I live a few doors down from an elementary neighborhood school. Even though the school mainly draws kids from about a mile radius, I rarely see more than a few bikes on the bike racks. Only a few kids walk to or from school along. But I do see a long line of cars at the begining and end of the school day (actually, the school will keep the kids until 5 or 6 to accomodate parents' work schedules). There may be a few kids who come from miles away, but not enough to explain the long line of cars.
I started riding to school in the third grade (eight years old). My parents treated requests for rides with complete disdain. I might have well have asked to eat only desserts or for a house of my own. Walking or riding to school was something kids just had to do in their opinion. And I'm grateful.
I'm not condemning all parents who drive their kids to school. As dedhed explained, many have perfectly valid reasons (including some schools being built on major roads). But there are so many parents chauffeuring their kids that it's clear that at least a few of them are overprotective dolts who just don't know when to let go.