Before the age of body cameras, police officers who pulled someone over needed a physical technique to leave evidence that they were there in case something went wrong or the officer went missing. They came up with a simple, yet effective technique to leave their fingerprints at the scene: touching the tail light of the car. This practice offered the ancillary benefits of scaring the driver into incriminating themselves if they were concealing weapons or contraband in the car, and cracking a potentially-intoxicated driver’s artificial composure.
That being said, new technology and the risk posed to police officers has meant this practice has, for the most part, been rendered obsolete. Now, cameras and other tracking devices fill the place of contact tracing through fingerprints, and police departments advise their officers to avoid this touch because it puts them directly behind the car, and therefore slightly more at risk.