The Daily Mail has a great article on the declining mobility of children in cities and suburbs. Included is a great map of the wondering space of children over three generations. Its gone down from about 6 miles in the early 1900s to about 300 meters today.

The reasons for this are pretty much what you'd expect: more roads that are more dangerous to cross, increased fear of child predators and just fewer places to go. But you do have to wonder how decreased walking mobility for children will affect them in the future, not just in health, but of their willingness to walk when they're adults.