Before people start putting their dukes up, hear me out.
I believe that Philadelphia has quite possibly the potential to redevelop an area that would be the largest seamless pedestrian friendly walkable area in the US. Of course this is in terms of street layout alone, there is no way that Philly will ever catch up to Manhattan's large array of amenities that brings pedestrians out, but I think Philly has an advantage that other cities would kill to have, in that the average street width ranges from narrow to narrow as hell! Look at these examples:
North Philly
http://goo.gl/maps/lAZd4
North Philly
http://goo.gl/maps/LbykX
Northwest Philly (around Germantown Ave)
http://goo.gl/maps/tZxp4
West Philly
http://goo.gl/maps/fa80z
South Philly (nearly all of it is like this)
http://goo.gl/maps/uv7Ll
These streets aren't anomalies, they're numerous in scope. I can't think of any other city that has large swaths of narrow streets like this far from the city. Also, they're not only walkable, they're absolutely hostile to automobile traffic, a distinction that puts it above its peers. I'm fully aware that Philly has other roadblocks in the way, such as vast areas of urban decay, swaths of unused industrial land, some wide thoroughfares, etc, but I think that there exists enough intact historical fabric to create one of the largest seamless walkable areas in the US.
What do you think?