For people to die in a traffic accident, there needs to be a traffic accident. They are usually caused by impaired humans, which means that they are very often involved in traffic accidents (basically, almost all of them have at least one party of the sort), whereas non-impaired people mostly do not participate in traffic accidents as often.

This is a discussion of chances and probabilities: not being impaired significantly reduces your chance of being in a traffic accident since being impaired significantly increases it. I am not sure what's unclear about that?