Once they started mandating helmets, post-crash fire was the next to be dealt with. I heard one of the senior Army aviation safety officers say that he can name off the top of his head every person who has died due to a post-crash fire associated with a crash within design limits. in US Army helos since he was commissioned (20+ years earlier). That's how he demonstrated what they achieved.The US Army based on data from the Vietnam War has made great strides in survivability, the greatest killer identified was post crash fire. Here is a paper published in the early 80's that is a good summary of their findings.
It's a **** shame that we haven't put a fraction of the attention into addressing a major killer. Then again, in EAB we are still facing a certain number of people who think that a shoulder harness is overkill.