Why do police officers touch the rear of cars during traffic stops? Tim Dees, Retired cop and criminal justice professor, Reno Police Department, Reno Municipal Court, and Pyramid Lake ... There are at least two reasons: Pressing down on the trunk lid or tailgate of a vehicle will close the latch if it's open. This locks in anyone who might be behind it, waiting to ambush you. Putting your palm print on the rear of the car ties it to you forensically if you are assaulted and the vehicle leaves. The palm print can be washed off, of course, but hopefully the vehicle will be captured before that happens.