Speed limits on motorways are all relative and how fast traffic is moving around you; if you're stuck in a traffic jam, doing a constant 30mph will seem to be incredibly fast and liberating.
70mph on UK motorways with clear conditions always seemed to be a bit slow -i don't mind admitting I would usually go 75-80mph.
Moved to the States.....
All of a sudden being able to do 60mph in a 55mph limit (usual speed limit on a major highway over here) seems to be pretty fast.
Problem with any speed limit is that the de facto limit always seems to be 5-15mph faster than the posted one. I think it's a poor argument to say that since most people do 80mph in a 70mph, the limit should be increased -all it means is that if the new limit is 80mph, people will go 90mph.
FWIW, I would have a 20mph limit through any urban zone. Just remember, back in the early 1900s
20mph was a galloping horse