Risk-Based Comparisons of Alternatives for NEPA

Focus Area

Air Quality


Air Quality






1-2 years

Research Idea Scope

Comparisons of alternative build and no-build scenarios in project-level air quality analyses for NEPA and conformity typically focus on comparisons of emissions and/or ambient concentrations of pollutants, namely greenhouse gases (GHGs), mobile source air toxics (MSATs), particulate matter (PM), and carbon monoxide (CO). However, the assessments for each pollutant class are not directly comparable and cannot be aggregated to provide an overall assessment of potential air quality impacts for each alternative. If the results for each pollutant class could be translated into a common risk-based metric (statistical risk of injury or death), then they could be compared and aggregated. NEPA alternatives could then be compared on a common basis, i.e., the potential air quality impacts in terms of aggregate risk. That aggregate assessment could then be incorporated into NEPA documentation for communication to stakeholders including EJ populations to support informed decision-making on a preferred alternative.

In this proposed study, a simplified screening-level approach for comparing highway and transit alternatives using risk as the common metric (e.g., statistical risk of injury or death) would be developed. That methodology would be applied in case studies involving the comparison of potential impacts for typical highway and transit alternatives, including cases involving different proportions and proximity of EJ populations. As the results may vary substantially depending on the project types assumed for each alternative, the study would seek to identify project types for which the trade-offs may be considered most significant including specifically for EJ populations, e.g., highway and transit/rail. Due consideration would be given to uncertainty in the underlying modeling or estimation procedures and how that might affect the conclusions for the analysis and ultimately NEPA decision-making. The study results could then be referenced in future NEPA documentation by state DOTs to provide appropriate context on the relative risk and trade-offs for each alternative to stakeholders including EJ populations.

Urgency and Payoff

State DOTs would benefit from better information on how to assess, compare and communicate to stakeholders the potential impacts of different highway and transit alternatives to air quality considering the aggregate effect of all pollutants, and how in particular to do so in cases in which there are substantial EJ populations in close proximity to one or more of the alternatives under consideration. This research is needed to support transparency and informed decision-making for NEPA.

Suggested By

Christopher Voigt

[email protected]