Traffic Studies Are Junk ‘Science’

Community leaders and the courts are putting too much faith into dubious traffic studies that always seem to predict more and more driving, a new study argues — but that could all change if our transportation culture shifts, plus makes a few small tweaks to the legal language that guides our development decisions.

Land use experts Kenneth Stahl and Kristina Currans explored why “Traffic Impact Analyses,” or TIAs, hold so much sway over U.S. planning decisions, despite the fact that countless examples have shown that most of them wildly overestimate future car travel. Nonetheless, the researchers say traffic studies and the carmageddons they project are reliably held up by NIMBY neighbors as evidence that leaders either need to cancel human-scaled projects, like new apartment buildings in walkable neighborhoods, or “mitigate” their negative impacts, by widening roads and adding parking — even though a century worth of data shows those strategies almost always do the opposite because they encourage more people to drive.

The paper was published in the Journal of the American Planning Association.

“These studies are absolutely pervasive in land use and transportation, even though they’re totally unreliable,” said Stahl, who is a practicing attorney and the director of the environmental, land use and real estate law program at Chapman University.  “They’re being used to require traffic mitigations that only induce more driving. … You’re getting terrible policy outcomes, and they’re based on analyses that aren’t reliable at all.”

Stahl and Curran say, though, that the rising tide of criticism of conventional traffic models may be pushing the courts towards a “tipping point” where judges (and the planning boards who anticipate their rulings) may no longer be able to give traffic studies such an easy pass.

Getting past that point, though, will require a combination of advocates speaking out to demand that TIAs receive more scrutiny, and policymakers shifting the language of laws that require “substantial evidence” that a development will be bad news for traffic, and instead require a “preponderance of evidence” that many members of the profession agree. New laws can also be written to force developers to meet meaningful impact standards unrelated to congestion, like Colorado’s new state law that requires state projects to significantly reduce projected pollution associated with automobile use.

“Hopefully this provides some ammunition to debunk some of these myths,” added Stahl. “If the neighbors are fighting a good projectsby saying, ‘This study says a new development is going to lead to 5,000 new trips, advocates can say, ‘Hey, wait a minute; we know that’s not true. We know that this study is junk.’”