Screening Procedures for Quantitative MSATs Analyses

Focus Area

Air Quality


Air Quality







Research Idea Scope

TERI Administrator Note: Selected as NCHRP 25-25 (70)

Screening level procedures for quantitative mobile source air toxics (MSATs) analyses for transportation projects are needed to provide state and other transportation agencies a ready and inexpensive means to meet emerging NEPA requirements and screen projects for potential MSATs impacts while minimizing resource requirements and delays associated with detailed quantitative studies (especially those in developing areas of science such as MSATs) that should be reserved for only the largest projects or those with the greatest potential impacts. One approach would be to develop standardized worst-case MSATs analyses for typical major improvements such as new freeways, additions to existing freeways, and major transit improvements. The worst-case analyses would be based on worst-case assumptions for traffic, emissions, dispersion and exposure. Both case studies and screening software could be applied for this purpose. State transportation agencies could then make use of the worst-case analyses in one of two ways: 1) by reference to the case studies for various sample highway and transit projects developed in place of a detailed quantitative study. For example, if a state were planning a new four-lane freeway, it could refer to an AASHTO or federal reference-standard worst-case MSATs analysis for a hypothetical four- (or even a six-) lane highway. The state could add local data such as traffic and land use information and make an qualitative argument that traffic, emissions and exposure (especially of sensitive populations) for their planned project are less than the referenced worst-case and therefore their project would have lower potential impact than that presented in the reference case study. 2) by making use of screening-level software developed by FHWA and/or AASHTO to facilitate the development by local jurisdictions of worst-case analyses for MSATs, similar in concept to the Cal3Interface software recently developed by FHWA for streamlining project-level worst-case analyses for carbon monoxide (CO). The software would allow the selection of various roadway configurations (number of lanes, skew angles, intersections, grade separations, traffic volumes etc) to use in the worst case analyses. Worst-case assumptions for dispersion modeling could also be similar to that for CO modeling and include standard worst-case meteorological and receptor location assumptions. Worst-case assumptions for exposure modeling would address land use and associated human activity levels.

Suggested Research Statement Text (From Chris Voigt, VDOT, Added Aug 2010)

General objectives:

Build upon the existing MSATs emission modeling capability provided by FHWA in their model EMIT, coordinating as appropriate with FHWA on their plans to update or enhance their EMIT model to better address MSATs using MOVES emission factors and additionally for any plans that might emerge in the course of the proposed NCHRP study to update or revise federal guidance for MSATs analyses.

The overall intent would be to advance the development of an enhanced version of EMIT to incorporate a screening capability for MSATs, including generation of charts and tables summarizing the results that could be directly incorporated into project reports and other public information materials.

A related objective may be to develop new or updated ADT thresholds or other criteria acceptable to FHWA for when the model could be used to meet NEPA requirements – in other words, not only create a screening model, but gain consensus on criteria as to when it could be used in NEPA documentation.

The proposed screening with EMIT could take as worst-case analysis approach, using worst-case traffic volumes and composition (truck and bus percentages) to generate B/NB comparisons and long-term (downward) trend lines. The qualitative text provided in FHWA guidance for MSATs analyses may need to be updated to suit.

The worst-case scenarios could include: new/expanded highways, transit service, and intermodal service, tested with various traffic compositions (truck and bus percentages). The user should be able to change some of the project particulars, e.g. before and after number of lanes, posted speeds, transit service levels, truck and bus percentages etc. The concept would be similar to the FHWA Cal3Interface model, which provides a similar limited set of options for worst-case modeling dispersion modeling.  

Draft Text: Screening procedures for mobile source air toxics (MSATs) analyses are needed to provide state and other transportation agencies a ready and cost-effective means to meet federal requirements for quantitative assessments for large projects (e.g., those over 150 thousand average daily traffic in the design year) and reserve limited resources for detailed project-specific quantitative studies for only the largest projects or those with the greatest potential impacts. The screening procedures must be practical for application to typical highway and transit projects and provide useful results for inclusion in report documentation and other materials distributed for public review. They should be designed to the extent feasible to be consistent with all applicable federal regulations and guidance for project-level (hot-spot) analyses currently in place or pending. 

