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Vehicle emission inspection and maintenance (I/M) programs are the US EPA’s primary policy to reduce on-road emissions of in-use vehicles. The EPA is preparing guidance for states on how to use on-road emissions measurements made by roadside sensors (remote sensing) to evaluate the effectiveness of their I/M programs. In its guidance EPA describes three methods of using remote sensing data for such evaluations: the comprehensive method, the step change method, and the reference method. We used an extensive number of remote sensing measurements in the Phoenix area to estimate program benefits under each method, in order to understand how each method measures emission reductions under an I/M program. The extensive number of measurements also allows testing the sensitivity of results obtained under the comprehensive method to the month in which the measurements were taken. We found that the comprehensive method estimates a small reduction in CO emissions (3.3±0.6%) from the I/M program; however, the step change method estimates no program benefit. A reference method analysis comparing on-road emissions of fleets in an enhanced and a basic program indicates that older vehicles have higher on-road emissions under the enhanced program than the basic program. Discrepancies between these results are discussed, as are several factors which appear to influence results under each method.