So I raised the threshold to at least 20,000 residents.

And again, with few exceptions, we find the pattern of high walking rates and major institutions of higher learning, military bases and areas of mixed use development.

So I then raised the threshold again to over 250,000 residents:

While all of these cities have colleges and universities and other major institutions, they are part of a very large mix and cannot alone account for why these cities are on the list. Even controlling for population density does not account for this distribution. It's clear that these are cities that grew to sizable populations before the automobile, which may explain why these major cities are on this list instead of Los Angeles, Phoenix, Atlanta, Houston and Dallas.

Surprising are two cities you might have expected to make this list: Chicago and Portland. They aren't that far off, but while both cities are getting a lot of credit for their green initiatives they don't seem to encourage walking to work as much as these cities above.

An even better question to assess walkability than % that walk to work would be the % that walk to the grocery store or pharmacy. We looked at Walkscore as a metric before and found it had flaws, but was generally useful.

From this very high level look at this census data and Walkscore, it would seem that there are two major factors that influence the walkability of a city or town.

1. Major Institutions: Colleges, Military Bases where people live in dorms/barracks close to their employment/education as well as dining/entertainment/social destinations
2. Pre-Auto City/Village Design: Places created before/without the need for automobiles with close mixed uses of residential/commercial/workplace/dining/grocery/education/entertainment.

The keys to both seems to be co-location of people's housing with the various destinations that they need/desire.

But there is a choice here that seems like one worth considering in greater depth. If we want to create a post-carbon society, creating more walkable communities seems like a major priority. But what kind of walking towns do we want?