FHWA currently provides a model, EMIT, that is very useful in the preparation of MSATs forecasts for NEPA documentation, as it greatly simplifies the development of detailed project-specific forecasts for MSAT emissions based on user-supplied traffic and fuel quality information. However, it does not have a screening version at present. Additionally, the EMIT model was generated in the era of emission modeling based on the MOBILE series of models from the US Environmental Protection Agency (US EPA) and, since it preceded the development and release of the new US EPA model MOVES2010, was not designed specifically for use with that new model. 

The following enhancements to the current version of the EMIT model are therefore of significant interest to state DOTs and others:

  • allow the input of emission factors or rates as appropriate from the new US EPA model MOVE2010 (and its successors) while maintaining, for a transition period as appropriate, the same capability for MOBILE6.2 factors;
  • add a worst-case scenario / screening functionality, described further below,
  • generate summary tables and charts showing MSATs forecasts and comparisons in a standard format that is appropriate for inclusion in project level reports (NEPA documentation) and other materials intended for public review. The summary charts and tables, which would at a minimum include build/no-build comparisons and long-term trends in emissions, would supplement the general text for qualitative analyses for MSATs taken as appropriate for the project documentation from federal guidance. The qualitative text from the federal guidance may need to be updated as appropriate for the worst-case screening scenarios. 

The screening functionality to be added to EMIT would be designed to generate worst-case emission forecasts for various typical highway and transit project using as appropriate worst-case traffic volume, traffic composition (truck and bus percentages) and fuel quality inputs. The project types would include new/expanded highways (restricted and unrestricted access), transit service, and intermodal centers. The new screening model would allow the input of select project- or location-specific values, i.e. allow some customization of the modeling as appropriate for specific projects. For example, the user would be able to change project particulars such as the before and after number of lanes, posted speeds, transit service levels, truck and bus percentages etc. The worst-case modeling concept would be similar to that incorporated into the FHWA Cal3Interface model for project-level modeling for carbon monoxide. The new MSATs screening model would generate standards sets of forecasts, including build/no-build comparisons as well as long-term trend lines for emissions for each of the MSAT pollutants, for inclusion in NEPA documentation. 

Note, while it is recognized that differences in traffic and emissions may occur at some distance away from a project, the proposed focus for the more generic worst-case analyses would be the immediate project vicinity. The more minor changes in traffic and emissions for roadways away from the immediate project area could be addressed qualitatively. 

[Optional] Recommendations for criteria to be applied for the selection of the new screening level approach for federal required quantitative analyses for MSATs would be an essential element of this project. The new criteria could include threshold values for average daily traffic volumes (e.g., between 150 and 250 thousand) along with supplementary criteria as truck percentages, bus percentages, incremental changes in these values attributable to the project, or other factors as appropriate.

Urgency and Payoff

The advantages of a federal/AASHTO lead in the development of case studies and ultimately screening level software include: 1. transparency and credibility, as the reference cases would not only be conducted by nationally recognized leaders in emission and exposure modeling, they would be subject to peer-review by national experts, 2. acceptance by the public, if the referenced worst-case studies are recognized as worse than what could reasonably be expected in most cases, and the studies have been conducted in an open process and led by national experts, 3. cost-effectiveness, as states would not have to unnecessarily and repeatedly conduct high cost detailed studies for their local jurisdictions if appropriate and accepted worst-case reference studies exist, 4. effectiveness, as the science will undoubtedly continue to evolve and the case studies will need to be updated, which can be systematically done (perhaps with scheduled periodic updates) at a national level, and 5. overall, similar to the experience with CO, a general worst-case approach for MSATs that incorporates applicable US DOT and EPA guidance and provides appropriate screening software would help states in the development of appropriate MSATs analyses while minimizing their costs and avoiding undue complexities and potential delays in their respective NEPA processes modeling would address land use and associated human activity levels.

Suggested By

Christopher Voigt, VDOT