I was thinking about this some more. We have had speed governors available in this country to a while now. I seem to remember them being almost common on some cars in the 80's. People hated them for the most part. I think the only place you might still see something like that is on fleet or work vehicles. I really can't see people putting up with that.
Also, speed limits vary from state to state and even within states so it would also require another national speed limit. That may be an environmentalist's wet dream, but no one else wants it.