SR2S 2007 State of the States Report

Very few children walk or bike to school today. But this wasn't always the case. In 1969, according to the National Household Travel Survey, approximately 50 percent of children in the U.S. got to school by walking or bicycling. By 2001, only about 15 percent of students traveled to school by walking or bicycling. As a result, kids today are less active and less independent. Over the past 40 years, rates of obesity have soared among children of all ages in the United States, and approximately 25 million children and adolescents are now either overweight or obese.

The increasing trend of driving kids to school also has other serious implications for health and safety. As much as 20 to 30 percent of morning traffic can be generated by parents driving their children to schools, and in the United States, motor vehicle crashes are the leading cause of death for children ages 3 to 14.
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Studies have shown that physical activity during youth plays a critical role in the development of strong bones, cardiovascular health and overall physical development. But today, sedentary lifestyles for both children and adults are, unfortunately, the cultural norm. In fact, on average, the majority of U.S. children now spend more time each day watching television and playing video games than they do in school.

http://www.saferoutespartnership.org/media/file/rpt_SRTSstates2007.pdf