For example, Outland Road in Plymouth, is four lanes wide, with broad pavement, light control, crossing points, and good lighting. The speed limit is 30, because there are lots of houses on one side, with park land and sports grounds on the other side. So at two o'clock in the morning, when the is very little traffic, it is still 30. A lot of drivers will try to get away with ignoring this limit, because at night it is pointless.
An example the other way, is Shalloak Road near Canterbury, where we used to live. A narrow winding country lane, with a railway crossing, multiple sharp bends and sharp corners, no pavement either side, no lighting, no lane markings, and a speed limit of 60, because us and next door were the only houses there. (There used to be four.) We had cars in our garden four or five times a year, cars on the railway at least one a year, one building demolished by a car that went into it, and another seriously damaged. There were frequent collisions on the road, but no one was ever going to be prosecuted for dangerous driving because they had not breeched the speed limit.
What we need, is a system of speed limits that is designed to reflect the nature of each road and the conditions that would generally by encountered on that road. 30 in a town centre, 40 elsewhere in a town, 40 also on rural roads that are too narrow to allow vehicles to pass at speed, or have multiple bends or other obstructions that might make the road hazardous at high speed.
Another idea which would improve road safety alongside an improved speed limit system would be to redesign the driving instruction system as well to teach people to drive according to the nature and conditions of the road, rather than just teaching control of the vehicle. This could be balanced by increased application of the charge of "Driving Without Due Care And Attention", which in itself would be better enforced by spot penalties rather than months spent dragging every case through the courts.