The dual message of the air-quality notices may be working at cross-purposes, according to senior author Harvey Miller.
"You are telling people, don't drive, but at the same time you're saying the air quality is unhealthy," said Miller, a one-time U. department chairman who moved to Ohio State University this year. "You're telling them staying in the valley is unhealthy, so what do they do? They head to the mountains."
Utah Department of Transportation officials acknowledged they too have documented poor public response to bad-air alerts, but new measures to reduce traffic on smoggy days appear to be paying off.
Last year, UDOT expanded its use of overhead electronic freeway signs to advise motorists of red and yellow alerts and worked with the Division of Air Quality to better forecast bad-air days and plan for them, according to agency spokesman Nile Easton.
Last winter, memorable for its particularly noxious inversions, UDOT documented a 3 percent to 7 percent decrease in traffic when air-quality notices were in effect, but the agency will continue scrutinizing traffic patterns this coming winter to determine whether such promising numbers are the result of a statistical anomaly or of a real trend.
"Changing public behavior regarding the convenience our vehicles bring can be difficult. But Utahns have shown a willingness to do so when they fully understand the impacts travel can have," Easton said. "We learned this lesson in 2002, when transportation worked better during the Winter Olympic Games than they had at any other recent hosting city. We've also seen great results during construction season as drivers have adjusted commute times."
Still, the study findings underscore the limits of "soft" policies that rely on voluntary action, particularly when they discourage walking, cycling and other activities that get us out of our cars.
"People may be acting out of personal health protection, at the cost of our air shed," said Erin Mendenhall, executive director of Breathe Utah. "Whether picking the kids up at school in the car instead of walking, leaving the valley to higher and assumed less-polluted elevations, or driving to the gym instead of running around the park, this research shows we're hopping in the car when we know we shouldn't."
Miller wondered if his findings point to the need for "hard" policies pushed by clean-air advocates, such as providing free public transit and lowering speed limits on bad-air days.
"You have to provide the capability for people to find alternative means of travel," Miller said. "An example would be to provide more public transit to the mountains."
Miller's team marshaled traffic information from UDOT's automated counters and correlated it with air-quality and meteorological data. Team members used readings at 28 traffic counters that remained in fixed locations over the course of the study period from 2001 to 2011. Most of the counters were in Salt Lake County, including at the mouths of Parleys, Big Cottonwood and Little Cottonwood canyons, as well as several in Davis County.
In winter, traffic counts on yellow-alert days were up relative to green days every day of the week, 10.5 percent on Saturdays, 12.2 percent on Fridays, and 5.8 percent Monday through Thursday. Traffic counts on red-alert days were also elevated but not as much as seen on yellow days.
"We see a strong signal here, but it is only a first step in this research," Miller said. "There needs to be follow-up doing behavior surveys that could suggest possible remedies."
His co-authors include graduate students Calvin Tribby and Ying Song, who moved to Ohio with Miller, and Ken Smith, a professor in the U.'s department of family and consumer studies.