walking and biking are good for both your health, and the earth's health. biking is the most energy efficient form of travel, and both are obviously co2 free.  most people can easily walk a mile in 20 minutes and bike 5 miles or more. even if you live too far from your frequent destinations, you can surely find other situations where it is possible to substitute walking for a short car trip. not driving has the compound effect of reducing overall traffic, causing other drivers to spend less time idling, reducing co2 emissions even more.