As Prometheus says, I've been led to believe that car crash victims are very often used to harvest organs for donation... it does make sense that they should derive from accidents rather than people who have passed away from illness, that's for sure. That said, I can't find anything to substantiate the idea, as widely accepted as it seems.
I didn't know about the correlation between seat belts and helmets... where did you find that? Would be interested to read further.
You have a point about reduced pressure on ambulance services, fiveworlds. The safety of self-driving cars is still a bit up in the air, but I think the consensus is that they'll save a fair amount of lives (as represented here by Slate: http://www.slate.com/articles/technology/future_tense/2016/12/self_driving_cars_will_exacerbate_organ_shortages.html).(With the alternative suggestion that there's still a long way to go represented by the Guardian: https://www.theguardian.com/technology/2016/jul/05/tesla-crash-self-driving-car-software-flaws).
However, getting to the hospital is no use if, when you get there, they're starved of necessary resources. I understand that transplant lists are a real problem, and growing...