As a former ,30 year, cop. who worked a load of traffic, both highway and local, during my career, I'll offer a couple comments.
Having investigated hundreds and hundreds of MVA's, I have never investigated one where a speed differential was the cause. For speed related incidents, it was always as a result of someone driving way too fast for traffic/road/weather/etc conditions. And it was the fault of that driver (barring someone else doing something stupid like cutting him/her off etc) and not that fault of speed differential - in other words the other person(s) who was/were driving the limit, or slower than the speeder, was not at fault.
Facetiously, two vehicles can travel at well over the speed limit, one immediately behind the other, and can be safe. What becomes unsafe is when something unexpected occurs (object in road, someone else hits the brakes, etc etc etc). That's why the powers to be set a speed limit, for those times when one or more drivers encounter something they do not expect.
And, there are a whole boatload of people out there who think they are a better driver than they really are, that they can travel faster than they should, can read their text messages.... and they usually find out the truth when, again, something they did not foresee, occurs.
While I personally think some limits in some sections are too low, I also realize they are set with the lowest common denominator in mind, those who should not be driving faster, due to age, experience and all those other contributors.
I could go on, but....
Marv