CrimeBeat Q&A is a weekly feature where reporters answers readers’ questions about local crimes and the law.

I was driving down Highway 101 when a CHP officer on a motorcycle began swerving across all three lanes and then brought traffic to a stop, only to ride off. What’s the purpose of doing that?

What the reader witnessed was a traffic break known as a “round robin.” It’s a maneuver used by police to control the speed of traffic across multiple lanes. An officer first turns on emergency lights and begins to swerve back and forth across highway lanes to slowly bring down the speed of traffic.

While it may seem random, it’s always used to address a traffic issue ahead.

A CHP officer used the round robin technique Tuesday morning to control the speed of traffic approaching a collision between a big rig hauling grapes and a truck towing a camper south of Ukiah on southbound Highway 101.

Round robins also are used to slow traffic down to clear a roadway obstruction, whether a dead animal, mattress or load of grapes.

“Round robins are never used for routine traffic control,” said Officer Chad Ramsey of the CHP Ukiah office. “Typically people see these and have no idea what happened because we cleared the road before they realized there was an issue.”

Drivers don’t slow down when there’s a CHP car or motorcycle on the highway shoulder with lights flashing, Ramsey said.

“We really have to get in front of traffic physically to bring down the speed,” he said.

But vehicles don’t always heed the warnings to slow down and often speed ahead of the round robin. In these cases the officer continues on with the maneuver but radios ahead for another officer to pull the scofflaw over, Ramsey said.

Speeding ahead of round robins not only puts a driver in danger of an obstacle ahead, he said, but also risks the safety of the officer who’s slowing down traffic.

Submit your questions about crime, safety and criminal justice to Staff Writer Nick Rahaim at 707-521-5203 or nick.rahaim@pressdemocrat.com. On Twitter @nrahaim.