Many American schools have resorted to "no-bicycle" policies, accelerating a trend of fewer and fewer children riding to school:
According to [the U.S. Department of Transportation] surveys, in 2009 only 13 percent of all children walked or rode to school, whereas in 1969 nearly half (48 percent) did. The remoteness of the new schools is not the only cause: Among students who lived within one mile of school 43 years ago, 88 percent walked or bicycled, while today only 38 percent do.
It's not just an American problem:
One British study found that over the course of four generations, the distance that eight-year-old children in one family (the Thomases of Sheffield, England) were allowed to roam from home had shrunk from 6 miles (for great-grandfather George in 1926) to one mile (for grandfather Jack in 1950) to half a mile (for mother Vicky in 1979) to 300 yards (for son Ed in 2007). Another study reported that, on average, today's children are two years older than their parents were when first allowed to do things like use public transportation, sleep over at a friend's house, or babysit for a younger sibling.