There was a larger discussion in a previous thread on this topic: https://news.ycombinator.com/item?id=45494730

Since then, [0] has been published and I think it's worth at least a skim. Since it's quite recent the introduction summarizes some of the most recent research.

The things that jump out at me are:

- [0]: Habitual users with baseline concentrations above legal limits perform just as well as habitual users with baseline concentrations below the legal limit, indicating that for habitual users, the legal limit doesn't have any relation to impairement.

- [1]: A study in Canada analyzed crash reports and blood tests to look at the state of drivers responsible for accidents. While alcohol had a very clear and statistically-significant influence on the risk of a driver causing an accident, THC did not.

To steelman the idea that THC causes accidents, [0] only looks at habitual users with baseline levels of THC and [1] only looks at non-fatal injuries.

My conclusion right now is that the number of drivers in accidents with THC in their blood is going up because the number of people with THC in their blood is going up, not because drivers who use THC cause accidents.

The law's assumption that this level of THC is evidence of impairment seems to be invalid.

The law would be better off measuring impairment in some way and perhaps intensifying penalties when an impairment test fails and the user has THC concentration above some threshold.

[0]: https://academic.oup.com/clinchem/article/71/12/1225/8299832...

[1]: https://pubmed.ncbi.nlm.nih.gov/31106494/

But the truth is that habitual users are always impaired. Source: former habitual user.

That works the same with alcohol - heavy alcoholic would probably need to be over the limit to feel ok and sober.

Perhaps habitual drug users should not participate in operating heavy machinery around people?

Even if your point is conceded which I will state as: "the impairment levels are too so that unimpaired habitual pot users appear impaired"

40% of the general population is not a habitual cannabis user. So whether they are "high" or not, folks with high THc levels are dangerous.

Yes, you cannot drive if you smoke cannabis at all. Seems fine, now just get them off the road.

And there are alcoholics who remain highly functional well over the legal limit.

Opening the article would have allowed you to see that the average was 30.7 ng/mL, it's in the very first bullet point!

To give some statistical context, as of 2023, about 16.11% of people in the US have used cannabis in the past year; per that same dataset, about 16.53% of Ohio residents. [1] Given that Ohio’s usage metrics align closely with the national mean, I think it's fair to use the state as a proxy for broader domestic trends.

Per more Pew Research data, also from 2023, Ohio seems to have an average, if not a less than average, concentration of cannabis dispensaries, compared to other states where CBD products were legal. Montgomery County, OH is located in the bottom-left quarter of Ohio, and sits in a region with lower dispensary density than many comparable U.S. districts. [2]

Given that usage metrics mirror the national mean despite a lower-than-average retail presence, I think this dataset is a pretty fair "middle america" benchmark.

[1] https://www.statista.com/statistics/723822/cannabis-use-with... [2] https://www.pewresearch.org/short-reads/2024/02/29/most-amer...

> I'm skeptical that the alcohol industry might be involved.

So you don't think that the alcohol industry is involved?

You sound like the people who were outraged when drinking and driving was first banned. They had all sorts of made up logic to get around wanting to consistently being a hazard to others.

You think 40% of the people in that area have THC in their system?

Of course you don't. So why make this argument?

Are you funded by Big Cannabis™?

Upvoted!

I’ve applied this same rule when reading news or watching tv - if your immediate reaction is shock, surprise, or anger, think of a list of organisations who would monetarily benefit [1] from that article or show etc.

Nobody does a news story for the fun of it unless it genuinely IS news, but it’s been shown [2] that over 70% is PR pieces trying to make you think one way or another.

[1] money talks, and bullshit walks

[2] Australia’s Media Watch ran a piece about this a few years ago

[flagged]

The biggest problem with drawing conclusions from the 40% number is that THC use correlates with other well established crash death risks like being a young driver or the use of other impairing substances.

The fact that legalization did not impact the crash rates is also a strong signal that THC itself is not causing the crashes.

