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Study Explains 70-Car I-4 Pileup

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By NEIL JOHNSON

njohnson@tampatrib.com

Published: January 9, 2009

TAMPA - A lot of dominoes had to topple to trigger the cannonade of collisions a year ago today that turned a rural stretch of Interstate 4 into a vision from some level of hell.

A diabolic mix of smoke and fog swaddled part of the highway in Polk County before dawn on Jan. 9, 2008, blinding drivers and causing pileups that entwined 70 vehicles, killing five people and injuring 38.

National Weather Service meteorologists and University of South Florida researchers combined to study the knitting of events that led to the worst accident - in terms of the number injured - on Florida roads in 22 years, caused when smoke mingled with fog.

"It was a series of incidents that led to a catastrophe," said Charlie Paxton, one of the meteorologists who worked on the study, which will be presented next week at an American Meteorological Society meeting in Arizona.

A computer model no longer in use, wind direction, a subtle wrinkle in the terrain and the time of year all played a part.

The first domino to fall was a computer forecast that led to a controlled burn spiraling out of control.

The Florida Division of Forestry used the model to forecast conditions when it issued a permit for a burn less than a half-mile from the crash site the day before the pileup.

The model predicted the lowest humidity that afternoon would be 60 percent, high enough to allow burning. When the fire started, the humidity was 63 percent.

But humidity dropped to 29 percent during the afternoon, low enough that wildfires can spread quickly, which is exactly what happened as a fire intended to cover 10 acres exploded to 500 acres.

Although the fire was controlled by late afternoon, boggy areas continued to smolder, releasing smoke into night air that was growing foggy.

Smoke mixing with fog creates what some call "superfog." Smoke particles give water molecules in the air a nucleus for condensation, creating a combination that's thicker than fog or smoke alone.

December through February is prime time for overnight fog in West Central Florida.

The weather service forecast for the day of the burn better reflected reality, Paxton said.

It called for patchy areas of dense fog at night. Temporary warning signs were posted along the interstate and Florida Highway Patrol troopers made checks on the roadway, though the study concluded neither measure was enough.

As temperatures dropped overnight and the humidity rose to nearly 100 percent, fog started forming over large portions of Polk.

Around sunset, the wind dropped to nearly nothing. But there was enough to nudge the smoke to the northwest where it mingled with the fog.

A small hill, only 15 feet high, trapped the mixture and funneled it south toward the roadway.

"That was a subtle change in the terrain but enough," Paxton said.

As the superfog drifted over the interstate, another small hill south of the road trapped the mixture over the highway, where even the light wind died.

"All it took was a couple minutes for the smoke to cover the road," Paxton said.

And there it waited for the first drivers to encounter a shroud so thick visibility dropped to a few feet.

Since the wreck, the Division of Forestry has stopped using the computer model and started using weather service forecasts when deciding to issue permits for controlled burns, Paxton said.

But preventing a repeat of this type of accident won't be easy. The smoke and fog covered too small an area to be detected by satellites the weather service uses to find areas of dense fog.

Portable sensing equipment would help, said Jennifer Collins, an assistant professor of geography at USF who worked on the study.

The weather service and USF plan to work on computer models relating to fires close to highways. Those models may be able to focus on areas small enough to predict the chance of smoke mingling with fog, Collins said.

Another possibility is installing Web cams in fog-prone areas that could be monitored by multiple agencies, the study says.

Reporter Neil Johnson can be reached at (813) 259-7731.

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