A good point, but I get the feeling that people don't quite realise what sort of forces were dealing with. It's not just about going twice as fast;
At 20 mph, a 1.4 tonne Audi S3 has 55,950 Joules of Kinetic energy
At 70 mph, a 1.4 tonne Audi S3 has 685,800 Joules of Kinetic energy. (3.5 x the speed, 12.5 x the energy of 20 mph)
At 80 mph, a 1.4 tonne Audi S3 has 895,440 Joules of Kinetic energy (30% more energy than the legal limit of 70mph, and 16x more energy than at 20mph)
At 140 mph, a 1.4 tonne Audi S3 has 2,741,870 Joules of Kinetic energy (4x more than at the legal limit, 50x more than at 20mph)
So, 140mph. Double the limit. You've got to dissipate 3 times more energy through the brakes to just to get back down to 70mph, as it takes to get from 70mph to a full stop. You need four times longer to stop as you do at the legal limit, and you're carrying the energy equivalent of over a pound and a half of TNT.
Even if you're only doing 90mph, you're carrying 1.13 million Joules of kinetic energy. Over 60% more than you were at 70mph...