The impairments of driving under the influence of alcohol have been extensively studied, but unless I have overlooked the literature it seems that the same investigations have not been carried out with THC.

[0] «Blood THC >2 ng/mL, and possibly even THC >5 ng/mL, does not necessarily represent recent use of cannabis in frequent cannabis users.»; https://www.sciencedirect.com/science/article/abs/pii/S03768...

I'm sure you can pick other counties in the US which have either very high representation of THC users or very low representations. Without knowing how other counties score in terms of driver fatalities and THC, this is not really very useful.

To me it sounds like an effort to paint THC as big and scary. But in my experience living in a few large cities, weed is rare - but lots of people go out, drink, and drive home one or more times per week.

ScienceDaily goes even further by rounding up to 50% and burying the location halfway down the summarization.

"Nearly half of drivers killed in crashes had THC in their blood THC-impaired driving deaths are soaring, and legalization hasn’t slowed the trend. Date: October 5, 2025 Source: American College of Surgeons Summary: Over 40% of fatal crash victims had THC levels far above legal limits, showing cannabis use before driving remains widespread. The rate didn’t drop after legalization, suggesting policy changes haven’t altered risky habits. Experts warn that the lack of public awareness around marijuana’s dangers behind the wheel is putting lives at risk."

Unless they publish who funded the study, I'm skeptical that the alcohol industry might be involved. It's absolutely in their best interest to paint marijuana as the devil (and take attention away from alcohol).

Obviously nobody should be driving with any impairment, but people do - driving tired, texting, even talking to passengers and turning their heads to look at the passengers while they talk! (Really, why??? I see people doing this all the time.)

Even if you dismiss all of the questions brought up in these comments like the use of mean levels instead of median, not accounting for tolerance of habitual users, or debates about the threshold for impairment, the 40% number in this study is without a doubt far higher than the number of people who have detectable levels of THC in their blood at any given time.

I see a lot of attempts to downplay the result of this study in the comments, but 40% having significant THC in their blood is a stunning statistic no matter how you look at it.

That could mean they all had levels far exceeding most state impairment limits, but it also could mean most of them had trace levels, while a few had levels way above 30.7 ng/mL. So, it says fairly little.

Also (FTA) “Researchers analyzed coroner records from Montgomery County in Ohio from January 2019 to September 2024, focusing on 246 deceased drivers who were tested for THC following a fatal crash”. That means there could be selection bias at play.

Finally, no mention is made on the levels of THC in the general population of of those driving cars. Both _could_ be equal or even higher.

I’m not sure one should blame (only) the researchers for these statements, though. Chances are they didn’t intend to find out whether THC use is a major cause of vehicle crashes, but only in whether legalizing THC use changed those numbers, and someone managed to get some more juicy quotes from them.

> In a review of 246 deceased drivers, 41.9% tested positive for active THC in their blood, with an average level of 30.7 ng/mL — far exceeding most state impairment limits.

Since COVID in CA, it feels like driving has become far more dangerous with much more lawlessness regarding excessive speeding and running red lights, going into the left lane to turn right in front of stopped cars, all sorts of weird things. But I can't tell if my anecdotes are significant. It seems that Ohio's impaired drivers have been consistent through the past six years though.

The other question I have - my prior is that a bad driver (tired, drunk, high) is something like 70:30 odds of killing themselves vs some innocent bystander dying because of their actions. I have anecdotally heard of several sad tales where some guy is on his Nth DUI and kills an entire family, while he walks away from the accident without a scratch. Meaning are the rates of fatalities involving THC actually higher, but the detectably inebriated person managed to walk away without dying.

That said, almost everyone I know that consumes THC has no qualms driving while doing it, and many of them also at work. It's a huge peeve of mine.

I thought this was common knowledge among physicians who have studied the subject.

That being the case, I'm not sure what the policy prescription should be here, if any.

I'm not sure how THC intoxication could be measured, but blood THC concentration feels like an incredibly crude metric compared to BAC.

50% of the people on this street get stoned before driving to work, every single day

dope isn't even legal here and even if it was DUI is wildly illegal

We can only cure this if we get serious about penalties because we can't undo murder and injuries

How about first time warning, second time weekend in jail, third time week in jail, fourth time month in jail, fifth time year in prison

Averages do not work that way! The average of 48, 48, 48, 3 and 3 is 30. The study findings remain interesting but the actual proportion of impaired drivers may be less than 40%.

He says that he gets cars in that reek so bad, he can smell them from three bays away.

I suspect that we'll soon be seeing a rapid "pullover-test," and that will probably knock that stuff down.

Interesting questions: * What is the baseline of consumption / THC level?

* what was the alcohol level in decesased drivers ? (e..g how many people only had alcohol, how many only THC, how many had both, how many had nothing.

* Are there other test scenarios where THC screening is mandatory that could help getting to the baseline ? Are there ways to get an approximate answer from sewers, like they did for Covid?

This paper would need to go into way more detail to be at all useful.

40% is a staggering number, which makes me suspect that all it measures is Montgomery County police's pretty good track record for deciding when to test someone for THC during an autopsy

  Over 40% of deceased drivers in vehicle crashes test positive for THC: Study (facs.org)
  281 points by bookofjoe 15 hours ago | flag | hide | 420 comments
                                                       =============

0. https://www.rnz.co.nz/news/national/581951/first-day-of-road...

    Researchers analyzed coroner records from
    Montgomery County in Ohio from January 2019
    to September 2024, focusing on 246 deceased
    drivers who were tested for THC following a
    fatal crash. When autopsies are performed,
    drug screening is typically part of the
    process.

For drunk drivers it’s rather easy to assess whether someone is impaired. With marijuana it’s not. So until we have a valid method of testing if someone is “too stoned to drive” we have to push back on any attempt to classify marijuana users as ineligible to drive.