Police patrols have been stepped up after youths hurled missiles at vehicles on the Sheffield Parkway.
Bricks and stones were thrown from a foot bridge on Saturday - hitting vehicles below.
Inspector Paul Ferguson, of the Sheffield East Safer Neighbourhood Team, said: “There were a number of incidents of young people throwing bricks and stones from a footbridge down onto unsuspecting motorists travelling along the Parkway.
“Clearly, this is an extremely dangerous activity and these actions could lead to disastrous consequences.
“Patrols have been focused on the area.”
Two teenagers arrested over the incidents have been bailed.
■ Anyone with information should call South Yorkshire Police on 101.