It's also unrealistic to assume that an attentive driver would have avoided the crash. Driving at speed, you have to make assumptions that other people will do

It's also unrealistic to assume that an attentive driver would have avoided the crash. Driving at speed, you have to make assumptions that other people will do the right thing. You pass cars all the time. If you assume that the car might side-swipe you, you wouldn't get far, and you'd be a hazard yourself. You drive on undivided highways all the time without driving as if the oncoming car might cross in front of you. And so on. We have to assume that others are minimally competent. That's the point of road laws, not just to control you, but to let you assume things about others.