Vehicle inspection & maintenance (I/M) programs

There is some evidence that vehicle emissions inspection and maintenance (I/M) programs reduce emissions from light duty cars and trucks^{4, 5, 6, 7, 8, 9, 10}, especially for pre-1985 model year vehicles¹¹. However, as vehicle technology improves and fewer high-polluting vehicles from the 1970s and 1980s remain in use, I/M programs become less effective at reducing local air pollution¹¹. Additional evidence is needed to confirm effects of I/M programs for modern, advanced technology vehicle fleets.

I/M programs can improve local air quality by reducing carbon monoxide, carbon dioxide, hydrocarbon^{4, 6, 7}, and nitrogen oxide emissions⁸, and particulate matter levels, especially among pre-1985 model year vehicles¹¹. Enhanced I/M programs, similar to Southern California’s program, can reduce older light duty motor vehicles’ hydrocarbon exhaust emissions by 14-28%⁷, and cars’ carbon monoxide emissions by 26%⁴. I/M programs may motivate pre-test repair work, improve repair effectiveness⁷, and create incentives for manufacturers to improve the emission characteristics of their vehicles, which would reduce emissions more than the amount attributed to existing programs⁵.

Available evidence suggests broad I/M programs can be regressive; vehicle owners with low incomes fail emissions inspections more often than vehicle owners with high incomes. Owners with low incomes tend to drive older, higher mileage vehicles, and vehicle models with poorer inspection pass rates; having a low income also appears to increase the likelihood of failing an emissions inspection, perhaps due to differences in preventive maintenance schedules, repairs, driving conditions, and driving habits between owners with low and high incomes. I/M programs that include a means test and resources for vehicle owners with low incomes may be more efficient, effective, and equitable in the effort to reduce mobile source emissions¹².

Variable timing I/M programs that target vehicles with a higher likelihood of being high polluters, based on characteristics such as vehicle age, odometer reading, engine size, or previous emission test results, can be significantly more cost-effective than non-targeted annual or biennial retesting programs^{6, 8, 13, 14}.

Historically, I/M program costs have varied significantly with program design, stringency, and test equipment. Currently, however, most I/M programs use lower cost on-board diagnostic computer tests for modern vehicles². Inspection costs account for over two-thirds of the total cost of most programs, the remaining costs for actual vehicle repairs are borne by vehicle owners⁵. In most states with I/M programs, vehicles that are not repaired cannot be registered¹; this may contribute to disparities in access to transportation.

Experts suggest multiple inputs such as program participation rate, vehicle identification rate, and effective repair rate, must be included in thorough cost benefit analysis to help decision makers determine which program design features and implementation requirements will provide greater benefits than costs of I/M programs¹⁵.

As of 2020, the U.S. Environmental Protection Agency (U.S. EPA) lists vehicle inspection and maintenance programs in 31 states and Washington, D.C.³.