bose-x
I am not sure that compliancewith the quad rule reduces the probability of a mid-air (implying a fatal accident).
It would make an interesting exercise in probability theory to work out whether IN IMC a mid-air is more or less likely between two objects flying head-on or the same two objects flying in the same direction.
I don't think the case is at all obvious. My feeling is that there is no difference at all.
(I am assuming that a 150kt plane going up the back of a 100kt one will be as good as a head-on)
If there is SOME visibility, then the quad rule should mean you have more time to see the other one. But given the poor visibility of GA planes (which is something all pilots and passengers notice pretty well right away) I don't think this is a factor unless the vis is a lot better than 250m. Also if somebody is being smart and is flying in marginal VMC with their landing/taxi lights on, you won't see those from behind. Also if somebody is closing with you at say a 20deg angle you probably won't see them anyway because few pilots twist their neck that far when looking for traffic.
I think by far the main benefit of the quad rule is for airliners: with TCAS you get a lot more time to act. But we all know that in GA a lot of people don't have transponders, or if they do they have them turned off. I flew today for 30 mins under an RIS, saw not one of 5 or so targets, and not one of them had Mode C on (I was VMC on top at FL050 and they were probably below).