Published in the Proceedings of the
9TH CRC ON-ROAD VEHICLE EMISSIONS WORKSHOP
San Diego, California
April 19-21, 1999

Tom Wenzel
Environmental Energy and Technologies Group
Lawrence Berkeley Laboratory
Berkeley, CA 94720

We have examined multiple years of I/M data from three states, Arizona, Colorado and Wisconsin. Although all three states use IM240 testing, there are substantial differences between the three programs in terms of emissions cutpoints, use of fast-fail, and test cycle. An interstate comparison of I/M failure rates by vehicle model does not result in good correlation among the states, particularly for newer vehicle models. This is in part due to different cutpoints used in each state, and the low overall failure rates among newer vehicles. Comparison of average emissions by vehicle model, however, results in quite good overall agreement among the states. The range in average emissions is large; the dirtiest models can have average emissions several times that of the cleanest models. Only a few models have substantially different average emissions in different states; we examine some of the causes of these discrepancies.

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