The issue for me, is once again that "Blanket" limits only help to re-enforce in the general public's mind (ie the thick stupid drivers who aren't paying much attention to the road anyway, the majority these days!) that speed limits are "advisory". There eyes tell them the road is clear, and "normilisation" tells them that yesterday, and the day before, and the day before that (x1000) they drove down this bit of road fine at 28 mph or whatever.
So, now, what do we do when we need to make drivers aware of a "real" hazard? No ones, going to be looking at the plethora of signs afterall, and even if they are, they will be mostly ignoring them.
In a modern car, for the most part, 20mph is and feels incredibly slow, so people tend to exceed that speed. In fact, plenty of studies have shown that we simply drive to our perceived level of risk (make things safer, we just drive faster etc).
The fundamental SAFE approach to all driving is to "LINK SPEED TO VISION" and blanket bans both undermine this, and remove the driver from the decision making process. Time after time you hear, after an "accident" either "they were driving over the speed limit, like a madman" or "It's fine, they weren't speeding afterall". Both of which completely fail to negate the fact there was an accident of course! (Whilst driving on average more slowly can of course limit the effects of an accident, if we apply that to it's logical conclusion, we need a blanket ZERO mph limit.....)
I am going to suggest that driving and our roads in the UK have a more than acceptable level of safety / risk. Afterall, when was the last time you got into a car and before you drove you thought "ooh, need to be careful today, i might be hurt/killed". I'd say you are approximately 1000 times more likely to be thinking "wonder whats for dinner" or "have i got time to pick up the drycleaning" etc.
And that really is the point. Accident are the ABNORMAL state of our roads. Unfortunately, those abnormal states carry a penalty for those involved, but rather than place some personal responsibility on ALL parties for their actions (drivers, cyclists, pedestrians etc) we just keep lowering speed limits and hoping the problem "goes away" before the road systems grinds to a halt.
In the UK, we have around 4.3 road deaths per BILLION Km driven. That is a low number, and means not crashing is the "normal" state for the average driver of course.
If we actually want to reduce that (low) number by any significant margin, we need to have proper driver education and testing, and a proper road safety curriculum for schools and colleges, not some arbitrary and practically un-enforceable signs blanketing our roads.