My DD2 is about to go into year 3, and while she qualifies for free travel (school is exactly 3km away), she will be walking some mornings. The route is reasonably suburban, only two main roads to cross, and they're only a lane each way with a crossing (one manned, one not).
DD1 (just finished year 4) and her friend started walking it in 2012, and they do it in half an hour. We recently walked 2/3 of the route and back home again (so 4km instead of 3km) and DD2 did it without a problem.
Of course, she will be walking it with at least 2 other (older) girls, if not up to 9 or 10 children altogether. I would not be happy for her to do it alone at this point.