The problem is that naive arguments about speed and laws based on fixed numbers don't really help anything except ease of prosecution.
Inappropriate choice of speed can obviously be dangerous, but what is appropriate to the conditions and what the fixed numerical speed limits dictate can be wildly different.
For example, the default speed limit in the UK on most residential roads is 30mph, but if you drive past a school at 30mph at the end of the afternoon when the kids are coming out and there are lots parked cars obscuring your view, that is still inappropriately fast for the conditions.
However, slower isn't always safer. On a motorway, the speed limit is usually 70mph, but in poor weather conditions and heavy traffic, everyone travelling at lower and similar speeds may be the safest choice. If you're the one driver insisting on unnecessarily doing 20mph when everyone else is doing a steady 40mph, you're also increasing the danger to yourself and others.
Another example is that the speed limit for heavy goods vehicles on many of our major roads was increased by 10mph in 2015, having previously been 20mph below what cars were allowed to do on the same roads. The results were notable for several reasons, among them that the actual average speed of those heavy vehicles only increased a little but the safety of the roads appears to have increased. This was attributed primarily to closing the gap in speeds between different types of vehicle and thus, among other things, reducing instances of queues behind and overtaking of the slower vehicles, both of which increase the risk of an accident.
Finally, we have the classic motorway speeding situation, where on an open road in good conditions, a competent driver with a suitable vehicle could do far more than the legal limit of 70mph with negligible increase in risk. Moreover, statistically our motorways are both our fastest roads and yet also among our safest. You could certainly print money by setting up a covert speed camera watching some stretches of motorway, but whether it is a good use of police resources if your objective is to increase road safety is a different question.