Corrections to Mileage Accumulation Rates for Older Vehicles and the Effect on Air Pollution Emissions

The MOBILE6 emissions model, currently under development, uses mileage accumulation rates that assume that the average 25-year-old car has been driven more than 337 962 km (210,000 mi), and that the average 25-year-old pickup truck has been driven more than 402 336 km (250,000 mi). Cumulative mileage is used in the model to calculate emission factors that increase with mileage due to air pollution control device “deterioration.” Odometer readings taken in previous studies by the Environmental Protection Agency and additional data indicate that average mileage accumulations may be much less [i.e., 201 168 km (125,000 mi)] than those used in the MOBILE6 model. This has the effect of overestimating emissions from older vehicles. A simple model is presented that accounts for scrappage of older vehicles as a function of cumulative mileage. This model predicts average cumulative mileage that is much closer to actual odometer readings (taken in Nashville, Tennessee) than the default values used in MOBILE6. The use of more accurate cumulative mileage values for older vehicles should provide improvements to the estimation of emissions from the vehicle fleet.