From the Centers for Disease Control and Prevention. Barriers to children walking and biking to school--United States, 1999.
D000063:Accidents, Traffic; D000293:Adolescent; D001642:Bicycling / Q000592:standards / Q000639:trends*; D002648:Child; D006801:Humans; D011159:Population Surveillance; D012574:Schools / Q000706:statistics & numerical data / Q000639:trends*; D014186:Transportation / Q000706:statistics & numerical data; D014481:United States / Q000453:epidemiology; D016138:Walking / Q000706:statistics & numerical data / Q000639:trends*