I posit that whether a search should be "local" or "global" is much more a factor of what is being searched for than who is searching. If I search for a barber I want local results, and I expect that that would be true for most people. If I search for a weird error message I saw while using my computer I want global results, and again I expect that is true for most people.

If true, then DDG could apply the same heuristics about location-sensitivity for everyone's searches and improve the results for everyone, without any tracking of detailed individual preferences.