Teenagers are gonna find a way to get dead at any/every speed. While something like 40-50 would be a wild speed to try to do on the potholed and crowded streets of NYC there are many rural roads as well as roads in more sprawled urban areas where being able to zip down the shoulder at such speeds where conditions permit is a worthwhile capability for the sake of both reducing travel times and speed differentials with automotive traffic.
That said, bicycle steering/suspension geometry is not made for that so it really is not the kind of capability a rider with any sense of self preservation will find themselves employing just anywhere.
But in reality there's a lot of untrained teens going different levels of too fast and that is something we should be trying to prevent.