Can More Frequent Testing Improve I/M Program Effectiveness? Analysis of the Effect of Annual Testing in the Phoenix IM240 Program

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In this report we examine the potential for additional emission reductions from adopting annual rather than biennial testing in vehicle emission inspection and maintenance (I/M) programs. We tracked a fleet of cars reporting for testing in two biennial cycles in the Phoenix IM240 program. Using an assumption of linear deterioration following a passing I/M test, we found that annual testing in the Phoenix program in 1996 would have resulted in an additional 45%, 48%, and 27% reduction in HC, CO, and NOx emissions, respectively, over what occurred in the biennial program. More stringent cut points under a biennial program would have resulted in greater emission reductions, but with higher fail rates. Tighter cut points can also be applied to the annual program. In fact, an annual program might ease the progression to tighter cut points by resulting in relatively lower failure rates per test than biennial programs.

A small number of cars were identified that were given three I/M cycles, each roughly one year apart, in Phoenix. This group of vehicles had higher overall emissions and may not be representative of the whole fleet. However, these cars appear to have the same HC and CO emissions, but slightly lower NOx emissions, after two years as cars tested biennially. This result suggests that our assumption that emissions after annual testing would deteriorate at the same rate as observed in biennial testing may be optimistic. The findings from our analyses suggest that test-to-test emissions variability is a limitation of existing I/M programs, and is preventing them from properly identifying some vehicles with broken or malfunctioning emissions controls and ensuring that they are repaired. In theory, more frequent testing of suspected high emitters could help address this problem. One strategy that may work is to use remote sensing to identify suspected high emitters, and require that they come in for off-cycle testing, as frequently as necessary.

Arizona remote sensing data provide additional evidence that a significant benefit results from annual testing of older cars. Remote sensing measurements made in Phoenix of vehicles tested biennially under the Phoenix enhanced IM240 program and of vehicles tested annually under the Tucson two-speed idle program show interesting results. While newer vehicles from Phoenix have lower emissions than those from Tucson, as one might expect from the enhanced program, the older vehicles from Phoenix have higher emissions than those from Tucson. Phoenix vehicles over seven years old had emissions 37% to 47% higher than Tucson vehicles of the same age.

We conclude that more attention should be paid to promptly identifying and properly repairing the high emitters found in the older portion of the vehicle fleet. 

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