An object was thrown from a footbridge over the A120 on Saturday.
The incident, which happened near the B1417 Rayne Road, near Braintree, 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.
Anyone with information is asked to contact police at Braintree police station on 101.