An HGV driver was struck in the face by a gas canister that was thrown from a bridge over the A1(M) near Baldock and smashed through his windscreen.
Police are now appealing for witnesses following the “incredibly reckless act” between junctions 10 and 9 at around 7.30pm on Saturday 10 December.
Two people were seen on the footbridge, which leads to Coachman’s Lane in Baldock, Herts.
The driver was not seriously injured and he managed to safely bring his vehicle to a stop before police were contacted.
Detective Sergeant Michael Ball, from the North Herts local crime unit, said: “This was an incredibly reckless act which could have resulted in serious injury or even worse to innocent motorists.
“Throwing any items from a height onto a moving carriageway is extremely dangerous.
“I’m appealing to anyone who saw anything suspicious in the area around the time of the incident or who was driving near the bridge at the time and may have captured dash cam footage to get in touch.
“Anyone with information is asked to email me directly via firstname.lastname@example.org”