Policemen often press their hand against a stopped vehicle, usually the taillight or trunk. Officers employ this practice as a safety precaution. This way they are leaving fingerprints and DNA that indicate the officer was in contact with the vehicle.
As Santa Monica Police Department’s information office stated, if something happen to the policeman the fingerprints will provide evidence that can identify the perpetrator’s vehicle.
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.
Today in most police cars video cameras are installed, but many police officers still practice this physical safety measure.
Earlier tapping the trunk or taillight was used in order to surprise the driver in the car. Tapping should prevent people in the car from hiding illegal substances and to help policeman in identifying anything suspicious.