> 40% having significant THC in their blood is a stunning statistic no matter how you look at it.

Yes, it is a stunning statistic.

So much so, that in itself it makes it worth questioning the results of this study.

If 40% of fatal crash victims had THC in their bloodstream, and only 20% of the general population did, that would imply a 100% increase in chances of dying in a car crash from having smoked marijuana. That's an absolutely massive risk factor, the kind you would expect to show up very, very clearly in any kind of statistical analysis of car crashes.

But the other thing I've seen a bunch of people cite in this discussion is that there has been no statistically significant increase in fatal crashes following marijuana legalization.

That would imply that either there was no statistically significant increase in drivers high on marijuana since legalization, or there was no statistically significant increase in the likelihood of causing a fatal crash from being high on marijuana.

Based on our knowledge of human nature, the former seems incredibly unlikely (yes, there would surely be some people who would have been smoking pot before who just stopped hiding it as much, but there would also, just as surely, be many people who had been interested in getting high before, but who had been intimidated by its legal status or had no idea how to find a dealer until there were dispensaries opening in every town).

The latter directly contradicts the implication of this study—but this is only one study, and may have methodological issues that we are unaware of.