The presence of THC in the blood is not a reliable signal for intoxication, so further research is needed to draw any type of conclusion.

Finally, it's also been noted that there are some sample bias concerns because the data comes from fatal crashes where it was determined that a drug test should be administered after the crash.

> 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.

No not at all. Let's do it this way:

What percentage of people who drive drunk have consumed thc within the time window of blood detection?

Now this is a more reasonable number

I really don't know why this number is significant. Accidents are situational and people who engage in situations where accidents are more frequent likely make other decisions about consumption and lifestyle which involve things like cannabis

Who cares?

You aren't going to elevate the behavior of the population by regulating a plant

The number of people who report using it is only a rough proxy for the number of people who are using it, though.

You can't drive at 12 though surely? And you have to account for the fact that young people are going to be more likely to die in crashes and more likely to use weed.

This kind of test seems silly. It's going to be far too hard to remove the confounding variables. Much easier just to give people different levels of weed and have them do driving tests. Directly measure their driving skill instead of doing it by shitty proxy like this.

Surely this has been done?

You really think there arnt 20% of people who use but don’t report? What a crock of shit.

The n for the group is only 246 though. That’s very biased depending on where the stats are from.

>focusing on 246 deceased drivers who were tested for THC following a fatal crash”. That means there could be selection bias at play.

that wording definitely sets off warning alarms for selection bias. but it looks like there were approximately 350 traffic deaths in montgomery county during that period [1]. that probably about lines up with 246 drivers dying during that period, so it seems likely they tested all or almost all deceased drivers.

[1] https://dam.assets.ohio.gov/image/upload/statepatrol.ohio.go...

They failed to present the average _age_ of the drivers as well. Young drivers are more often involved in fatalities than older drivers. This is clear if you look at the NHTSA's FARS database.

“Finally, no mention is made on the levels of THC in the general population of those driving cars.”

How do you propose gathering that particular data?

Came here to say most of this, also worth calling out the note at the bottom:

> Note: This research was presented as an abstract at the ACS Clinical Congress Scientific Forum. Research abstracts presented at the ACS Clinical Congress Scientific Forum are reviewed and selected by a program committee but are not yet peer reviewed.

My guess is when it gets to peer review, one of the reviewers will request at least mentioning these limitations. As it was only an abstract, it’s possible the paper itself does mention these limitations already as well.

>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

NYC has had the same effect since COVID, and over the last year or two it's gotten to the point where every single light at every busy intersection in Manhattan you get 2-3 cars speeding through the red light right after it turns. I bike ride a lot so I'm looking around at drivers a lot, and for the most part the crazy drivers seem to be private citizens in personal cars, not Uber or commercial/industrial drivers.

> I was guessing it was any amount of positivity, and may be close to the population level, but it's actually impairment levels of THC:

A lot of people are trying to debate the impairment threshold or argue about mean vs median, but 40% of deceased drivers having this much THC in their blood would be a notable result for basically any sample of people for anything other than a music festival or something.

The number of people age 12 or older who report any THC use at all, even once, in the past year is around 20% (or less depending on the survey). Having 40% of a group register levels this high is a very eye opening result.

I think it's lawlessness overall. For instance, consider San Fransisco traffic citations. Went from around 11k in 2014-2015 steadily down and then fell off a cliff during covid but never recovered (around 1k in 2023).

I remember the sad story of Eric Garner who was killed in 2014 while being arrested for selling loose cigarettes in Staten Island. Today, at least in NYC, you see people parked out in front of the same corner every day selling weed and loose cigarettes. Same people, out in the open. I'm pretty sure that's not a sanctioned dispensary.

Just shows how much things can change in ten years. For whatever reason, police and prosecutors just gave up in enforcing any kind of laws. Seems like an overreaction to whatever problems we had with criminal justice

https://www.reddit.com/r/sanfrancisco/comments/11nbnxw/san_f...

https://en.wikipedia.org/wiki/Killing_of_Michael_Brown

The numbers do appear quite staggering. It can't just be the dead drivers - there must be similar numbers of stoned drivers who are causing accidents, maybe killing others, while surviving themselves.

