Tapping the Tail Light is a Police Ritual
The act of tapping or touching a tail light goes back to the humble beginnings when police officers started patrolling the highways and streets. Even before the invention of dash cams, tapping or touching the tail light is a police ritual.
Source: Mental Floss
Cops believe that tapping the tail light is a way to honor the officers that came before them. Even without a valid reason, they consider the act a ritual that is necessary to make the tradition alive. For some, it’s a ritual of passage where junior cops just follow their seniors’ tapping habit.