@ Real Ale
Mr Real Ale. Whilst I agree with your user ID, I disagree with some of your arguments here.
Motorway barriers are generally placed (in my experience, at least) either side of the carriageways. The difference in lateral momentum between a blow-out at 70mph and a blow-out at 100mph is minor. The momentum of a car at 100mph is also significantly less than a 44-tonne truck travelling at a legal 56mph. Obviously, if they started putting barriers across the motorways then the difference in speed would become significant , mostly down to stopping distances though (at what distance should these lateral barriers be visible?)
Additionally, the engineered design-speed of motorways in the UK is, and always has been (since the very first miles were opened up over half a century ago) 100mph. That is why cars don't fly off the motorway at every corner, even though they are mostly driving at over 70mph.
The 70mph limit was chosen as this was about as fast as a typical car could go in the early 60s, with 50bhp, drum brakes and cross-ply tyres, and this would reduce the speed differential from the Aston Martins and Jags who were burning up and down at 150+ to a more manageable amount.
The problem of lift at high speeds exists with SOME cars, mostly due to poorly-designed cooling air flow (the MG TF, for instance, exhausts cooling air underneath the front subframe and thus tends to become nose-light at about 120mph but will still take UK motorway corners at 140mph albeit less confidently). However the earlier poster forgets the venturi effect that is also present at the low altitude of a car's underside, which tends to suck the car down on to the road surface, and also the downforce generated by other body components (the bonnet, and rear spoiler if present).