Final answer:
Police officers touch the back of a car during a traffic stop to leave their fingerprints, which can serve as evidence in case of an incident. It's a routine safety tactic to ensure officer safety, check that the trunk is closed, and prepare for any unexpected actions from the vehicle's occupants.
Step-by-step explanation:
Cops often touch the back of a car during a traffic stop as a precautionary tactic. By doing this, the officer leaves their fingerprints on the vehicle which can serve as evidence that the officer was at the scene with the specific car. This procedure is implemented for officer safety and is a routine part of the training they receive. It can be used for identification purposes in case the traffic stop takes an unexpected turn, such as if the officer is harmed or the vehicle flees the scene.
Approaching a stopped car is one of the most vulnerable times for an officer, as they are exposed to traffic as well as the unknown potential actions of the occupants of the vehicle. By placing their hand on the car, they also ensure that the trunk is closed, as an open trunk could potentially hide a threat. Furthermore, this action is intended to ensure that no one is hiding in the trunk, and if someone were to jump out, it would give the officer a slight reaction advantage.