An approaching hazard? I live near a country lane with slow written on the roads at one part. It is to warn that a concealed road is coming up, also it floods there very often so it might be that too. Its a national speed limit so everone does a fair bit of speed down there, and as you go round the bend, a road entrance pops up quite suddenly, very dangerous for cars merging onto the road.
I was just wondering since I seemed to recollect the original belief (first post) but a corner near my house has SIX (count 'em!) SLOW signs painted in the road from each direction.... and a big red sign that says "Road Narrows On Corner! DEAD SLOW"
People still fly through it cutting the apex.
Buses regularly travel thru it and dont fit on one side of the road.