As far as driving goes, any amount of drugs or alchohol is going to reduce reactions times, in addition to any impaired judgement or ability to control the vehicle. Even a couple of 1/10ths of a second in increased reaction time is enough to make the difference between braking in time and hitting another car or pedestrian/etc.

The running red lights thing is crazy. I think at it's height, I would maybe see 3 people do this in a single 20 minute drive.

And not like running a late yellow, but a full on my-light-is-green-and-there's-a-guy-in-front-of-me-sideways

It has dropped a bit now though.

It's the same everywhere. It seems like police have just stopped enforcing traffic laws. Multiple times per week someone runs the red light in front of my local police station, in full view of an officer in their car, and nothing ever happens. Same with the multiple near-misses I see every week. They don't care, and since there are no consequences, there are no longer any traffic laws. Couple that with the mass psychosis afflicting the US, nobody seems to care about anything and just drive as fast and hard as they want to, and fuck absolutely everyone else.

There’s no reliable way to determine impairment from a blood test. At most, this says that ~42% of people used it recently and/or frequently enough for metabolites to be present.

https://forensicresources.org/2021/marijuana-impairment-faq/

Traffic fatalities increased during the pandemic[1]. AAA released a study examining the effects in 2024[2].

[1]: https://pmc.ncbi.nlm.nih.gov/articles/PMC10149345/ [2]: https://newsroom.aaa.com/2024/08/the-pandemics-tenacious-gri...

The most galling and pervasive offense, though, is TEXTING. The rampant texting while driving is killing pedestrians (and other drivers), leading to oft-cited statistics about the failure of "Vision Zero" and the increase in pedestrian deaths. Not to mention the millions of hours stolen from us all by people BLOCKING TRAFFIC while texting.

We should not tolerate the ignorant and ineffectual response from lawmakers on this issue. Year after year, they refuse to do the right thing: make texting a DUI-level offense, with the same penalties. You could even argue that texting while driving is worse than DUI: Drunk people suffer from impaired judgment; sober people texting have decided to endanger and steal from everyone else while in full command of their faculties. It's despicable.

If you're in San Francisco, the city essentially stopped issuing traffic tickets when COVID started. It's no wonder lawlessness increased. https://datawrapper.dwcdn.net/xMUFt/mobile.png

Before this year I had only seen 1 wrong way driver in 30 years. In the last year alone I've seen 6! I saw one person going the wrong direction in a round-about. Another person going over the inner portion of a round-about. People stopping in the road for no reason. It's insane. The strange driving patterns is indeed a major issue. I thought it was maybe a Gen Z thing, but often times these people seem to be between 30-50 in age.

Edit: no offense to Gen Z with my earlier comment btw. My reasoning was maybe we're failing younger generations with drivers ed so the blame would be on us anyway.

Also I've seen these strange patterns in many states in the last year+: Oregon, Washington, Wyoming, Idaho, California

I got a dog during COVID and I'm not sure if this is a related issue but the number of times I've had people not brake but accelerate as we're crossing the street or flash their high beams or try to drive around us at the last moment is insane.

there are days where it happens multiple times during one walk and weeks where it happens at least each day of the week.

i'm actually a car guy but when i drive if i see any pedestrians i always slow down and i take it even easier if i see they have small children or dogs since either can randomly stop or dart away.

We've seen the same uptick in reckless driving in CO since Covid. Reddit Denver complains about it all the time. I think it's happening everywhere, and it's not clear why.

