I did a speeding seminar about 10 years back.
Apart from the patronising merchent banker of a police officer (no offence to ex coppers on here) that gave the first part of the lecture, I must admit it was a real eye opener.
The main thing I remember was a fact given by some advanced driving instructor.
It goes ............... 2 identical cars driving down the road.
No1 is doing 70 mph
No2 is doing 100 mph
A lorry jack knifes up the road and No1 car stops with a inch to spare.
What speed is No2 car doing as it hits the lorry ?
The answer is 70 mph
Got to admit that thought slowed me down to the point that I've had a clean licence now for 6 years.
As for speed limits on a motorway.
The way I see it is if you do more then 100 mph in any conditions, you deserve to be nicked.
It doesn't matter how good a driver you are, you can't always compansate for the muppets on the road.
Put more traffic coppers on the road with more power to nick the dangerious / careless drivers and impound there cars on the spot.
Imho it not so much speed of the car that is the problem but f*ckwits that drive them

Also make everybody retake there test every 5 years with a full medical included.