I’ve always been fascinated by devices that allow you to do something illegal and get away with it without making that activity any safer. Radar detectors are a great example. They aren’t collision detectors. They don’t warn you if your speed creeps up. They just tell you if someone who can cite you for speeding is in the area. Really, the radar detector's main function is as a priority-clarifier.
I owned a radar detector for a short time. I left it at home one day and got burgled. It was the most valuable thing they took (I was broke at the time). When the police asked what was missing, and I started the list with “Radar detector,” their enthusiasm for helping me decreased noticeably.