Illinois State Police said not much can be done to prevent people from throwing rocks on interstate highways.

Three separate rock throwing incidents have happened in the Chicago area within the last week.

Guy Tridgell, an Illinois Department of Transportation spokesman, said drivers should call police if they see anything.

“These do happen from time to time in areas where there’s a lot of foot traffic,” he said. “There might be a correlation with the weather getting warmer, too”.

Tridgell said chain link fences have helped in high traffic areas, but the department doesn’t plan to install any at recent trouble spots.

At least seven cars were damaged from thrown rocks along I-57 between Kedzie and Sibley roads on Sunday. And last week, two teens were charged with throwing rocks along the Skyway.

State police said the incidents aren’t related.