As someone who regularly travels fairly long distances on S/C A-roads, like West Cumbria to Newcastle on the A69 (100 miles) or even within West Cumbria itself (whitehaven - Carlisle or Barrow (50 miles)) I feel mightily miffed about this! It's not just the extra travelling time. That would be a relatively minor irritation rather than anything else
IFI could "average" 50 along such a route. The truth, of course, is that it's pretty much impossible. No, the nasty, insidious side of this, for me and others like me at least!) is the fact that it is actually a reduction to 40! The reason is that the big HGVs are already limited to 40 on such roads. The roads are twisty and scenic with few overtaking opportunities. This has 2 effects:
1. A queue will build up behind each artic, made up of drivers who would go faster were it not there but who can't be bothered for the sake of another 10MPH. Nevertheless, they won't leave enough of a gap between one another for people in a hurry to "leapfrog" during the few safe overtaking opportunities. Of course, what they'll quickly realise is that the HGVs won't "average" 40 either. They'll drop to 30 or even 20 on the uphill bits!
2. Overtaking in those areas where it is safe to do so, without exceeding the new speed, limit will take longer. This will be inherently MORE dangerous (anyone who thinks otherwise is welcome to drive as slowly as they like on the wrong side of the road and see how long they last)! Not only will this increase the time exposed to danger, but it will ensure that fewer motorists can get past the obstruction (HGV / caravan / horsebox / tractor / whatever) at each of the suitable overtaking opportunities. Again. this will lead to frustration and, I imagine, some rather more risky overtaking manoeuvres elsewhere.
Of course, transiently exceeding the speed limit whilst overtaking was never a problem to any half-sensible traffic cop...
...but we're not dealing with them any more

. The safest places to overtake are the long straights with good visibility. These, coincidentally, are also the best places to put the cameras / park the scamera vans.
