It's a truism that walking — as with most forms of exercise — is good for you. John Howard credited his longevity as PM, in part, to daily walks; Albert Einstein walked to sharpen his mind; writer Henry David Thoreau needed four hours a day to preserve his "health and spirits". But walking isn't just good for your physical and mental health; it's also good for the economy. And we're talking billions. Aside from savings to the public health system, urban planners and economic analysts argue that the more "walkable" your community is, the more economically viable it is.