Evidence of a particularly negative feedback loop.

In mid-winter, a heavy fog can descend over Salt Lake City. Caught between the mountainous Wasatch Range and the Great Salt Lake, fine particulates end up trapped within this layer of warm air, which then hovers over the densely populated valley that's home to roughly 80 percent of all Utahns. Salt Lake's fog has contributed to some of the worst air pollution in the country.

But there's a chance that state officials' efforts to warn residents about these dangerous conditions — through a daily green-yellow-red air quality rating — may have backfired. The system was intended in part to discourage driving at times of high pollution. But on days with red and yellow alert levels, it turns out that people drove more, according to a study of ten years of traffic counter data culled from the Utah Department of Transportation.

The study, first reported by the Salt Lake Tribune and set for publication shortly in the journal Transport Policy, looked at data from 28 traffic counters in Salt Lake and Davis counties from 2001 to 2011. The data covered both the winter, when small particulates known as PM 2.5 are a problem, and the summer, when ground-level ozone can be an issue, according to lead author Harvey Miller, now a professor of geography at Ohio State University.

Overall, researchers found that traffic was heavier on days with yellow air alerts in both the summer and the winter, while on red air alert days car traffic decreased slightly in the summer but grew during the worst of the winter months. Miller says that this is the problem with "soft policies" like air quality warnings, which convey information but leave individuals to interpret the best course of action. Being told that the air is bad makes it more important than ever, but also more unpleasant than ever, to ditch the car in favor of walking or public transit.

"You can imagine people going through this calculus. They have this balance between environmental responsibility and their own health," Miller says.

Even more compelling is the finding that the level of car traffic stayed relatively constant at downtown trackers but swung far more dramatically at the traffic counters located on the edge of the valley, at access points for recreation areas like Big Cottonwood Canyon and Little Cottonwood Canyon. Gaps were bigger on Fridays and Saturdays as well, when flexible schedules allow for weekend trips. On yellow alert days in the winter, traffic was up relative to green-day baselines 5.8 percent Monday through Thursday, 10.5 percent on Fridays, and a full 12.2 percent on Saturdays.

Similar studies have found less variation in individual transportation habits after air quality alerts in places like Chicago and Atlanta, but Miller says that residents of these cities don't have easy access to recreation areas that lay above the smog.

"I’m a big environmentalist, but on the weekends my wife and I would go to the mountains," he says of his own time at the University of Utah. "It’s too compelling: you’re in this unhealthy air, and you know that this fresh air is a half-hour drive away."

The intriguing findings hinted at in this study could force a rethinking of how government officials educate the public about pollution and their own role in creating it. It seems like a textbook example of that Econ 101 tragedy of the commons — the air's getting worse, but each Salt Lake City resident is better off escaping the smog in their car, on their way out to the mountains.

For a place with chronic pollution like Salt Lake City, there are a few examples to follow to deal with this kind of problem. At one extreme, they could follow the lead of Santiago or São Paulo, which have both implemented driving restrictions on high-pollution days. By forbidding cars with certain registration numbers to drive, cities have had some success in reducing pollutants in the air.

At the other extreme, Miller suggests that an important part of the solution will be providing the infrastructure that will allow people to escape the city without relying on their cars. Building better public transit connections between the city and the mountains—and providing the education that will encourage Salt Lake City residents to use it—could significantly reduce this auto-centric flight from the city each weekend, he says.

Discovering the best-fit solution for Salt Lake City will require further research, Miller says, using his study as a jumping off point. Next steps would involve a more intensive, behavioral approach to understand how people actually interpret the red-green-yellow air quality alert system.

"We don’t really know whether people actually heard these alerts or they just looked out the window and saw the smoggy air and said, I need to get out of the valley."

Top Image: This Jan. 23, 2013, file photo, shows a poor air quality sign is posted over a highway, in Salt Lake City. (AP Photo/Rick Bowmer)

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