Earlier quoted context omitted.
Walking and biking infrastructure and livable/walkable cities/neighborhoods are key pieces to health from youth into old age. Just because it doesn't now exist in the United States doesn't it mean it can't or should not.
The population density in the United States is far too low to support this. Increase the population of the U.S. to a billion or so, then maybe it will work.
Do you believe that the existence of exurbs and rural areas is what's preventing cities from pursuing less fragile means of transit? Why?