Jut to dip my oar in:
NWW is right about the cause of accidents, perhaps adding the odd act of God eg trees falling, mechanical failure at the wrong time, and its accidents that cause deaths, but that doesn't change the calculations because we're dealing humans.
As backdoc says there is a trade off between the opportunity cost of time lost and other externalities of driving slower (in areas where you could be driving faster) and lives saved. Yes I know its a horrible calculation but we do it all the time: NICE uses Quality Life Years when calculating whether or not a drug or treatment should be available on the NHS. The last time I heard a figure it was that Network Rail values a life saved at ?1m. So the question is how many fatalities are we prepared to put up with and at what cost.
But there's another area to consider: Risk mitigation. We've made driving and being a passenger incredibly safe and the survival numbers must be trending towards zero by now, or would be if drivers didn't feel so safe*. That added safety: seat belts, air cushions, roll cages, crumple zones etc means that drivers can be more inattentive or drive faster for the same level of risk if they didn't have all those safety features.
I'd add that drivers going slower are also less attentive either through frustration, boredom or the belief that they aren't in danger, but the numbers do show the accidents are more survivable for pedestrians.
If you really want drivers to slow down and pay attention, strip out all the safety features and put a 6" metal point in the middle of the steering wheel.
I'm not against 20mph zones in principle, just blanket legislation. We visit Germany a lot and they have them outside schools and kindergartens, old people homes and the like and also for noise reduction between 10am and 6am in some villages. (Its a Federal country so States differ).
*One of the biggest drivers of survivability is response times, but that's the same for pedestrians.
We also value people's time quite highly cf the huge amounts to be spent on HS2 to save 10 to 20 minutes between Birmingham and London (And yes I'm aware the Dept for Transport screwed up the numbers and then had their fingers on the scales, but the premise still stands).