Today I attended an excellent lecture by my friend, Austin Lawyer Jamie Spencer, who discussed defending Marijuana-only DWI's. The takeaway from the lecture was that there is apparently no correlation between blood levels of THC and impairment. Which means that the most important piece of evidence in a Marijuana-only DWI case will be the videotape, as it should be.
Interestingly, Jamie discussed a specific 2015 NHTSA study that indicated marijuana seems to impair driving, but that the specific blood level doesn't correspond to the level of impairment:
At the current time, specific drug concentration levels cannot be reliably equated with a specific degree of driver impairment.
A little bit of internet research found an even more current 2017 report to Congress from NHTSA which says the same:
Despite the variability in results, this research has demonstrated the potential of marijuana to impair driving related skills. It does not show a relationship between THC levels and impairment.
So the problem that Jamie is highlighting is not that there's not enough research into whether marijuana in general can impair drivers, it's that unlike the blood alcohol measurement ("BAC"), a measurement of THC in the blood is simply not a proper measure of the level of impairment.
When we receive lab reports regarding blood THC levels in DWI cases, the reports indicate the levels of the psycho-active THC compound ("delta nine") as well as the metabolytes of THC (hydroxy and carboxy), with numerical values assigned to each. The clear implication of the NHTSA studies is that none of the numbers are meaningful in terms of predicting impairment.
The question then becomes, since the state must prove impairment, but the blood levels do not correlate with impairment, how are they relevant?
Numbers have special importance in courtrooms because numbers imply precision and meaning. Jurors know and trust important numbers; they know that the BAC number of ".08" is significant, just as their own cholesterol numbers need to be "under 200." Jurors expect to see numbers correlating to blood tests in their own lives, as well as in the courtroom. By showing jurors the non-correlative THC levels, are courts allowing the state to mislead jurors?
As Jamie put it so well, we criminal defense lawyers have a job to do--we must educate. Educate the judges, educate jurors and every once in a while, educate skeptical prosecutors about junk science like THC blood levels.