Skip to Content
Streetsblog USA home
Log In
Car culture

Why Traffic Studies Are ‘Junk Science’ — And Why We Rely On Them Anyway

12:33 AM EDT on April 4, 2023

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."

All those big, scary numbers in a typical traffic study, of course, aren't totally made up, as urban planning professor and study co-author Currans can attest. They're typically based on the number of vehicle trips that similar land use choices have generated in that region the past, often as outlined in the Institute of Transportation Engineers’ Trip Generation Manual, which provides industry-standard estimates for how much driving various new development types, like grocery stores or stadiums, is "likely" to induce based on national data.

Many agencies, though, don't adjust those blunt-instrument models for complex local realities — the researchers pointed out that one study found less than 40 percent of surveyed East Coast agencies use local data or control for local contexts" — or acknowledge that the Institute's rates are typically based on "vehicle-oriented contexts with free and unlimited parking and little to no multimodal access" and offer "severely limited" data on urban areas.

Nor, critically, do they stop to question whether all those new driving trips even should be accommodated — or whether the frustration of sitting in a traffic jam is just the push some drivers may need to walk, bike, or take the bus rather than drive.

Nonetheless, Stahl says that when traffic studies are presented to decision makers in quasi-judicial settings like local planning commissions, they're usually deemed to have risen to the legal standard known as "substantial evidence," simply because they've been rubber-stamped by a prominent industry group — regardless of whether they've met the actual legal definition of "substantial evidence" that is "credible and reliable," which TIAs plainly aren't. And on the rare case that a NIMBY neighbor uses a traffic study to sue the developer of a new apartment building in a walkable neighborhood for theoretically wrecking his car commute, real judges are likely to favor the status quo by recommending that the project be cancelled.

"The courts aren’t experts in land use and transportation planning; they don’t want ot have to rigorously review every traffic study," Stahl added. "If the industry’s been using [this standard] for years, that’s good enough. ... It’s circular; everyone in the industry uses it because they know that’s what courts will accept, and courts accept it because everyone in the industry uses it."

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.'"

Stay in touch

Sign up for our free newsletter

More from Streetsblog USA

Tuesday’s Headlines Are Getting Warmer

EVs and renewables are not going to be enough to stave off a climate catastrophe, scientists are warning officials at an international conference.

December 5, 2023

How (And Why) To Start a Delivery Bike Revolution

Delivery vans and trucks are responsible for nearly a third of urban emissions, and a lot of congestion and traffic violence, too. Here's how cities can replace many of them with clean, safe cargo bikes.

December 5, 2023

“Watch Out, Amsterdam”: Santa Monica Cuts Ribbon Opening Ambitious Curb-Protected 17th Street Bikeway

Santa Monica's recently completed 17th Street bikeway improvements have a "region leading design" featuring Southern California's first protected "Dutch-style" intersections, plus concrete curb protection, and makes great connections to the city's growing bikeway network.

December 4, 2023

Monday’s Headlines Are For the Children

For the 1 billion children who live in cities worldwide, the streets are too dangerous for them to play outside.

December 4, 2023
See all posts