The old dear who pulled out on me last week on an unrestricted road then proceeded to pootle along at 22mph won’t ever get caught by speed cameras but she is clearly a horrifically dangerous driver.
In principle you could set a speed camera to capture anyone driving below a threshold speed. In practice, defining that minimum speed would be almost impossible. There'd be some justification for using 25mph on motorways, but even that is dubious.
It’s horribly Germanic to have arbitrary laws and then enforce them rigidly. It’s really not English. Are we going to criminalise jaywalking next, to improve safety?
Probably yes. The self-driving car lobby are very keen to criminalise crossing the road, because it causes problems for them. If they get to say that people crossing the road are automatically in the wrong, it's not the software company's fault when they get killed, and the cars don't have to figure out if they need to stop for them.
Those who drive above the speed limit cause crashes, and those who drive well below it can also cause them.
There will always be road accidents1 for as long as there are road vehicles, because there are too many environmental variables to completely eliminate risk. But if they happen at lower speeds, they'll do less damage. There's therefore a tradeoff between letting people get where they're going very quickly on clear, good quality roads, and stopping people who don't know better killing themselves and others on busy, poor roads.
Ironically, the less car-dependent the country is, the easier it is to have clear, good-quality roads and to enforce high driver standards. Which means that people who want to blast along at 120mph should want everyone else to take the train and get out of their way.