A lot of the roads where I live are having their speed limits reduced from 60mph to 50mph, or even 40mph. The reason being is that accidents between vehicles, or between vehicles and property, are too high for the amount of traffic using those roads.
As for motorway tiredness or complacency, this is a natural phenomenon of this sort of carriageway. Do the same speed on a dual carriageway or a single carriageway, and you are naturally more alert. In fact, the more bends in the road, the more alert you become. I don't understand why the motorway puts you to sleep, but it does, which is why I hate motorway driving.
(Probably something to do with brain stimulation. Just driving in a straight line, following the car in front, gives you a false sense of security, whereas negotiating bends and deviating from a straight line keeps your brain active).
While I do occasional speed, to pass a vehicle that may be doing 67/68mph, I tend to keep to speed limits. I have also been involved in a collision so know first hand how speed can affect the outcome of accidents. Ironically, it doesn't matter how good a driver you are or how safely you drive, if the other person isn't driving safely, there's bugger all you can do about it.
What I have also noticed is that the more powerful the car, the more it tends to be driven in a respectful manner. The less powerful the car, the more likely that the idiot behind the wheel will try to demonstrate that their car is more powerful than it actually is. Always makes me laugh when a Corsa tries to burn up a BMW 530d, and fails.
In my mind, some speeds should be reduced. Anyone stupid enough to speed through a village or town deserves to lose their license.
I used to be a proponent of increasing the speed limit on motorways to 80mph, but to be honest, with the congestion and poor road manners, anything over 70mph could prove dangerous. I suppose what infuriates me more than anything on motorways are those idiots who, once they are passed you, pull in without indication and without giving you amble space. They seem to think that it is their right to 'cut you up' just because they are ahead of you. Have they never heard of the '2 second rule'?
Nah. Keep the speed limits the same.