NCHRP 25-55 made ground-breaking recommendations for the design and implementation of an enhanced model evaluation process (EMEP) for air quality analyses for transportation, including in particular that model evaluations be conducted for the regulatory modeling chain (RMC) of traffic emissions and dispersion for project-level air quality analyses, that the RMC evaluations be conducted for the intended regulatory purpose of showing compliance with the regulatory tests (NAAQS and B/NB), that multiple technical criteria appropriate for regulatory models be used in the evaluations in addition to accuracy (which is typically the focus), and that state-of-the-art field studies be conducted to collect representative data for all typical transportation applications (project types, configurations etc.) that are subject to regulatory requirements for modeling. These recommendations and more were cited in the December 2023 AASHTO comment letter on proposed revisions to the Guideline on Air Quality Models. An EMEP for transportation is needed for two primary reasons: 1) good science, as currently the RMC has not been validated (which makes air quality analyses open to challenge), 2) to meet the challenges posed by increasingly more stringent national ambient air quality standards (NAAQS) for fine particulate matter (PM2.5), which will make clearing projects for conformity and NEPA much more challenging technically.

Implementing the NCHRP 25-55 recommendations will be complex and resource intensive. Before it can be implemented, consensus among state DOTs will be needed on its key elements or framework, including the methodologies to be applied for the field studies and the associated modeling and analyses, the priorities for the transportation applications (project types, configurations etc.) to be evaluated, and the format and content of the deliverables (e.g., an EMEP template for reporting). Budget cost estimates would also be needed for the prioritized transportation applications covering both the field studies and the modeling and analyses. For the EMEP template, the results from NCHRP 25-55 would be reviewed and updated as appropriate for consistency with the recommended methodologies, with the final report serving as a template for future EMEPs. Additionally, consensus on the consultation and coordination process to be used in future EMEPs would be needed.

The proposed approach for implementing NCHRP 25-55 is therefore to first inform and gain consensus among state DOTs and other key stakeholders on these key elements, with due consideration for any updates or advances in technology or methods since the time that NCHRP 25-55 was conducted. Once consensus has been obtained, model evaluations can be conducted on the prioritized transportation applications following the consensus-based methodologies for the field studies and modeling and analyses, with reporting following the EMEP template and consultation and coordination following the agreed process. An update on attainment status and margins and the associated ability to use the NAAQS and build/no build tests for state DOTs affected by the revised PM2.5 NAAQS will also be provided.