Driving cars releases tons of CO2 into the atmosphere (per car). There are 300 million cars on the road in the States. CO2 is a greenhouse gas that contributes to climate change.

75% of U.S. adults are considered overweight, I would think that walking would first help this problem, as well as be a catalyst to other beneficial habits, including eating less meat and more vegetables, which would further reduce the amount of CO2 released from agriculture, as well as slim people's waistlines.