Risk-Based Comparisons for Air Quality of NEPA Alternatives

Focus Area

Air Quality


Air Quality






2-3 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 specific pollutants, including 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, i.e., morbidity and mortality), then they could be compared and aggregated. In addition to potential direct impacts, the analysis would also consider potential indirect and cumulative impacts to the extent feasible and appropriate. NEPA alternatives could then be compared on a common basis, i.e., the potential air quality impacts in terms of aggregate risk. Those aggregate assessments could then be incorporated into NEPA documentation for communication to stakeholders including EJ populations to support informed decision-making on preferred alternatives. As cost and complexity are major challenges for formal detailed risk-based assessments, a simplified method and set of case studies are needed for optional application and reference respectively in NEPA analyses by state DOTs.

Accordingly, the objectives of this proposed study are to develop a simplified means to assess and report the relative risk of potential air quality impacts for NEPA alternatives (including assessments of potential impacts to environmental justice (EJ) populations) and apply that simplified methodology in case studies that may be referenced by state DOTs in their NEPA analyses. The simplified methodology would not involve economic valuations, just assessments of morbidity and mortality. The case studies would compare the potential impacts for typical highway and transit alternatives, including ones involving different proportions and proximity of EJ populations. As the results may vary substantially depending on the project types assumed for each alternative, the proposed study would identify project types for which the trade-offs may be expected to be the most significant, e.g., highway and transit/rail, and compare the results for those project types to those for project types where the results may not be as substantial, e.g., two highway alternatives. 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. State DOTs could optionally apply the simplified methodology in project-specific air quality analyses at their discretion. In either case, the air quality risk for NEPA alternatives could also be compared to other risk assessments for each alternative, such as those commonly reported for safety.

Urgency and Payoff

State DOTs would benefit from better information on how to assess, compare and communicate to stakeholders the potential impacts to air quality of different highway and transit alternatives in a way that characterizes the aggregate effect of all pollutants and, considering the increasing regulatory emphasis, how in particular to do so in cases in which there are substantial EJ populations near 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]