Are Traffic Studies “Junk Science” That Don’t Belong in Court?

Jurisdictions rely heavily on traffic impact analyses (TIAs) to predict the traffic impacts of projects and calibrate appropriate mitigations. But TIAs are also litigation tools: Jurisdictions use them to satisfy courts that their land use decisions are supported by substantial evidence, or evidence that is credible and reliable. The problem, as we discuss in this Viewpoint, is that TIAs are not consistently credible and reliable. We explore some common criticisms—and provide a brief overview of a growing literature—regarding underlying vehicle estimation methods in practice that demonstrates the ways in which TIAs are widely flawed. Historically, courts have not expected much from TIAs, but our analysis shows a tipping point in which courts may begin to question whether conventional TIA methods constitute substantial evidence, suggesting an important need to innovate and adopt new data and methods in practice.