projects and has increased pressure on statewide metropolitan transportation agencies to ensure that transportation and air quality plans are coordinated. Rigorous requirements for regional and local air quality modeling of transportation systems are included in the final conformity rule, and agencies are working diligently to meet these demands.

The research community and practitioners alike have raised significant questions as to the reasonableness and accuracy for requiring specific modeling procedures -- large uncertainties in the modeling process are prevalent. These inherent uncertainties are likely to lead to erroneous transportation planning decisions, and possible mis-allocation of local, state, and federal moneys.

This paper briefly addresses and identifies some of the more significant uncertainty issues associated with the transportation modeling process in context of the demands set forth by the Conformity Rule. The uncertainties listed are not meant to be comprehensive, but instead aims to shed light on the 'missing communication link' between technical experts and policy makers. Modeling requirements and procedures set forth in the Conformity Rule do not reflect the magnitude and depth of current modeling deficiencies.

Solutions to these modeling uncertainties can be provided with advancements in both transportation activity and emissions modeling procedures. It is important for future modeling revisions and upgrades to explicitly include estimates of uncertainty in computations, so that decisions makers can re-shape existing policies -- making them more sensitive to explicit uncertainties.