This astounding finding was the result of "new research from former University of Utah geographers, who found traffic has increased on days when alerts are issued for elevated levels of particulate matter [especially particulate matter 2.5 microns or smaller in size, also known as PM2.5] in the winter, as well as of ozone in the summer", writes Brian Maffly.
Messages regarding air quality and voluntary reductions in vehicle trips are not only ineffective at reducing traffic but apparently increased average daily traffic levels, especially on yellow days," reports the study, accepted for publication in the journal Transport Policy. "Much of the increases in traffic were … on roads leading to mountain environments that offer respite from the poor air quality."
Senior author Harvey Miller explains why these well-intentioned messages are self-defeating:
"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 don't appear to be influenced by these findings, as their own measurements last winter showed "a 3 percent to 7 percent decrease in traffic when air-quality notices were in effect".
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.
Maffly concludes by writing that "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."
Other regions impacted by air pollution have their own version of "bad air alerts". In the Bay Area, the Bay Area Air Quality Management District issues "Spare the Air Days" in summer and asks motorists to reduce driving and use alternatives to reduce ozone pollution. "Spare the Air-Winter" is aimed at refraining from wood burning to reduce particulate pollution.