A bag of rock salt has been dropped through the windscreen of a moving van from a motorway bridge.
It was being driven by a 40-year-old man between junctions 5 and 6 on the M4, near Langley, Berkshire, on Tuesday when the 5kg sack was hurled down.
A group of teenagers was seen on the footbridge before one of them pushed the sack over the railing.
Police said the "reckless" act smashed the windscreen and the driver had been fortunate to escape injury.
PC Callum Brown, from Thames Valley Police, described the behaviour of the youths as "stupid", and said it could have had "extremely serious consequences".
"It is very fortunate that the driver of the vehicle which was hit was uninjured and was able to safely pull over to the hard shoulder," he added.
"I would like to hear from anyone who was driving on the motorway at the time and saw a group of two or three teenagers on bicycles on or near the footbridge."