Sleeping policemen are more effective than real policemen when it comes to speed enforcement. Currently 80% of the population admit to breaking the speed limit so putting up a sign that says 20mph would probably be headed by 1 in 5 cars who probably live on the road.

Speed bumps on the other hand are obeyed by nearly all as there is the fear of the constant penalty of wrecking your suspension. The same has to be said of working speed cameras, we have all been overtaken by some idiot only to find ourselves stuck behind him a few hundred yards later going 25mph through a 30mph speed camera.