Don't get me wrong, I am not advocating drug use,for driving, but I also am far more aware of the effects of drugs than most, because I have had far more than most.

Hence while all the studies can give you some indication of the effects, it is in reference to which drug is more harmful that we entered this discussion, and alcohol is far worse than pot, no contest.
Also, what easel said about THC being detectable in the body for a long time, because it is stored in the fat particles, I think, does raise the valid point of were they stoned when they had their accident, or at some time in the previous weeks, we just don't have enough info to say.
Also, I can say from experience also, that people tend to be far more reckless when pissed, than stoned, if you are drunk you drive faster and sillier, if you are stoned, you usually slow down and (at least try to)concentrate more.
Maybe a reactive agent could be included in legal pot which would indicate if one had smoked it in say the previous four hours, which would aid police sobriety testing.
We don't want to add to the road toll by unleashing people who are not in a fit state to drive, onto the roads, but pretending it doesn't happen won't help.