Youths spotted throwing missiles at cars from a bridge in Sheffield are being hunted by South Yorkshire Police.
They were spotted throwing an object at a car on the A57 Mosborough Parkway on Thursday night.
It is the second incident reported in the area recently.
Police patrols have been stepped up in a bid to catch the offenders.
Inspector Jason Booth said: "Incidents like this are incredibly dangerous and place motorists and their passengers at serious risk.
"Someone could get seriously injured or worse as a result of this reckless behaviour and we have increased our patrols in the area to try and identify those responsible.
"If you have any information about who might be doing this, please give us a ring and let us know."
Call South Yorkshire Police on 101.