Hmm, this seems like a promising idea, though the limiter probably would need to change depending on highway/local streets. Because having a 70mph limit on a local road doesn't make sense,thats still way too fast. Going at a speed of 70 on a 60mph limit is going to feel limiting, but going at 70mph on a 40mph or less is going to give the thrill of speeding (which is even more dangerous).
So unless it can change limits depending on where the person is, (gps connectivity? bluetooth gps? something) its not going to really make things any safer, and infact it'll probably make teens more dangerous.
And of course this is a generalization of teens, but I'm speaking of those who are prone to speeding intentionally, not those who don't (for the latter it shouldn't really matter whether they had a limiter or not because they don't speed, right ?
)