"Seven of the 10 most walkable cities sit on large bodies of water. With a coastline checking expansion, available land had to be used more intensively. (As the map makes clear, there were considerable natural limits on San Francisco's physical growth.) Intensive land use means density, and density generally means walkability."
"Second, the walkable list is dominated by Northeastern and West Coast cities that are comparatively old, at least by American standards. Six of the 10 most walkable cities were among the 20 largest urban places in 1900. By the 1950's, these cities were largely mature; collectively, they grew only 1.5 percent in population between 1960 and 2000."
"In short, with some admittedly notable exceptions (such as the interesting case of Portland), they just don't seem to be building walkable cities any more. The tricky part is figuring out how, and whether, we can take steps to remedy this."