People drive at over 70 because they're impatient.
Besides, plenty of people have cars that can cope with going over 100 these days, all the fancy BMWs and stuff that's on the road these days. Maybe not your family hatchbacks, but having two classes of cars that go at different speeds would introduce dangers of its own. And we've already got the lorries and coaches that can't really go much faster.
( Moon Girl Technologies horrendous beanbag, Tue 16 Jun 2009, 13:28, archived)