Have you ever wondered if you're meeting the same people every day on your daily commute? Over half a million people ride the Warsaw Metro daily, but if I arrive at the station at roughly the same time every day, surely there must be others with similar schedules? But even after almost a year of taking the same route, I couldn't recognize any familiar faces. So I decided it was time for a more scientific approach.
A surprisingly large number of people leave their phone's Bluetooth in discoverable state (I find it strange, because on most phones I've seen the default behavior is only to become discoverable for 120 seconds or so when pairing). I thought if I scanned for Bluetooth devices every time I rode the subway and saved the results, I could then check if I ever saw the same device on different days. So that's what I did, I made an application for my phone that does a Bluetooth scan and logs all devices it sees in a SQLite database. That was not ideal yet, because I still had to trigger the scan manually every time (I didn't want to keep scanning all the time, because that would quickly drain the battery and also I'd get a lot of false positives from my neighbors, coworkers etc.). Happily, there is a solution. Living in a city, just about the only time I lose cell phone signal is when I get on the subway. So that's a good condition to wait for to trigger the scan (there will be occasional false positives, but they don't bother us much). Obviously, this only works if your commute involves riding the subway and not, say, a bus.
Here's the source code of the application if you'd like to play with it.
So are there any strangers that happen to commute at the same time I do? After just two weeks of the experiment, with the help of a SQL query:
select addr, count(*), max(sightingtime)-min(sightingtime) y, group_concat(sightingtime) from sighting group by addr having y > 3600 order by y desc;
I found two persons that I shared a subway ride with on two different days. Unfortunately I can't really say much about them, except for the fact that one of them had an LG phone and the other a Samsung.