Pretty sweet idea!
These vests actually displays your mph on your back:
That's just what this Speed Vest does. Equipped with electroluminescent wiring on the back and wheel speed sensor, it displays a bright numeric readout that states just how fast you're moving in no uncertain terms. It runs on a 9-volt battery for six hours, which is plenty of time to really show off how fast you can get going down those hills.
Now can really get jealous when they zoom past me in traffic!
Hmm... I know a certain smug-one in the office that is getting this for his bday... via