When: 20 or so years ago
Where: Castle Hill, Cambridge
Who: Me (a student back then), and a Pleeceman
More scene-setting: Cambridge has just the one contour line. Castle Hill is a major student residential area, and all the teaching facilities are in the centre of town, at the bottom of the hill.
I was a-hurtling down the hill, across the lights (at green), and across Magdalene Bridge. Pleeceman stepped out into the road about 50-80 yards away from me, looking straight at me, hand outstretched making the "STOP" gesture.
I continued at speed. Pleeceman remained in the "STOP" stance. With some 8 yards to go, I "dropped the lot" (a railway term), and came to a dead stop less than a foot from the officer.
His hand dropped, and in a voice calmer than I bet he was feeling, he said "Your brakes work, Sir. Thank you, you may continue.", he stepped out of the way, and waved me on.
So it was just a random check on students' bikes' brakes.