Transportation projects collectively valued at billions of dollars in geographic areas subject to PM2.5 conformity requirements face challenges in being cleared for the National Environmental Policy Act (NEPA) given ongoing revisions to the National Ambient Air Quality Standards (NAAQS) that may drop below the national average concentration trendline and put many new areas into nonattainment. While the NAAQS test may be challenging to pass, as margins with background concentrations are substantially reduced, the build/no build (B/NB) may be worse as it may not only preclude the construction of new highways but may also limit capacity additions to existing highways.

The objective of this research is to seek consensus among state departments of transportation (DOTs) and their partners on the methodologies and consultation processes to be applied in a critically needed enhanced model evaluation process (EMEP) for the air quality regulatory modeling chain (RMC) of traffic, emissions, and dispersion. An EMEP can be implemented as a process for individual project types or as an ongoing program by an agency or research institution. To date, the RMC has not been evaluated for the project types required by regulation to be modeled, with the primary challenge being the cost and complexity of the field studies to obtain the requisite data. The lack of validation raises concerns about accuracy and the resulting potential litigation of NEPA clearances, which are heightened by ongoing revisions to the NAAQS for fine particulate matter (PM2.5) that will place an increasing premium on accuracy for the RMC.

Successful implementation of an EMEP will require consensus among state DOTs on its framework, including the:

·       Methodologies to be applied for field studies and associated modeling and analyses;

·       Priorities for the transportation project types, configurations, operating conditions, and settings to be evaluated;

·       Format and content of the deliverables (e.g., an EMEP template for reporting);

·       Budget cost estimates for EMEPs for reference in scoping and budgeting future EMEPs for each project type, configuration, etc.; and

·       Consultation and coordination processes to be used.