The label on a Michigan pre-roll gives you a THC percentage, a batch number and a test date. It doesn’t give you a time when you’re safe to drive. People read that missing line into it anyway, and the gap between what a label says and what we assume it says is where a lot of bad calls about cannabis and driving impairment start.
Can a THC test prove impairment? No. A THC test shows exposure, not impairment. Michigan’s Impaired Driving Safety Commission concluded in 2019 that no blood THC level reliably indicates impaired driving, because THC falls fast while its effects linger. That’s why Michigan has no legal THC limit and impairment cases rest on wider evidence.
Cannabis has been legal for adults here since the 2018 ballot vote. It’s sold in licensed stores from Lansing to Jackson, tested by licensed labs, sealed and printed with numbers. That legitimacy does something quiet to people. It makes the product feel finished and measured, something you can plan around. The driving part never got that treatment.
So I’m going to keep five things apart here, the way we try to on every Terp517 piece: what a lab measured, what researchers have shown, what the law requires, what people report feeling, and what’s still unknown. On driving, those five pull in different directions. The honest answer lives in the space between them.
The number Michigan went looking for
In 2016, Michigan lawmakers created the Impaired Driving Safety Commission inside the Michigan State Police and gave it one job. Find a scientifically supported THC level that could work as a legal limit, the cannabis version of 0.08 for alcohol.
The six-member panel had the right people in the room: a forensic toxicologist, law enforcement, researchers in traffic safety and cannabis pharmacology, and a medical-cannabis patient. It also ran roadside oral-fluid testing with state police.
In March 2019 it reported back, and Michigan Public covered the findings. The commission said there’s no scientifically supported THC threshold that indicates impaired driving, because blood THC and impairment line up poorly. It recommended against setting a limit at all and pointed to roadside sobriety testing instead.
I come back to that report a lot. A state went looking for the number everyone assumes exists, asked the right experts, and came back saying the number doesn’t hold up. That isn’t a loophole. That’s the science.
Why THC doesn’t act like alcohol in your blood
Alcohol is predictable in a way cannabis isn’t. Blood alcohol rises, peaks and falls roughly in step with how drunk someone is. That’s why a breath number can stand in for impairment in court.
THC breaks the pattern. The National Highway Traffic Safety Administration laid this out in its 2017 report to Congress on marijuana-impaired driving. Blood THC peaks around the end of smoking and drops fast. Impairment peaks later and fades more slowly. The report describes people with high THC and little measured impairment, and people with low THC and clear impairment. Its conclusion: per se THC limits aren’t meaningful the way BAC limits are.
Then there’s the body. THC is fat-soluble, so it lingers. The Michigan commission noted that frequent consumers can carry measurable THC long after the effects pass. It also warned that because THC clears the blood quickly, a sample drawn later at a station or hospital can badly underestimate what was there at the wheel. Edibles run on a different curve again, slower to start and longer to finish.
| What we have | What it doesn’t tell you |
|---|---|
| Blood THC at the moment of the draw | How impaired the driver was behind the wheel |
| THC in an oral-fluid swab | How long ago cannabis was used, or how much |
| The THC percentage on a label | How strongly that product will hit you today |
| “I feel fine” | Whether your reaction time and attention agree |
What Michigan law actually asks
People get this part wrong in both directions. Michigan has no per se THC limit for drivers. That doesn’t make legal cannabis a free pass. The 2018 adult-use law says plainly that it doesn’t allow anyone to operate a vehicle while under the influence, and Michigan’s operating-while-intoxicated law covers more than alcohol.
The state’s highest court set the frame years before adult use. In People v. Koon (2013), the Michigan Supreme Court held that a registered medical patient couldn’t be convicted just for having THC in his system. Prosecutors had to show he was actually under the influence.
So the legal question in Michigan matches the scientific one: was this person impaired? A lab number is one piece of evidence toward that answer. It isn’t the answer. You can read more about how we cover state rules on our Michigan cannabis regulation page.
A THC number is data. It isn’t a permission slip, and it isn’t a verdict.
When a science question becomes a legal one
One of the biggest mistakes in cannabis conversations is assuming a lab number tells the whole impairment story. THC testing can provide evidence about cannabinoid exposure. An actual DUI investigation may also involve:
- Driving behavior
- Officer observations
- Field sobriety evidence
- Chemical testing
- Toxicology
- Video
- Timing
- Other substances
- Individual case facts
And the rules change at the state line. Michigan’s legal market doesn’t follow you on a road trip south. South Carolina still has no medical cannabis, decriminalization or legalization law, according to the Marijuana Policy Project’s state tracker.
If someone is actually facing a drug-related DUI allegation in Horry County, interpreting those facts becomes a legal issue rather than a cannabis-education question. Johnny Gardner Law, a Conway firm that defends DUI cases across Horry County, says its attorney holds a 2010 NHTSA and IACP certificate of instruction for the same standardized field sobriety testing course officers are trained on. Someone needing individualized advice could independently speak with a Grand Strand DUI lawyer. No lab report, and no article on this site, can do that job for you.
What people report, and why feeling fine isn’t proof
The most common pushback on this topic sounds like this: I know my limit, I’ve been doing this for years. Part of that is real. The Michigan commission noted that frequent consumers need much more THC to feel the same effect, which is one reason a fixed blood limit misfires.
What researchers haven’t shown is that tolerance protects the attention and reaction time that driving needs. That part remains uncertain, and uncertain isn’t the same as safe. The commission also flagged that THC combined with alcohol makes impairment worse. A beer and a joint aren’t two small risks. They’re one bigger one.
If you want the chemistry behind why THC behaves so differently from person to person, our cannabinoids section is where that work lives.
My bottom line
Here’s the principle I’d hand anyone buying legal cannabis in Michigan: do not use a THC number, or a feeling, as permission to drive.
Plan the ride before the session, not after. If you’ve used cannabis and there’s any question, the car stays parked. Call a rideshare, ask a sober friend, take the couch. With edibles, give it longer than you think, because that curve runs late. And when you travel, the law where you’re standing is the one that counts.
The lab did its job when it measured the THC. Whether you should be driving was never a question that number could answer. That one’s yours.