Chris Carroll and his wife, Liz Mechem Carroll, encourage their two girls to ride their bikes to and from the Upper Nyack school near their home. At 7 years old, Sophia is a little young to go the half-mile on her own, so one of her parents goes with her and 5-year-old Lucinda.The trip is short, but the parents want to foster the good exercise habits and sense of independence that come with the daily ritual."I do remember the first time I was allowed to ride my bike into another neighborhood," Chris said. "It was freedom; it really was a big deal."Upper Nyack has bike racks and Carroll's daughters have locks, but riding a bike to school isn't as easy as it once was. And walking — which is how 90 percent of kids got to school in the 1960s — is the preferred mode of transport for less than 30 percent of kids who live within a mile of school, according to the Westchester County Department of Transportation.Walking or riding to school was once a pivotal, if utilitarian, part of a student's active life. In less than a generation, however, that mode of transport has changed as more students get individual rides. At the same time, childhood rates of obesity have skyrocketed, tripling for high school kids since the 1980s.Naomi Klein, the principal planner for the Westchester DOT, said her own fourth-grade son is often the only student the crossing guard at his White Plains school helps cross the street."The crossing guard said if he doesn't walk, there is no one else," Klein said.There are reasons why kids don't take advantage of this daily form of exercise, including parental concerns over traffic and safety, as well as a more general and low-level concern over ill-intentioned strangers. Cora Greenberg, executive director of the Westchester Children's Association, said the region is diverse and that real dangers vary."Some people are overly paranoid about their children in relation to reality, and others are struggling to keep their children safe in environments that really are dangerous," Greenberg said.When Robin Winter wanted a bike rack installed at Robert E. Bell Middle School in Chappaqua, she found unexpected opposition. As a child, she had ridden her own bike to school in Scarsdale — on the streets in the days before helmets — and she wanted sons Jacob and Noah to have the same experience.She wrote a few opinion pieces and tried to get the community interested in the nationwide Safe Routes to School program, which provides funding for biking and walking paths, but to no avail."A lot of people have said it's too dangerous," Winter said. "And I tell people to just try going the speed limit, and it really does make a difference."In New York City, which deals with many of the same traffic issues more urban areas such as Mount Vernon and Yonkers do, the group Transportation Alternatives found that installing speed bumps around schools made a difference. Amy Pfeiffer, Transportation Alternatives' program director for Safe Routes for Seniors, said it makes as much sense for kids to walk now as it did a generation ago."We've seen a pretty good increase in cycling," Pfeiffer said. "The more people we can get doing it, the better."Instead, many parents drive their children to school every day, increasing the traffic around the schools. Klein said that about 25 percent of morning traffic is school commuters, which increases the danger in the school zone itself.Sheryl Turk, a 28-year-old teacher at Pearl River High who commutes 10 miles on her bike from Nyack every day, said there aren't many students who take advantage of the bike racks outside the school, and that things have changed since she rode her bike as a student."People are not used to seeing bikes on the road, so I have to be more aware of the cars," Turk said