In case I missed it, did that study control for location? I know that the other study stated that it did not have much (if any) background information on the candidates, but did say that infection rates for the parasite are higher in villages and small towns. I think that this is a significant lurking variable in any traffic accident analysis, and needs to be considered: In my experience, rural folk are less exposed to some of the more "creative" driving situations than those who live closer in.