Developing correction factors for reference fleet. The case of Atlanta I/M program evaluation.
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Roadside vehicle remote sensing is a common method to evaluate criteria pollutant emissions from vehicle fleets. A popular application of this method is the evaluation of the effectiveness of emissions reduction treatments (e.g. vehicle Inspection and Maintenance (I/M) programs) by comparing the emissions from the treated (experimental) fleet to those of an untreated (reference) fleet. Since no two large vehicle fleets are identical, the reference fleet is produced synthetically by application of correction factors. Current federal guidance assumes that closely located experimental and reference fleets behave similarly if normalized by model year distribution and Vehicle Miles Traveled (VMT). However inconsistency in the results from these evaluations questions whether these controls are sufficient to estimate emission reductions attributed to I/M with an acceptable level of uncertainty. In addition to age and mileage accumulation, emissions from vehicle fleets can be influenced by differences in a variety of other factors including differences in fuels and usage patterns and socioeconomic factors that can influence fleet composition and maintenance trends. This study evaluates the influence of variety of these factors in determining the reference fleet correction factors for the Macon and Augusta Georgia fleets for evaluation of the Atlanta, GA I/M program over a twelve-year period from 1998 to 2010. A series of "null" experiments were conducted to evaluate factors that might affect emissions across vehicle fleets of the same model year distribution. The results showed that in addition to model year and VMT, the reference fleet also required normalization by vehicle type, Vehicle Specific Power (VSP) and, in certain cases, fuel composition to produce a reliable reference fleet correction factors.