Cities across the U.S. are turning their attention not only to green education, but to how students get to school. Forty years ago, half of all students walked or bicycled to the schoolhouse. Today, that number has dropped to 15 percent, while 60 percent of youths are toted in a car. The shift, brought on by fears of traffic hazards and stranger danger, has contributed to increases in other problems: obesity rates, traffic congestion, vehicle accidents, and air pollution around schools. In an effort to encourage students to transport themselves to school with their own two feet, many municipalities are seeking funding for more sidewalks, safer bike lanes, and other pedestrian-friendly measures, while parents are organizing walking versions of carpools. One potential downside: Groups of walking kids may be highly susceptible to homework-eating neighborhood dogs.