The whole reason I posted this thread was because a driver in front of me hammered on the brakes due to being flashed. I was doing 30, in a 30 and had to drop to 15 as that's how much he slowed. We then crawled along at about 20 until we passed the mobile camera ( operator standing on the pavement, dressed in fluorescent yellow from head to toe).
Once out of sight of the operator the twit in front of me booted it, very quickly pulling away, but still in a built up 30 zone. He was a couple of hundred yards in front of me when he hit the brakes very hard for a pedestrian crossing. I know that he didn't manage to stop for the crossing as it is on a raised hump and I saw the car rise up. There were people standing waiting to cross.
I don't warn oncoming drivers, I think it is deplorable to do so, and think it risks people's lives.
It's another symptom of the abysmal driving mentioned, and the problem with people driving fast is who gets to decide whether they're capable to do so, or not.
Keep to the flaming limit!