I think youth culture may have something to do with it as well, not just commuting; especially since the original article based its statistics on the rates of teenagers getting licenses, and I'd be surprised if those are primarily based on commuting preferences. In my dad's generation, having a car at 16 was socially a big boost, while I think it's just not as important anymore for teenagers. There was a heydey of "teenage car cool" in American culture with drive-in theaters and the like, and a long plateau where other car-centric things (like suburban malls) were at the center of teenage social life, but it's not clear that's (as) true anymore.