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Implications of the Implementation of the MOBILE6 Emissions Factor Model on Project-Level Impact Analyses Using the CAL3QHC Dispersion Model

1. Summary of Objectives of Study

The release of MOBILE6² marked an important milestone in making available additional facility functional classes, increased number of vehicle classifications and improvements in emission estimates, now based principally on trip starts and length. However, the implication of how these and other changes will affect project-level analyses has not been explored. It is anticipated that the project-level analyst will require additional guidance in selecting MOBILE6 modeling options. In addition, most of the improvements in MOBILE6 have been directed at improving regional emission estimates, not project-specific estimates. Thus, the changes in MOBILE6 may inadvertently undermine the validity of the project-level assessment procedures.

The study has been designed to provide further insight on the implications of the use of MOBILE6 in project-level analysis. The project has three major objectives:

• Inform practitioners in the transportation community about the most critical and sensitive input parameters in MOBILE6 affecting project-level transportation and air quality analyses
• Identify under what conditions the use of MOBILE6 may lead to project-level problems and identify possible approaches to developing necessary inputs.
• Identify potential screening threshold procedures for project-level studies.

The principal findings from the study can be briefly summarized as:

• Application of MOBILE6 for freeways in the near future years coupled with high traffic volumes and high background concentrations may present problems not currently demonstrated with MOBILE5.
• Intersections with high start fractions and high volumes appear to have a strong potential for exceeding the carbon monoxide (CO) standard.
• Intersection modeling will need to change "worst case" modeling receptors from intersection-based to using a mid-block location. For mitigation, the traditional approach of increasing intersection capacity to achieve higher average speeds may result in overall emission increases.<
• It appears likely that detailed modeling can be excluded for both intersection and freeway locations with level of service (LOS) E or better under a wide variety of conditions, especially when looking beyond the near-term period (2015 or later).
• Potential problem locations requiring detailed modeling still remain. These sensitive locations include: locations in very close proximity to very high volume freeways; locations with an extraordinary rate of start emissions, such as near a park-and-ride lot or central business district (CBD) parking garage; a fleet much older than the national default age distribution; and locations with an unusually high background concentration.