I think its down to the driver. I live in the country and drive to the conditions. 60 in some places and as low as 30 in some places. Thing is most of the time if you go to fast round a corner, youre not going to hit anyone else, youre just going to roll it into a ditch, only damaging your own wallet. If it were to be lowered to the lowest speed on the road then youd have to put it down to 10. You have to creep round some corners in my area they are that narrow and blind.
I can only see that lowering the limit will annoy people getting stuck behind people. It wont be enforced so no point.
You can slap a speed limit of 150mph it won't make a difference, its the what the road and conditions allows you to do and most lanes round here are so narrow with high hedges that anything over 40mph would be suicidal anyway. Especially when there's farm machinery, horses, cyclists and walkers using the roads too.