An object, believed to be an air pellet, was fired at a car as it drove under a flyover on the A120.
The silver BMW, driven by a 51-year-old woman from Dunmow, had its windscreen smashed in the incident near Stansted Airport.
A man dressed all in black was seen on the bridge at the same time, at around 7am on Friday.
The driver, who had just turned off from the M11, was not hurt.
Anybody with information is asked to call Pc Red Leeson at Saffron Walden police station on 101 or Crimestoppers anonymously on 0800 555 111.