Why don't we just have a suitable / sensible speed limit that we all stick to and is actually enforced? Would love to understand the social process that has occured where speed 'limits' are now seen as flexible. (I include myself in that sometime)
The simple and logical answer is that a speed limit is only as good as the read-outs (the speedometers) in cars.
Increase the speed-limit to 100mph, and you have the 10% rule (acceptable variation in speedo), so technically, someone doing 110MPH could not be prosecuted.
I believe the law is 10% + 2. So, at Motorway limits (70MPH) the 'allowed' speed is actually 79MPH.
On all the motorway roadworks at the moment and the SPECS set at 50MPH, I bang the cruise on an indicated 55MPH and I've not had a ticket yet. Bear in mind I'm probably only doing 51 or 52MPH. I'd guess the SPECS are set to report at 57MPH. In that, I am well below the limit (at the indicated 55MPH).