Repair avoidance and evaluating inspection and maintenance programs

On-road remote sensing measurements of vehicle carbon monoxide (CO) emissions in Denver, CO, have discovered vehicles that, after failing a state inspection and maintenance (I/M) test, are avoiding repair through sales to individuals who register them outside the program but still drive the vehicles in the I/M area. This reduces the emissions benefits from the program and makes it difficult to analyze program benefits by comparing the emissions of eligible vehicles with ineligible vehicles since the program is increasing the emissions of the ineligible vehicles. On-road emissions measurements of CO collected in 1995/1996 after the first year of Colorado's new centralized biennial I/M program showed a pattern of high emitting vehicles, which were unable to pass the new test, being moved out of the program but not out of the area. We predicted that if this were the case the observed pattern would disappear for CO data collected 1 year later. New measurements made in 1996/1997 confirmed this prediction.