There are 2.3 million people living in America with no car and without a supermarket within a one-mile radius. People living in these "food deserts" are often obese and unhealthy because they're stuck eating junk food from the convenience store. Slate has prepared an interactive map of the counties with the most people who live in food deserts as defined above.
The ones least likely with a car are also the most likely to live in neighborhoods where you need one.
[Link to Food Deserts]