A Retrospective Analysis of Heavy-Duty Vehicle Tailpipe Nitrogen Oxides Emissions Standards

This paper presents a retrospective analysis of the Environmental Protection Agency (EPA) 2007 regulations targeting NOx emissions from heavy-duty vehicles. We replicate EPA’s on-road emissions model and compare the assumptions used in its analysis—vehicle sales, scrappage rates, NOx emission rates, and vehicle use—with actual outcomes in 2022. This comparison evaluates the accuracy of EPA’s assumptions and their long-term impact on NOx reduction estimates, providing a basis to assess the accuracy of the similar methodology used in the recent 2022 standards. We find that EPA’s most significant prediction error was overestimating scrappage rates of older vehicles, which led to underestimated emissions both with and without the policy; on net, this resulted in an underestimation of emissions reductions by 0.52 million tons. Conversely, EPA underestimated miles traveled by older vehicles, which, on net, overestimated emissions reductions, as these high-emission vehicles traveled more than expected. Anticipatory sales effects before 2007 had minimal effects on emissions in 2022. Although certified emissions have consistently been below required standards, this discrepancy had only a minor effect on EPA’s overall emissions predictions.