Review of Sierra Research Report “Development of a Proposed Procedure for Determining the Equivalency of Alternative Inspection and Maintenance Programs”

The Sierra report proposes a methodology for states to use to demonstrate that their Inspection and Maintenance (I/M) programs are obtaining emissions reductions similar to those estimated by the MOBILE emissions factor model. The proposed methodology involves comparing fleetwide test average emissions of a state I/M program with those of a benchmark program (Sierra suggests using the Arizona IM240 program for the benchmark program). The fleet emissions are determined based on the final (passing) IM240 test results of a stratified random sample of 1,600 vehicles. Average fleet emissions are determined based on the age distribution of the fleet (Sierra suggests using three age groups, pre-1975, 1975-1980, and post-1980). The fleet emissions are then compared with the fleet emissions of the benchmark program, reweighted using the evaluated program’s vehicle age distribution.