At somewhere out in the sticks, maybe - if I pop up at Chatham, then I've either come from/via Rochester or Gillingham (or walked into the station) with 100% certainty. But Oxford Circus is just too big a nexus of possible start points to get to there, at least IMO. It's a high enough volume you could probably do meaningful traffic analysis on the whole population of data, but linking that back to the individual is problematic if you don't have something else to link them to location X, such as a facesfriend who comes from there....