Just an idea... Sometimes it happens we have to present to the user the information which is easier reachable, not what has a shorter radius distance.

What about having the option to compute time proximity. Which is would not be radius distance, nor any other spatial distance. It would be the time needed to reach one point from another, taking into account the streets you have to walk, the speed limits, etc etc.

It could be computed using Google or Bing get direction webservice, which both take into account the streets you have to walk, real-time traffic information, lights, speed limits, etc etc.