They put them where the limit is most inappropriately low because they are the most profitable places to put them. You would find them on roads through housing estates and around schools if safety really was the priority but despite what the mouth breathers believe, most people are sensible so they will not make much money at all.
someone who is likely to break the law where it is "safe" is likely to break it anywhere?
Sorry but I can't even begin to work out the logic behind that.
I never 'speed' through the local estate or residential streets where it is unsafe but do I stick to a regimental 40 on fender lane or the tunnel approach road?, do I heck. And that does not mean I drive like Schumacher either before I get accused of being a maniac.