Why don’t license plates have glow-in-the-dark letters? Sure, it would really only be useful to cops, but still... it’s easy enough. If someone’s plate lights are out, the plate will still glow, and it will be recharged by the headlights of traffic. This also keeps them perfectly visible when the vehicle is parked at night. Would probably also show up better on security cameras at night, which is easily the best benefit I can think of. As far as I know, there aren’t any drawbacks.