If you have ever driven in Ohio the driving behavior is absurdly different than what you see in places like CA. People rarely speed. They willingly let you cut them off. And there are reasons for that too. Here is a map of where you are most likely to get a speeding ticket (1). The only place in california I've seen any enforcement of speeding or traffic behavior is along the 395 corridor in the Owens valley. Incidentally this region comes up in the map. It simply does not happen in LA county at least. Well, maybe they slap a speed ticket onto a pursuit case on top of everything else. But no sit and wait radar taking that I've seen.

1. https://i.redd.it/f898arvdx6je1.jpeg

> Since COVID ...

I was noticing driving getting worse before COVID.

It is the plague of narcissism and individualism out there (which doesn't just affect "boomers" but also every millennial and zoomer that dreams of doing nothing other than becoming an "influencer" and posts their life on their Instagram).

Social media, low attention spans, cellphone and driving distractions, narcissism, and "fuck you, got mine" culture is going to wind up being to blame. It is a population-wide axis II personality disorder.

Non-scientific anecdote here but I feel like the lawlessness is due to lack of enforcement for traffic violations vs an uptick in weed usage. Possibly covid scrambled our brains and promotes aggressive impulsive behavior. Maybe cops are afraid to pull people over because they might inadvertently shoot them and become a national news story. Maybe the adoption of big trucks makes people feel invincible (two distinct trucks on my drive have cut in line at a left turn lane with a red arrow and crossed 2 lanes opposing traffic at different lights, what would possess someone to do that(?))

Whatever the cause I feel in my gut that if our police did basic standards enforcement people would think twice about lawlessness. I’m in Pennsylvania.

It is crucial to consider correlated variables in their correct context. This finding does not even imply impairment.

A low emotional intelligence driver, one with depression or low self worth, perhaps a psychological pathology like narcissism or nihilism. This is the type of person to initiate vehicular homicide. Intoxicant intake is a SUBSET of this group of variables.

The archetypical homicidal driver would of course have exceptionally high representation in cannabis use, and also likely cigarette use, and probably nitrous oxide but they don't measure that.

EDIT: what I will say is that dab culture is something beyond traditional cannabis use, and I could absolutely theorize that dab use in a vehicle is the new drunk driving.

reading the paper, I’d say this is a case of hoofbeats meaning horses- people are just getting high and crashing.. Although, this seems like a case where the average is very vulnerable to a ‘spiders georg’ type distortion, especially because of the tolerances people build.

Wow, pretty much no one I know drives under any influence regardless what they use.

I wonder how many of these people were under the influence of alcohol and other substances.

> Feels like a low sample size

Its not a sample, it is the whole universe of analysis. (If you treat it as a sample of, say, US drivers killed in accidents in the same period, then errors due to sample size are probably the least of its problems.)

That's genuinely frightening and possibly explains a lot about people on the road these days.

Everyone I know, a pretty successful group of people, have no qualms driving when stoned.

I don't think it is that simple. I would wager $$$ that dead drivers tend to skew younger, mostly young men who think they are great drivers, drive way too fast, pass with little margin, etc. Young people probably skew higher for THC use as well.

Having said that, I think that effect explains only part of the 40%, but can't explain all of it.

> It would give better context to a dead drivers testing positive for THC.

No it wouldn't.

People make those excuses because it's weed, but you would have never posted that on an article about alcohol.

> “An average level of 30.7 ng/mL generally means those people must have consumed marijuana at some time close to driving. This isn’t about residual use; it’s about recent consumption.”

If we are at 40% of the population being high at any given moment I think we are having extremely serious societal problems around mental health. Occasional use is not a big deal IMHO, but if a person is spending 40% of waking hours impaired that person has some serious unmet psychological needs.

> 103 drivers (41.9%) overall tested positive for THC, with yearly rates ranging from 25.7% to 48.9%.

The statistics for this seem suspect at best, I'll believe it once it's peer reviewed

Smoking weed is terrible for you. Either drink or do psychedelics.

> We can only cure this if we get serious about penalties

Saying that in the country with world-leading mass incarceration mostly due to its decades long “war on drugs” which has very much not cured drug problems is a perfect example of putting ideological preconceptions ahead of reality.

