Implementing NCHRP 25-25 Task 104 to Eliminate CO Modeling for NEPA

Focus Area

Air Quality


Air Quality






Under 1 year

Research Idea Scope

NCHRP 25-25 Task 104 recently completed updates to templates for a programmatic agreement (PA) for project-level air quality analyses for carbon monoxide (CO). The templates were designed to be customized by state DOTs using local data to the extent desired and executed with their respective FHWA Division offices, at which point the PA may be applied to clear projects for air quality without requiring the time and cost of project-specific CO modeling. This study would support implementation of PAs by state DOTs across the nation by providing assistance in customizing and executing the Task 104 templates for their respective jurisdictions. 

As part of this implementation effort, an option will be provided to state DOTs to make a simple but highly effective enhancement to the PA to cover all projects for NEPA using a weight-of-evidence (WOE) approach, instead of just those projects for which worst-case modeling was conducted for Task 104. In this approach, long-term downward trends in CO emissions and ambient concentrations at the national level and the long experience of state DOTs in modeling transportation projects are given weight along with the worst-case modeling results from Task 104 to make the case that CO modeling for purposes of NEPA may reasonably be eliminated for all projects as the applicable national ambient air quality standards (NAAQS) may reasonably be expected to be met in all cases. 

This implementation effort would therefore involve two steps: 
1) update the Task 104 template PA to add a weight-of-evidence approach that would eliminate the need for CO analyses going forward by state DOTs, using information already compiled in the Task 104 TSD for this purpose, while retaining the ability to screen projects based on modeling as already provided in the Task 104 template PA, and 
2) assist state DOTs across the nation in customizing and executing the PA (with or without the WOE option) for their respective jurisdictions, most of whom are expected to apply the PA without detailed state-specific modeling. For any state DOTs that may still prefer to update the Task 104 modeling using state-specific inputs, significantly greater effort would be involved, which generally would require state DOT resources. 

The modification to the PA to add the WOE option would be expected to add only two or three pages of text, and rely on information already compiled for the Task 104 TSD. For comparison, the Task 104 template PA exceeds twenty pages as it includes detailed appendices for modeling results, so the addition would be relatively minor. 

The Task 104 Technical Support Document (TSD) documented WOE considerations but focused on the “traditional” modeling-based approach for the PA. The Task 104 PA and TSD both however highlighted that the compounding effect of multiple worst-case assumptions provided a substantial safety margin for the modeling, which effectively translates to the NAAQS being met by a wide margin. That wide margin with the NAAQS is supportive of a WOE approach that would extend coverage of the PA to all projects. 

For reference, the FHWA 1987 Technical Advisory 6640.8A* that governs NEPA assessments for CO states “A microscale CO analysis is unnecessary where such impacts (project CO contribution plus background) can be judged to be well below the 1- and 8-hour National Ambient Air Quality Standards (or other applicable State or local standards.)” It does not specify modeling, project-specific or otherwise, as being needed to make these determinations. A WOE-based approach therefore suffices for such determinations.

Urgency and Payoff

State DOTs across the nation that implement the PA would benefit from reduced time and cost for unnecessary CO analyses. The implementation of a national template WOE-based PA would help standardize CO clearances across the nation for purposes of NEPA, facilitating review and approval by FHWA Division offices.


Suggested By

Christopher Voigt Virginia DOT (804) 371-6764

[email protected]