as I already said
Amersham hill is one bad spot for this, there are two national speed limit applies signs yet drivers continue to do 25 - 30mph down there. Would you drive at 30mph on a motorway? No you wouldn't, so why do it here? There are two signs, white with black stripe, one on each side of the road... look at them! It's frustrating enough trying to get through the terrible traffic outside the beacon school at 8am, only to be held up by captain slow down the hill as well.
And often there is just no point in driving at the speed limit. For example, what is the point of going up to 60 down Amersham Hill into Chesham,, when you know perfectly well that there will be a queue at the bottom?