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

4 Recommendation of Best Practices

Based on the work done in this study, a set of general recommendations or best practices has been developed. These suggested practices represent the general consensus among current practitioners and results presented earlier in the study. These suggested practices should complement or enhance existing guidance for project-level analysis when using the MOBILE6 model.

4.1 Inputs to MOBILE6 Most Critical for Project-level Analysis

The practitioner should attempt to specify the local I/M program in with as many details as possible. Applicable details include: the model years for the program, the stringency level, waiver rate, compliance rate, inspection frequency, vehicle covered and type of test. Where multiple I/M programs exist within a local area (e.g., multiple state programs), the practitioners should attempt to estimate the fraction of vehicles participating in each program at the location of the project-level analysis and develop a weighted emission rate.

Information should be developed specific to the local fleet age distribution and mileage accumulation (i.e., the local registration distribution data), as this data may have significant changes in the emission rate. If the registration data is not representative of what the on-road fraction is at the project location, adjustments should be made for the outside fleet through local surveys.

It is recommended that the practitioner coordinate with other agencies involved early in the process of air quality assessment. In particular, coordination should be made with the department of motor vehicle registration for local registration data and the air quality management agency to ensure consistency with the state implementation plan.

4.2 Recommendations on Local Screening-Level Assessment Procedures

Before applying the CAL3QHC model in a screening-level analysis, it is recommended that the agency review the current procedures to assure that the project is assessing the potential worst case conditions. This may be accomplished by following these procedures:

Closely examine the speed assumptions in the screening procedure in light of the changes in MOBILE6 due to minimum emission rates at 30-35 mph for speed curves, much lower idle emission rates and increased moving emission rates.

If the project includes a roadway segment (e.g., freeway) and an intersection setting, examine both, as the freeway setting may show higher concentrations than the intersection.

In applying the intersection model, place receptors at mid-block locations away from the corners of the intersection, as these are more likely to show the highest concentrations.

Strongly consider estimating the future background concentration based on an estimate of the regional VMT growth and MOBILE6 emission factor for the future year and then scaling from the current background concentration.

4.3 Recommendations on Exceptions to Screening-Level Assessment

For certain project-level settings it may be necessary to examine the use of MOBILE6 default assumptions or general model input procedures. The practitioner should consider the following questions when deciding on the appropriateness of the default or general model inputs:

Will the project setting experience a higher than typical start fraction? Is the projected location in the nearby vicinity of parking garage(s), regional shopping center(s) or large park-and-ride lot(s) for public transportation? If so, consider adjusting the MOBILE6 model inputs to characterize these higher emission rates.

Consider adjustments for project settings where the local fleet is suspected of being much older and/or having a higher proportion of light-duty vehicles than the proposed registration distribution.

4.4 Revisiting current mitigation strategies

It is recommended that historical strategies for CO mitigation be revisited in light of the changes with MOBILE6. In particular, the practitioner should consider the applicability of

Increasing the roadway capacity to achieve higher speeds. With the use of MOBILE6, this may lead to increased overall emission rates at the intersection and increased concentrations.

Optimizing the signal cycle timing. With the use of MOBILE6, this may have negative impacts on air quality if speeds are increased and the moving emissions are the dominant contributor to the maximum concentration.

Updated: 3/17/2015
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