Replace distance function with driving directions

[openbrian]

I think this may give better results, especially where a district was crossing the Chesapeake Bay. Though when it comes to clustering, it becomes harder because k-means is in pl/pgsql and that's where the distance function is used.

[openbrian]

I check with the pgrouting folks, and they don't have an all-nearest neighbors function yet. They suggested i start working on it by checking out the driving directions algorithm.