What I want to point out is that there are also some influences coming from the fact of wearing an helmet. I'm thinking about 2 aspects have heard of (sorry no reference) :
It has been proven by metric analysis (reference needed) that the vehicule pass closer by a bike if the cyclist wear an helmel. As to say "He is protected, so there will be less damage in case of accident"
The cyclist may feel "protected" in a way he/she drives with less attention.
Ok, it not really an answer in itself, but this is an aspect of the question that should be mentioned.