Well, the cars may be better, but the human driving it isn't. As speeds increase, stopping distances increase exponentially- and your reaction time stays the same. New cars may be able to stop better than old ones but not so much better that they make 100mph safer than 70mph in an old car.
I don't have a problem with people speeding on the motorway, I occasionally do it myself, but don't kid yourself that it's no more dangerous than travelling at the speed limit, because it is. A good driver doing 100mph is probably safer than a bad driver at 70, but the good driver would be safer still if he were doing 70.