When you see an emergency vehicle off to the side of the road with lights flashing, you're supposed to move over a lane or slow down.
It's been the law in Michigan since 2001.
But what if that squad car pulled over isn't writing anyone a ticket.
What if it's a trap?
That is to say, a vehicle with lights flashing just so officers can watch for drivers who don't switch lanes or hit the brakes.
In a two-hour Lansing Police Department sting last week, 40 drivers drove into just such a trap.
"I have a hard time with that," driver Laura Olson told us.
Olson wasn't one of the 40. And she understands the point of the law: emergency responders have been hit and killed by omcoming traffic.
But she says it just doesn't seem right when there's no real emergency.
"Forcing people to move over when there's nothing going on runs the risk of causing accidents," Olson said.
But if you think you might get out of the ticket because the traffic stop wasn't real, the law, it appears, is not on your side.
"There's nothing I can find in the statute that says there's anything wrong with what they're doing," said Ron Bretz, a professor at Thomas M. Cooley Law School in Lansing.
Bretz says the state law doesn't say anything about whether the emergency vehicle has to be responding to an actual emergency.
Lansing City Attorney Brigham Smith, who may be prosecuting the tickets, said he thought the practice was fair. Even if the traffic stops are staged, Smith says, drivers are still breaking the law.
Ignorance, of course, is not typically a defense.
But Bretz says drivers might have been able to get out of a ticket when the law was newer by arguing they weren't aware of the law.
That would not work now, he said. "It's been six years. People have to learn: we have to pull over."
If they don't, they could get a city traffic ticket or even face misdemeanor charges under state law.
"You can get up to a $500 fine and 90 days in jail," he said.
Cautionary words for drivers -- since similar stings could happen again.