I e-mailed the Sightline Institute last week when I made this post about a walkability map of Seattle to learn more about the map. Here's the response that I received:
There is more about the walkability maps in the 2006 edition of the Cascadia scorecard, which is available online here: http://www.sightline.org/publications/books/CS2006/CS06
The underlying data for that map is based on a proprietary index, developed using data on population, street connectivity, and the local mix of commercial floorspace. The creators of the data looked at all of those factors in their studies looking at the relationship between city design & active transportation. The information was developed by Dr. Lawrence Frank from University of British Columbia, and Dr. James Sallis of San Diego State University and Dr. Brian Saelens of Cincinnati Children's Hospital. It was funded by King County and the NIH.