A passenger in a car was injured when a brick thrown from a bridge over the M25 motorway in Surrey smashed the vehicle's windscreen.
The 29-year-old woman suffered minor injuries when the brick struck her arm at about 1945 GMT on Thursday.
Surrey Police said she was travelling on the anti-clockwise carriageway and was passing under the Ockham Lane bridge near Cobham.
Det Con Fiona Clare said the incident could have been fatal.
She said: "I cannot stress too strongly just how dangerous it is to throw objects from motorway bridges.
"Given the speed and volume of traffic on the M25 this incident could easily have had tragic consequences."