Police patrols have been stepped up after youths hurled missiles at vehicles on the Sheffield Parkway.
A number of bricks and stones was thrown from a foot bridge on Saturday – hitting vehicles below and shocking motorists who raised the alarm.
A police investigation into the incident is under way.
Two teenagers arrested over the incidents were quizzed over the weekend before being bailed pending further inquiries.
Insp Paul Ferguson, of Sheffield East Safer Neighbourhood Team, warned a fatal accident could have been caused by the vandalism.
He said: “There were a number of incidents of young people throwing bricks and stones from a footbridge down on to 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.”
■ Anyone with information should call police on 101 or Crimestoppers on 0800 555 111.