Evaluating Vehicle Emissions Inspection and Maintenance Programs by National Research Council

Author:National Research Council
Language: eng
Format: epub
Tags: Environment and Environmental Studies : Policy, Reviews and Evaluations
Publisher: NATIONAL ACADEMY PRESS
Published: 2001-11-15T16:00:00+00:00

In the SIP process, MOBILE is used to estimate what are referred to as SIP credits. States use the model to estimate the emissions reduction in a future year with implementation of an I/M program (or changes to an existing I/M program). These SIP credits based on MOBILE are only an estimate of the real emissions reductions. Actual emissions reductions from an I/M program can be measured only with real data from vehicles that have and have not been through the program. SIP credits are very important to states, because if they do not accumulate enough credits to demonstrate future-year attainment, they can be penalized economically by withdrawal of federal transportation funds and limitations on new construction requiring environmental permits. On the other hand, if states claim too much credit for I/M and the emissions reductions are not fully realized, then progress toward attaining clean air standards is hindered.

It should be noted that MOBILE estimates emissions factors in grams per mile by vehicle class (e.g., passenger cars, light-duty trucks, and heavy-duty diesels). To estimate on-road mobile-source emissions, these emission factors are then multiplied by estimates of vehicle miles traveled (VMT) by vehicle class. In most urban areas, VMT estimates are derived from transportation demand models. This chapter addresses issues in the MOBILE estimates of I/M program effects. There are just as many issues and problems in the estimation of VMT, but coverage of these issues is outside the scope of this report.3

3See EPA (1992c) for guidance on development of VMT forecasts.

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