I would have thought it would be difficult to prove that (currently) speed does not kill when in an Australian regulatory framework that sends you a bill for $500+ for going just a couple of km over and loss of licence once you get 4 such bills. Further the safety of cars has developed such that accidents where speed is involved are less of an issue to occupant safety that previous years.
As you cannot design cars to prevent red light running, phone use, and other moronic events, these behaviours then become the dominant statistic. If you remove all speed related road rules, my hunch is that speed would become the dominant statistic.
Whether 5km/hr over really matters…who knows, but a line has to be drawn, and someone will always drive faster than that line. But I do know that part of the road I am on which used to have several bike/car fatal accidents has had none of significance since the limit was reduce from 50 to 40, and a rule to keep 1m from cyclists applied.
No scientific fact in any of this, just opinion.