Vehicle inspection & maintenance (I/M) programs

Vehicle emissions inspection and maintenance (I/M) programs require scheduled testing of a vehicle’s tailpipe and evaporative emissions to determine the effectiveness of its emission controls. I/M programs are federally mandated under the 1990 Amendments to the Clean Air Act for areas with particularly poor air quality; program areas are identified based on criteria such as air quality classification, population, and/or geographic location (US EPA-IM resources). Programs can also be implemented at the discretion of state and local governments. Program design varies by location, although most I/M programs for modern, advanced technology vehicles use on-board diagnostic computer tests (US EPA-Vehicle OBD, US EPA-State IM Programs).

Expected Beneficial Outcomes (Rated)

  • Reduced emissions

Other Potential Beneficial Outcomes

  • Improved air quality

Evidence of Effectiveness

There is some evidence that vehicle emissions inspection and maintenance (I/M) programs reduce emissions from light duty cars and trucks (Dehart-Davis 2002*, Harrington 2000*, Washburn 2001*, Eisinger 2005, Samoylov 2013, CVEIMP 2001*, Austin 2009), especially for pre-1985 model year vehicles (NBER-Sanders 2017). 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 pollution (NBER-Sanders 2017). 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 (Dehart-Davis 2002*, Washburn 2001*, Eisinger 2005), and nitrogen oxide emissions (Samoylov 2013), and particulate matter levels, especially among pre-1985 model year vehicles (NBER-Sanders 2017). Enhanced I/M programs, similar to Southern California’s program, can reduce older light duty motor vehicles’ hydrocarbon exhaust emissions by 14-28% (Eisinger 2005), and cars’ carbon monoxide emissions by 26% (Dehart-Davis 2002*). I/M programs may motivate pre-test repair work, improve repair effectiveness (Eisinger 2005), and create incentives for manufacturers to improve the emission characteristics of their vehicles, which would reduce emissions more than the amount attributed to existing programs (Harrington 2000*).

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 emissions (Wessel 2020*).

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 (Samoylov 2013, Moghadam 2010*, Bin 2003*, Washburn 2001*).

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 vehicles (US EPA-Vehicle OBD). 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 owners (Harrington 2000*). In most states with I/M programs, vehicles that are not repaired cannot be registered (US EPA-IM resources); 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 programs (Li 2017).

Impact on Disparities

Likely to increase disparities

Implementation Examples

As of 2020, the US Environmental Protection Agency (US EPA) lists vehicle inspection and maintenance programs in 31 states and Washington, DC (US EPA-State IM Programs).

Implementation Resources

US EPA-IM resources - US Environmental Protection Agency (US EPA). State and local transportation resources: Vehicle emissions inspection & maintenance (I/M).

US EPA-Vehicle OBD - US Environmental Protection Agency (US EPA). Vehicle emissions on-board diagnostics (OBD).

Citations - Evidence

* Journal subscription may be required for access.

Dehart-Davis 2002* - Dehart-Davis L, Corley E, Rodgers MO. Evaluating vehicle inspection/maintenance programs using on-road emissions data: The Atlanta reference method. Evaluation Review. 2002;26(2):111-46.

Harrington 2000* - Harrington W, McConnell V, Ando A. Are vehicle emission inspection programs living up to expectations? Transportation Research Part D: Transport and Environment. 2000;5(3):153-72.

Washburn 2001* - Washburn S, Seet J, Mannering F. Statistical modeling of vehicle emissions from inspection/maintenance testing data: An exploratory analysis. Transportation Research Part D: Transport and Environment. 2001;6(1):21–36.

Eisinger 2005 - Eisinger DS. Evaluating inspection and maintenance programs: A policy-making framework. Journal of the Air & Waste Management Association. 2005;55(2):147–62.

Samoylov 2013 - Samoylov AV. Improvement of the efficiency of vehicle inspection and maintenance programs through incorporation of vehicle remote sensing data and vehicle characteristics. Atlanta: School of Civil and Environmental Engineering, Georgia Institute of Technology (GT); 2013.

CVEIMP 2001* - Committee on Vehicle Emission Inspection and Maintenance Programs (CVEIMP), Board on Environmental Studies and Toxicology, Transportation Research Board, National Research Council. Evaluating vehicle emissions inspection and maintenance programs. National Academies Press; 2001.

Austin 2009 - Austin TC, McClement D, Roeschen JD. Evaluation of the California smog check program using random roadside data. California Air Resources Board, California Bureau of Automotive Repair. Report No. SR09-03-01. 2009.

NBER-Sanders 2017 - Sanders NJ, Sandler R. Technology and the effectiveness of regulatory programs over time: Vehicle emissions and smog checks with a changing fleet. Cambridge, MA; 2017.

Wessel 2020* - Wessel RJ. Policing the poor: The impact of vehicle emissions inspection programs across income. Transportation Research Part D: Transport and Environment. 2020;78.

Moghadam 2010* - Moghadam AK, Livernois J. The abatement cost function for a representative vehicle inspection and maintenance program. Transportation Research Part D: Transport and Environment. 2010;15(5):285–97.

Bin 2003* - Bin O. A logit analysis of vehicle emissions using inspection and maintenance testing data. Transportation Research Part D: Transport and Environment. 2003;8(3):215–27.

US EPA-Vehicle OBD - US Environmental Protection Agency (US EPA). Vehicle emissions on-board diagnostics (OBD).

US EPA-IM resources - US Environmental Protection Agency (US EPA). State and local transportation resources: Vehicle emissions inspection & maintenance (I/M).

Li 2017 - Li Y. Evaluating and improving the effectiveness of vehicle inspection and maintenance programs: A cost-benefit analysis framework. Journal of Environmental Protection. 2017;8(12):1541-1566.

Citations - Implementation Examples

* Journal subscription may be required for access.

US EPA-State IM Programs - US Environmental Protection Agency (US EPA). Vehicle emissions inspection and maintenance (I/M): Tampering, warranty, state websites and program evaluations.

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