Maybe we can declare our intent to eliminate drug use with harsh penalties using a metaphor, like going to war against them. That should do the trick.

> 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

Those laws exist, and often result in people who should be receiving treatment spending years of their life in prison.

Someone who gets 5+ DUIs isn’t likely to be deterred by schemes like this

It's interesting. You begin by describing the circumstances and then conclude the problem must be fixed by changing individual behaviour.

50% of fatalities are from the 8% of the driving population that refuses to wear a seatbelt.

https://www.trafficsafetymarketing.gov/safety-topics/seat-be...

If you do an adequate job of looking for confounders you'll likely torpedo the study because it will turn out to be something already known.

No, only conference abstract available.

https://journals.lww.com/journalacs/fulltext/2025/11001/trau...

The number of people who use any THC at all isn’t even close to 40% thought. The highest survey numbers I found had 20% of people reporting any use, even once, in the past year.

There’s no way to normalize a result of 40% of a population sample having significant THC concentrations. That’s way higher than any conceivable sample of the general population.

Kind of a bad test since people know the goal is to drive well, most accident happen when people are complacent.

From what I understand, the effect of legalization (or decriminalization) on the amount of cannabis use is not straightforward at all. You'd have to factor that in somehow.

I remember reading that in an accident with one party intoxicated and the other not, the intoxicated party is more likely to survive because they are more relaxed and can take an impact better. But of course sober people are less likely to get into single car fatality causing accidents I'm sure.

I don't think the user population changes much whether it's illegal, "medical," or legal.

Yeah, drug use is also influenced by social and economic status, which also influences driving risks. People of lower socioeconomic status drive less safe cars on less safe roads for longer commutes. This is something valid to evaluate with a drug like THC which is detectable long after use. It would be nice to see the distribution of levels detected and not just the average.

Are they necessarily smoking and vaping cannabis though? My vape is visually pretty similar to a tobacco vape, and vaping doesn't usually have much odor either way (unless it's scented vape juice, but I'm not terribly worried about cognitive impairment from bubble gum).

Obviously the study is not claiming that rates of THC use in general remain the same.

One possible reason: the “new recruit” people who are now willing to use cannabis BECAUSE it is legal are also rule-following by being willing to stay off the road after using it. Perfectly plausible to me.

Just that you see it more doesn't necessarily mean it happens more

That could mean that THC is not causative, just coincidence.

With this information alone, no we cannot tell.

For example if we took random samples of the population and tested them for marijuana usage, what percentage would test positive?

Next, this study is only talking about marijuana testing, how many of the same group also tested positive for alcohol (or other impairing drugs). Lets make up fake numbers and say 60% of total fatalities had alcohol or other impairing drugs and the overlap between them and marijuana use was 80% then marijuana is rather insignificant.

We have to have all the details so we don't fall into a base rate fallacy.

Well, its the wrong universe of analysis to make that claim and there is no comparative measure of alcohol exposure in the same universe of analysis so it also fails to provide a basis for any alcohol/THC comparison, so, no?

It is entirely possible THC is more dangerous for vehicle safety than alcohol and that wouldn’t surprise me. But based on this study alone? No.

And likewise, what share of that 40% were killed by the other 60%? Fault was not evaluated here.

> I can smoke a lot of weed and not “feel high” yet I would test off the charts on this test.

> 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.

I agree. As someone who regularly consumes 250mg of edibles daily at a minimum, I’m sure my levels would be off the charts on a constant basis, even when sober. With the tolerance I currently have, it’d take a ridiculous amount to put me into a state where I felt driving wasn’t safe.

It should be the other way around. If there’s no good way to test, then no driving with any THC in blood.

> 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.

You’re not really going to win anybody over to the legalization side when you basically say that people can consume as much THC as they want and drive without any penalties because of testing limitations.