Heck, I even heard of a case in LA (a couple of years ago) where a pregnant woman was on a talk show saying that her unborn child was the passenger, so she should be allowed in the HOV lane. A minor problem there, in CA it's a violation to have two people wearing the same seat belt and you can't have two people behind the steering wheel.
Why ? The stickers and ther number are linked to my vehicles Vin # .... If somebody copies them and sticks them on there vehicle how does it affect me negatively - aside from possible extra vehicles in the car pool lane ......
Lots of Prius sighted in Santa Clarita lately... it's an unusual day I don't see three or more on my commute now.