First. The speed limits on motorways are ludicrous. 70 mph - it's a joke. The problem the people who take a pious view is that the limit is not enforced. No-one will ever get fined for driving on a motorway up to 80 mph, possibly 85. I even go past police at 80 mph with no problem.
Make the speed limit a sensible limit and then enforce it if you want respect for the limit, but leave the crass comments such as "If you speed you shouldn't be on the road".
Secondly, I recognise the need for speed limits through road-works. In a rush hour, having slower traffic actually speeeds the flow, less stop-start. However, if I'm driving to Debyshire on a Sun eve through the roadworks and there's 3 lanes with virtually no cars on the road and no workmen about, does it really matter if I go though at 70. Well, really. If your only argument is "Well, it's the law", then don't bother, as the people who made the laws are the same ones that made the rules for parliamentary expenses, so they don't exactly command respect from yours truely.
Actually the laws are made by people who base them on the work done by experts, people who factor in the physics of speeding cars, human reaction times, probability of accidents, and so on.
With unattended works around there's no way of guaranteeing the road conditions are safe for a higher speed, what may look fine may indeed not be, both because of decieving looks and because of deteriorating light.
With empty roads there is an increased chance of bleeding to death if you have an accident and there's no one around to call for an ambulance etc while you're lying there bleeding out.