Are Traffic Impact Studies Still "Junk Science"?

Traffic Impact Analyses (TIAs) have become a central component of the land-use decision process. Municipalities use TIAs to measure the amount of traffic a project will generate and identify appropriate mitigation measures to accompany development. While TIAs are meant to provide reliable evidence to support land development approvals, many planners and engineers argue that they rely on an outdated approach and generally overestimate vehicle traffic in urban areas.

TIAs Face Credibility Challenges in Court

In "Are Traffic Studies 'Junk Science' That Don't Belong in Court?," (Journal of American Planning Association), Kristina M. Currans and Kenneth A. Stahl investigate whether TIAs methods meet the substantial evidence standard of review that courts use to assess expert testimony.

The substantial evidence standard demands credible information, similar to what courts require from experts. Conventional TIAs, which frequently rely on travel demand rates from the Institute of Transportation Engineers (ITE) Trip Generation Manual, often fail to match this benchmark. Less than 40 percent of surveyed East Coast agencies control local contexts in their analyses, and this is problematic considering historic ITE data often overestimates vehicle demand in urban contexts by ignoring factors like demographics and multimodal travel options.

In Table 1, Currans and Stahl identify several issues with conventional TIAs reliant on ITE data. For instance, the data is mostly representative of suburban contexts, predominantly vehicle-based rates, which is often outdated, and ignores changing land use and transportation technologies. Nonetheless, courts generally accept the ITE methodology as the industry standard and have done little to scrutinize its reliability.

Courts Question ITE Manual's Reliability in TIAs

While the ITE manual instructs TIAs to use data with caution and test local contexts, they often fail to ensure the accuracy of their findings under local conditions. Currans and Stahl show that some courts are starting to question the validity of incomplete ITE data as agencies and practitioners around the country adopt innovations. For instance, the Washington State Supreme Court rejected a TIA because it presented no local data to the Bellevue City Council. While this is the first case of a court rejecting the ITE method, this development signals that stricter standards for TIAs may be on the horizon.

Practitioners have several choices to improve TIAs should courts raise their expectations, for instance:

• Test the relevance of ITE data to local contexts instead of relying on national averages.
• Travel Demand Management (TDM) strategies can also be used to inform vehicular traffic for local areas, especially in urban contexts, reducing reliance on inaccurate estimates.
• By tracking how individuals travel using non-automobile modes, planners can gain a more accurate understanding of transportation needs before development.

Some states and cities have begun to adopt more holistic approaches to TIAs. In California, planners now estimate the contribution to greenhouse gas emissions from vehicle miles traveled on a regional basis to understand the environmental consequences of development.

The shift to analyzing personal travel instead of vehicle trips in some states has also facilitated the transition to demand management instead of vehicle accommodation. Furthermore, cities like New York, Washington, and Los Angeles have invested in collecting localized travel data to capture previously excluded contexts.

These changes are in alignment with the argument that the substantial evidence standard requires a more thorough analysis from TIAs. If these assessments are intended to serve as sound evidence for discretionary determinations, they must make projections attuned to urban contexts that can withstand judicial scrutiny. They should account for technological and social changes affecting travel behaviors, such as micromobility, disamenities from growing vehicle traffic, and the impacts of emissions among other issues.

Ultimately, Currans and Stahl provide important insight into emerging issues with TIAs. Moving forward, practitioners relying on conventional TIA methods will need to enhance their approaches to estimating the transportation impacts of development projects. If courts begin to take the substantial evidence requirement more seriously, the current industry standard will likely be insufficient. Fortunately, agencies and analysts have numerous innovative examples in practice to consider.

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