Sorry if I'm misinterpreting the /s here, but:
> If only there were an objective reality to know which car was in the wrong /s
This, but sincerely! Problem is there isn't an objective reality to know which car was in the wrong. There's only each participants' recollection of the event—both flawed. She thought her inattention was the cause of the wreck, he thought his failing to yield was the cause. Who is right? Both of them? Just her? Just him? 80% him, 20% her?
There is no objective way to answer any of this. What happened during that accident only exists today in the minds of the drivers. Sure, you can look at photos of the aftermath and determine that this fender was bent, or that airbag deployed. But you can't get a similar (objective) level of understanding about the actions of the drivers leading up to the wreck.
When they met, he learned that she was not paralyzed (and that his understanding of the past was wrong in that aspect), and she learned that he blamed himself rather than blaming her. By comparing notes, they both got a better version of "the past" in their minds, but there's still room for more variances and disagreements on what exactly happened.
There may be an objective reality. But there's no way we can completely understand it. We can only know what our limited senses tell us. As time marches on, those senses get recorded in memories, and further distorted. For the tiny slice of events that get recorded in audio, video, contemporaneous notes, etc.; we have even better understanding. But those methods of capturing events still leave a lot out. You can hear the words, but not the thoughts. You can see the actions, but not the motivations. You can read the written diary, but there are no entries there for things considered irrelevant by the author.
There are untold numbers of events that occurred millennia ago that we know absolutely nothing about. But they contribute to how the world works today just the same. Long after the cause is lost to time, the effect chain keeps adding links.