An object was thrown from a footbridge over the A120 yesterday.
The incident, which happened near the B1417 Rayne Road, occurred at around 2.30pm.
A driver reported that she had to swerve when a small piece of wood was dropped from the footbridge.
She believes the object was thrown by a youth, so police are asking parents to ensure that it is not their children committing this dangerous act.
Police immediately searched the area, using both local officers and the force helicopter, but no-one was found.
Chief Inspector Rachel Nolan said: "This is serious and it is fortunate that this incident did not result in any injuries. These children may think it is just a game but it is very stupid and dangerous."
"I would ask parents to please talk to your children and make sure that it is not them committing this offence.”
Anyone with information is asked to contact police at Braintree police station on 101.