The Trouble With Traffic Studies: Why Bad Traffic Predictions are Making Our Cities Worse and What Courts Should Do About It

Abstract

Land use decisions today almost invariably require predictions about the amount of traffic a new development will generate, but these predictions are often inaccurate because the predominant method used to predict traffic impacts tends to significantly over-estimate vehicle traffic. As a result, land use decisions predicated on such inaccurate traffic predictions end up perpetuating automobile dependency and generating hostility to new housing development, deepening a severe housing crisis throughout the country. Planners, lawyers, developers, elected officials and courts mostly seem content to ignore these obvious problems, but their complacency may not last much longer. This article argues that the conventional, vehicle-oriented method for predicting traffic impacts does not withstand judicial scrutiny under any of the standards of review applicable to land use decisions. Therefore, we conclude that courts should review predictions of future traffic with greater skepticism. Indeed, the United States Supreme Court recently heard oral argument in a case, Sheetz v. County of El Dorado, 84 Cal. App. 5th 394 (2022), appeal docketed, No. 22-1074 (U.S. argued Jan. 9, 2024), that may require courts to review traffic predictions more closely. Our research should encourage members of the public to push back against traffic studies that over-estimate traffic impacts, and spur agencies to explore and adopt more reliable methods for estimating the traffic impacts of new development.

Keywords: traffic studies, land use, exactions, Sheetz v. County of El Dorado, substantial evidence, rational basis, fairly debatable, special assessments