I used to have a KOM at 53.1km/h, though have now been beaten by 1s. Admittedly that was only over 0.9km and there was a decent tailwind on the day I took it (the chap who beat me was on the way back from a time trial, so presumably on a proper aero bike – though I have to admit I once took my TT bike out Strava hunting 😳 )
I’ve still got a flat 1.0km KOM at 56kmh, but the juniors I train with are doing a lot of TTT training on that road so I don’t see it lasting much longer.
52kph for a mile seems pretty exceptional to me, the nearest I can find for comparison is a fairly flat one near me(perhaps very slight uphill). 640 riders KOM 48kph my best is 45kph and that was with a tailwind, two other riders and a concerted effort.