Drive reader Doug was headed east on Interstate 94 in the vicinity of the southbound Interstate 35W exit in downtown Minneapolis on a recent Saturday morning when he noticed a highway patrol car with its siren on and lights flashing in the westbound lanes making odd driving maneuvers.

"The police car was 'swerving' left, then right, then left crossing all the lanes each time," he said. "There was not a lot of westbound traffic at the time, but the cars behind, maybe a quarter of a mile or more behind, were all slowing down. What was the purpose of the officer repeatedly crossing all of the lanes? Was it a warning of some kind?"

To motorists like Doug, it may have looked like the trooper was drunk or driving very erratically, but the officer was doing what's called the "Wiggle," said Lt. Tiffani Nielson of the State Patrol.

Ladders, vehicle parts, cargo and other material jettisoned from vehicles can obstruct traffic and have the potential to do great harm. A 2012 report from the U.S. Government Accountability Office found that such items lead to about 51,000 crashes annually on the nation's roads. More than 10,000 people are injured and 440 die as a result of vehicles striking something that came off another vehicle or an object lying in the roadway.

So it's imperative that somebody clear away wayward debris as quickly as possible, and often that job falls to a state trooper.

Of course, stopping on a freeway and stepping into traffic whizzing at 55 miles per hour — does anybody drive 55 anymore? — isn't a safe thing to do. You might recall that in February, a Blaine man was seriously hurt when he stopped on I-35W in Roseville and was hit by a car while trying to remove a recliner from the freeway.

To clear out space, troopers will activate their siren and lights and zigzag across traffic lanes to get traffic behind the squad to slow down. It's similar to when a yellow caution flag comes out during a Formula One race. Racers have to hold their position behind a pace car.

Drivers who see a trooper doing the Wiggle should stay behind the squad. Motorists should stop when the squad stops because that is the signal that the trooper is getting out of the vehicle to pick up debris or help a tow truck move a disabled vehicle to a safer location.

"They should not try to squeeze by on the shoulders because the trooper is likely going to use one of the shoulders to move the debris," Nielson said. "Lots of people go around on the shoulder and then the flood gates open. The trooper will resolve the situation quickly, because we know it's dangerous to stop on a freeway, and will get traffic moving again ASAP."

What to do when stuck in traffic

Reader Amber asked what action drivers should take when they are stuck in bumper-to-bumper rush-hour traffic on the freeway and police are trying to get through. Move or just stop?

Again, Nielson provides the answer.

In most cases, the patrol will stay to the left and ride along the stripe marker between the furthest left lane and the adjacent lane to the right. Motorists in the far left lane should move onto the left shoulder while traffic in the adjacent lane should move to the right.

"It's sort of the parting of the seas," Nielson said. "That is where there is the most room for us."

Follow news about traffic and commuting at The Drive on startribune.com. Got traffic or transportation questions, or story ideas? E-mail drive@startribune.com, tweet@stribdrive or call Tim Harlow at 612-673-7768.