15 minutes, eh?
From my days in the USAF, to prove you were fit, you had 15 minutes to walk/jog/run a mile and a half!
So, is this 15 minutes, on foot, by bicycle, by scooter, or by automobile?
How do you secure such a city?
So in the Air Force, if you can jog a bit faster than a brisk walk for 15 minutes, you are considered "fit"?
They are in China and have started implementing them such as in Oxford, UK, whereby they have partitioned the city with gates and you are only allowed to drive anywhere outside of your section 2 times a week without special permission. They have cameras to scan your license plates, face, etc., and the idea is that you can walk anywhere you need within 15 minutes, so your “territory” is really just a few square miles.