It’s a bit metaphorical, nobody knows. Even with through investigation, knowing both sides stories, you can’t do anything but speculate

I did find the movie slow and everything was revealed at the end without actually spelling it all out because no matter what, you’ll never know the whole truth

In this case, both parties were severely emotional, drunk, angry, suicidal, depressed they weren’t fit for driving
The dark road, the deer, everything was set up for the “accident” and that’s what an accident is, unplanned, any small element can cause it

The what ifs are as strong as the clues left by the encounters

The daughter blamed herself, the wife feared blaming herself, the mom was left with so many burdens taking care of a sick husband

Does the “truth” change anything? How does it effect the events

Yes they were unfit for driving, yes an unexpected deer was like a domino effect, move away from deer, move away from oncoming car, too angry to focus…. The daughter running away, the truth, the pain, no matter overthinking, over feeling, things need to be let go