It means to use posted speed limits to decide what is a safe speed, rather than to evaluate the actual risk or at least have some model for what makes a number unsafe other than what the sign says. Your method, picking a number based on pedestrian survival rate, wouldn’t be that.
Ah, but posted limits are a terrible signal of safety, at least in all the states I've lived in.
One state likes speed traps for out of towners, so the speed limit is 55mph (and not enforced) on 1.5 lane gravel country roads, but 35 for four lane divided roads just outside city limits.
The other state decided to upgrade the roads in affluent cities/towns so existing speed limits would be 10-20 mph too low, but then didn't raise speed limits afterwards. So you need to know what the road budget was a few decades ago in order to compute safe speed from the speed limit signs. (Or, just use common sense...)
Exactly so. And those particular examples (lowering them for revenue or aesthetic purposes) can actually mean decreases in safety and wellbeing, beyond the ordinary dangers of arbitrary set speeds, as stakeholders internal or external to the police department pressure officers to spend too much time tending the road.
