A Retrospective Analysis of Heavy-Duty Vehicle Tailpipe Nitrogen Oxides Emissions Standards
This article presents a retrospective analysis of the Environmental Protection Agency’s (EPA) 2007 nitrogen oxides regulations for heavy-duty vehicles and finds that EPA underestimated emissions by 0.12 million tons.
This paper presents a retrospective analysis of the Environmental Protection Agency’s (EPA) 2007 NOx regulations for heavy-duty vehicles. We replicate EPA’s on-road emissions model, replacing assumptions on vehicle sales, scrappage rates, NOx certification rates, and vehicle use with observed 2022 outcomes to test their accuracy and long-term effect on NOx estimates, providing a basis to assess methods used in setting the 2022 standards. After replacing all assumptions with data, we find EPA underestimated emissions in policy and no-policy scenarios by 0.71 and 0.59 million tons, with a 0.12 million ton error in their difference. The largest error came from overestimating scrappage of older vehicles, which underestimated emissions in both scenarios, on net understating reductions by 0.52 million tons. EPA also overestimated total vehicle miles while underestimating older-vehicle miles, on net overstating reductions by 0.51 million tons, largely offsetting the scrappage-related error. Assumptions on sales and certification rates had minor effects.
