Why do cops touch the rear of a car? It is a shock tactic to the passengers inside the vehicle. When a police approach and touch a car, the drivers and everyone inside the car will usually stop whatever they are doing, just to not look suspicious. The tapping on the car’s rear is sort of an indication that the officer is close enough to see whatever the people inside are doing, so there is that psychological tactic in play. The people inside the car would then be stunned on what to do and stopped them from hiding any visible malicious items, maybe drugs or alcohol. This practice of touching the tail light or the trunk has proven to be effective, especially in arresting drugs peddlers and owners, and intoxicated drivers to name a few.