The Impaired Driving Safety Commission was established by 2016 PA 350 to research and recommend a threshold THC bodily content for per se impaired driving. In March 2019, the commission issued its report. The Commission reviewed the thresholds imposed by other states that have legalized medical or recreational marijuana and examined the science of THC absorption and metabolism as well as the behavioral effects of cannabis. The report explained the difficulties of assessing measured blood THC levels and impairment given differences in cannabis ingestion methods and drug tolerance issues:
The consequence of these results for setting per se limits is that blood ∆9-THC can fail to detect impaired drivers (when blood levels are low and impairment is high). It can also inappropriately flag unimpaired drivers or chronic users whose blood levels are higher in general … even when not impaired.
Given these difficulties, the Commission recommended against establishing a threshold of THC bodily content for determining driving impairment. Instead, it recommended both use of roadside sobriety tests to determine whether a driver is impaired and additional training of law enforcement in detecting impairment.
One criticism of this approach is that it gives officers too much discretion. Brett Rendeiro of Butzel Long noted that for “[c]ommunities that are historically marginalized or have concerns about police tactics, this is an area that would be of great concern.”
There are some testing technologies in the works to assist officers in determining impairment. In the meantime, Carol Flannagan, a University of Michigan professor who was on the Commission, offers this advice for drivers:
The best advice that I’ve seen is if you feel different, you drive different. If you are feeling affected by anything you’re ingesting …, it is presumably affecting your cognitive skills that you need to drive.
To hear what a drug recognition expert has to say about recognizing marijuana impairment, check out the 2018 Drunk and Drugged Driving Update.