The limit could easily be raised without the vast modifications people are claiming, but a) it's not a very green thing to do, power required increases with the square of the speed, b) 80 makes sod all difference to even long journey times in the overall scheme of things and c) sure some things may then fall outside spec, but actually most items on the motorways are rolling improvements as and when they need to be upgraded. Lots of the existing barriers etc were not up to spec a decade ago and are an ongoing improvement. It's still highly unlikely. As Steven says, the one thing we suffer from is overcrowding of the roads, the way to get more cars through per unit time is to slow them down and get them closer together - simple as that.
The wire rope barriers are actually pretty damn good at their job, which is to deflect the car back into the carriageway they left at a shallow angle rather than not move at all. This video demonstrates it clearly:
The last thing you want is a barrier that's such a hard impact it kills the occupant, but also you dont want them passing through to the next carriageway - the ideal is the barrier reducing the energy of the impact and pushing them back across.
Steven - whats this about concrete barriers, loads of new barriers going in where I drive, most of them being plain old armco still?