Moving cars damaged by stones or pellets on A120
Stones or pellets have been reportedly thrown or fired at vehicles on roads in Essex.
Police received 13 reports from people travelling along the A120 and B1383 Stansted Road at Birchanger between 20 September and 11 December.
The Essex force is working with Hertfordshire Constabulary after other reports close to the county border.
Officers are carrying out "high-visibility" patrols in the area.
Insp Colin Cox said: "If these were deliberate acts, then those responsible are putting drivers... at serious risk of harm.
"Fortunately, no one has been injured during the incidents we are investigating."