I helped a friend move some furniture across the city recently (using car club van, my friend driving), and we dutifully tried to keep to the speed limits, which varied along our route.
As neither of us drive at all frequently, one thing that we did notice is that, if you are not familar with a particular route, it is actually quite hard to tell where the speed limit changes: the larger change signs can often easily be hidden by parked vehicles or buses in bus lanes at the exact moment that you need to see them, and painted roundels on the road also aren't particularly visible either if the road is busier. To drive safely, you're usually also more focusing on other people on the road than trying to keep an overly careful eye on signs at the side of the road or below your normal field of view.
You have to continue for a few hundred metres further to be able to definitively check for the presence (or absence) of the repeater signs (some of which may also be blocked from your view) to be sure whether the limit has, or has not, changed, and all at the same time trying to look out for direction signs and junctions where you need to turn.
Obviously we took the stance that we would assume that it was still 20 until we were sure otherwise, but it did make for a slightly more stressful journey than it perhaps should have been. (One thing that might perhaps help a little would be, on faster roads, which often have traffic islands or central traffic lights, for reminder signs to be added to these, as the view of these is less likely to be obstructed?)