Thick smoke from a blaze set to reduce wildfire risks blinded motorists Wednesday on the main highway across northern Arizona near the Grand Canyon, causing numerous accidents as haze clouded the freeway for hours and reduced visibility to 20 feet, officials said.
The fire to burn dry brush and trees was set Tuesday in the Kaibab National Forest that surrounds Flagstaff, and officials knew that a wind shift overnight would send smoke toward Interstate 40.
But they were surprised that it did not dissipate as predicted, said Brady Smith, a U.S. Forest Service spokesman.
Multiple collisions with minor injuries to motorists and passengers were blamed on smoky haze that settled over the highway for about five hours. Authorities closed I-40 for hours to prevent more accidents.
Police had not immediately determined whether the poor visibility was the cause of a fatal accident after a vehicle was sandwiched between two tractor-trailers before dawn, Arizona Department of Public Safety spokesman Bart Graves said.
Controlled burns are frequent events in Arizona this time of year as forest managers work to thin brush and trees that can present major wildfire risks in summer months.
Smoke from the fires can cause a nuisance for residents and tourists who flock to the Flagstaff area this time of year to look at fall foliage only to encounter a haze overhead.
Grand Canyon officials warned this week that controlled burns on at the canyon's North Rim could produce smoke visible throughout the national landmark. And burns to the south in the Prescott area earlier this month prompted complaints from residents about poor air quality.
Kaibab National Forest officials had announced Tuesday that the burning operation would produce visible smoke along the highway, and Smith said electronic signs cautioned drivers about the potential hazard.
The planned fire was set west of Flagstaff and forest officials thought there would be enough air movement overnight Tuesday into Wednesday to dissipate the smoke.
But the area at this time of year experiences temperature inversions allowing smoke to be trapped close to the ground and hover over the highway, said Cory Mottice, a National Weather Service meteorologist in Flagstaff.
"It almost always gets trapped after dark," he said. "It's just a question of where the wind blows it."
Forest officials thought weather conditions would vent smoke near the freeway more than it did in low-lying areas, Smith said.
"I believed they used good judgment based on the conditions and the information that they had," he said.
Traffic was diverted before dawn until about 9 a.m. on I-40 between Flagstaff and the Grand Canyon gateway town of Williams to an alternate route that added about one hour of travel time for motorists.
The highway was reopened when the smoke dissipated.