A windscreen was smashed after a gang of youths threw a stone at a moving car on the A45 in Northampton.
Police were called at around 3.35pm this afternoon (Wednesday) to a report of a cracked windscreen.
A spokesman for Northamptonshire Police said four boys were seen throwing a stone or a rock at the car from a bridge above the A45 near the Great Billing junction.
The driver is said to be shaken but not injured.
Officers have searched the area but there is no sign of the four suspects.
Witnesses or anyone with information can call police on 101.