Minnesota's new school bus safely law is in effect. Here's what to know.

If a school bus ahead of you starts flashing its red lights, Minnesota law requires you to stop — even if the stop arm hasn't swung out yet.

That change took effect in March, and as students across Minnesota head back to school Tuesday, the State Patrol and the Minnesota Department of Public Safety are reminding drivers it's the rule for the first full school year.

Under the old law, drivers had to stop only when a bus had both its red lights flashing and its stop arm extended — an interpretation the Minnesota Court of Appeals confirmed. Safety officials said that left drivers room to roll past in the seconds before the arm deployed, so lawmakers rewrote the statute. Red lights alone now mean stop.

"School is starting now, school is in session," said Minnesota State Patrol Lt. Brian Reu. "Slow down, pay attention to what might be happening with that school bus."

The scale of the problem is hard to miss. In the state's most recent one-day survey, 3,472 school bus drivers counted 514 vehicles illegally passing their buses — in a single day. Law enforcement wrote more than 2,000 stop-arm citations statewide last year.

You must stop at least 20 feet from the bus and stay stopped until the lights stop flashing. The requirement applies whether you're behind the bus or, on an undivided road, coming the other way. Drivers who don't stop face a misdemeanor and a minimum $500 fine.