Interesting result but, at the risk of "over thinking" the report given we don't have the details, a few questions arise. Was he riding the same time every day such that similar if not mostly the same drivers passed him or was this one long ride, was he able to maintain the same distance from the curb or lane divider for each ride, few riders can, at least within a narrow range. What type of street and what is the frequency of bicycle riders on the street. What was the mix of cars vs. trucks. While safety of rider may be an issue, it is likely more involved, i.e. competition for the road, why is he in "my" lane, he is slowing me down, he is not following the traffic rules, bikes should be on the sidewalk, damn hippie anyway, etc.
As someone who has ridden both motorcycles and bicycles in traffic, I would question the results, the median distance and range might be more revealing.That not a small number of drivers would try to get as close to me as they could always amazed me, this was more of a problem riding in So. California than in more bike friendly Seattle. It also mattered if bicycles were common on the route. I think there are too many other variables, without seeing the study, to place the variation solely on the helmet. Female vs. Male might make a more interesting if not valid study.
