A 40mph speed limit is exactly that, a limit meaning one should not drive above/faster than that speed.
It does not mean that you must drive at that speed. Any other interpretation would mean that you should get a speeding ticket for travelling at 41mph and also a ticket for travelling at 39mph, because you are not driving the limit.
Yes, I too have been annoyed by people driving 15-25mph in a 40mph zone and as annoying as it may be, it is their right and furthermore they are driving within the legal limit, as they should do. Maybe if they are pulling a dangerous load, it may be the safest speed at which to drive.
I would rather have them drive at that speed and feel in control than force them to drive faster and not be in control. Are you also planning to ban all learner and newly licensed drivers from the roads until they can drive faster?
That can only occur with practice and confidence. There is absolutely nothing wrong with driving 30-plus mph where the speed limit is 40mph, it is a perfectly acceptable speed. In terms of road safety we have a problem with speed, not slowness.
What is far more dangerous are the fools
i) who tailgate and leave insufficient stopping distance between themselves and the car in front;
ii) who blindly overtake at inappropriate places on the bypasses, and other roads at 50-plus mph, where they cannot properly see the oncoming traffic and narrowly miss a collision, causing the oncoming driver to have to swerve dangerously;
iii) who blatantly drive through red lights, as if the light is not intended for them, to mention just a few
With all of these bad driving behaviours to contend with on a daily basis, the slow annoying driver who is legally driving within the speed limit is only a problem to our impatient nature and is only dangerous to, those who are so impatient that they react dangerously, and any victims those dangerous drivers leave in their wake.
Maybe what a previous writer is trying to suggest, is that on certain roads there should be a minimum driving speed required, as is the case on many US highways. However, given the nature of our roads I hardly see this as necessary. It is the rare person who will drive at the snails pace of 0-20 mph without good reason, eg towing a dangerous load or some sort of car trouble and trying to get to a garage or learning to drive and these people are far more rare than the other extreme, which is commonplace.