State safety officials say tragic accidents such as the pileups that claimed three lives Wednesday can be avoided if motorists practice one simple rule: Slow down.

"Fog banks do hit suddenly, but usually the fog becomes denser (as you approach it). You should slow down when you see the first traces of fog," said Gary Whitney, spokesman for the Utah Department of Public Safety.But that advice only works, Whitney said, if everyone abides by it.

The problems begin when one driver doesn't slow down. He suddenly sees brake lights a few yards ahead from cars that decided to decelerate once they couldn't see. He panics and slams on the brakes to avoid hitting the slower vehicles up ahead. But he has screeched to a halt on a freeway where cars that haven't hit the fog are cruising along at 60 mph. They won't notice that traffic in the dense fog has come to a halt until it is too late.

"If you slam on the brakes, you can count on getting hit from behind," Whitney said. "Once it's gone that far, not even gradually slowing down" will help motorists avoid a collision.

The Utah Highway Patrol speculates the above scenario of a fog-related pileup is what may have happened in a series of multivehicle accidents Wednesday morning on a stretch of I-215 in North Salt Lake.

Because fog appears in pockets, Whitney said, it is difficult to anticipate it and warn motorists of the hazards.

While Utah Department of Transportation officials have a record of fog-related accidents on I-215 where it crosses the Jordan near 6200 South, the section where Wednesday's pileups occurred - between the Redwood Road and I-15 interchanges - had no history of fog hazards.

"Our people say they can't recall a fog-related accident on this section of freeway in the past 10 years," said UDOT spokesman Kim Morris.

But the Jordan River runs next to the interstate's Redwood Road interchange and is about two miles north of a persistent fog problem on Beck Street. Morris said the Beck Street fog problem was reduced when nearby hot springs water was piped away from the area.

Short of educating motorists on how to deal with fog, Morris said there is no way to avoid fog-related hazards on I-215. "It's not practical to talk about relocating the freeway or the Jordan River."

Thousands of northern Utah motorists had to fight driving in heavy fog early Thursday, but conditions should begin improving, according to the Salt Lake office of the National Weather Service.

Clouds began forming above the fog early Thursday, and westerly air flows are expected during the weekend and into next week. It will be cloudy with continued hazy conditions Friday with a slight chance of snow late in the day.

High temperatures Thursday were expected to be in the mid-20s. Temperatures will be in the midteens Thursday night and near 30 degrees Friday, said William J. Alder, meteorologist in charge of the Salt Lake office.