If you've ever been pulled over, or if regularly watch police shows like Cops, you may have seen a police officer touch a car's taillight when approaching a stopped vehicle.
Identification of Vehicle
Officers often press their hand against a car, typically the taillight or trunk, in order to leave fingerprints and that indicate the officer came in contact with the vehicle, according to the Santa Monica Police Department's information office. Officers employ this practice as a safety precaution. Should the officer be attacked or "go missing" the fingerprint will provide evidence that can confirm the perpetrator's vehicle.
Although video cameras are installed in most police cars, many officers still practice this physical safety precaution.
Older police teaching methods advocated tapping the trunk or taillight in order to surprise the driver in the car. This was thought to prevent those in the car from hiding illegal substances and to help the officer in identifying anything suspicious.
Police officers are no longer advised to tap on the taillights of cars, as this gives the officer's position away, making them more susceptible to attack.