10 ms·

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

by Youden 10mo ago

There was a larger discussion in a previous thread on this topic: https://news.ycombinator.com/item?id=45494730 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?login=false https://academic.oup.com/clinchem/article/71/12/1225/8299832... [1]: https://pubmed.ncbi.nlm.nih.gov/31106494/ https://pubmed.ncbi.nlm.nih.gov/31106494/

seizethecheese 10mo ago
Sure, but in this study 40% of people had very high THC concentrations. Is it even remotely plausible this is the baseline population level?
- bookofjoe 10mo ago
  DEAD people
  - graeme 10mo ago
    If the population of dead drivers with over the limit THC is 40%, and this dramatically exceeds the population average, that would strongly suggest the THC level IS an indicator of either: 1. Impairment from THC, or 2. Worse than average driving and risk management skills in those who use the drug
    - sfn42 10mo ago
      Right, so it might just mean that reckless drivers are more likely to smoke weed.
    - mindslight 10mo ago
      The second seems eminently plausible with the correlation between driving skills and drug use both being due to higher risk tolerance.
    - machinationu 10mo ago
      2. young people are worse drivers and more likely to use
    - Retric 10mo ago
      The expected number seems to be about 20+% (depending on assumptions) so this is higher than expected but not drastically so. Critically, people are more likely to get in accidents later in the day and after drinking both of which also correlate with relatively recent cannabis consumption.
    - hattmall 10mo ago
      It's seat belts. People who die in wrecks are overwhelmingly not wearing seat belts. I would think marijuana users as a group probably have average seat belt usage, but people who don't wear seat belts probably have much higher than average marijuanna usage. Roughy 92% of people wear seat belts. But that 8% of people that don't wear setbelts makes up 50% or more of all fatalities. From my personal experience it seems easy to me to assume that 90% of the people that don't wear seat belts also use marijuanna.
metadope 10mo ago
> 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 This. In just the short time since legalization, recreational use in my immediate vicinity (a small Ohio village, one stop light) has decidedly, undeniably increased. I offer only an anecdotal observation, but my evening walks around town are now accompanied by a dank potpourri of skunk scents, representing I-don't-know-how-many strains of Sativa... Indica to me that at least 30% of the population here is puffing.
- rayiner 10mo ago
  Same thing in DC. I thought the new development in DC where we lived had a major sewer gas problem. Turns out it was marijuana.
  - boston_clone 10mo ago
    Genuinely asking - had you never smelled cannabis before living in DC? To claim the odor is mistakable for sewer gas is borderline funny, unless you’re slyly trying to name a new strain.
    - rayiner 10mo ago
      I had never smelled it before. It smells identical to sewer gas to me—I know what that smells like because our house was missing several drain traps.
      - boston_clone 10mo ago
        Fascinating. I wonder if this is a genetic thing similar to people who sense soap for cilantro, except with terpenes instead of aldehydes.
        cj 10mo ago
        I mean weed really doesn't smell good. If you're not turned off by the smell, it's a learned pleasure. Similar to how nearly every child will dislike the taste of alcohol, yet after drinking for a while they'll learn to tolerate or enjoy it. It can be a very overpowering smell. When an odor overpowers, it's harder to discern one scent from another.
dang 10mo ago
Thanks! Macroexpanded: Nearly half of drivers killed in (Ohio County) crashes had THC in their blood - https://news.ycombinator.com/item?id=45494730 https://news.ycombinator.com/item?id=45494730 - Oct 2025 (125 comments)
mattmaroon 10mo ago
But this paper specifically rules out legalization as cause because the numbers didn’t significantly increase after legalization.
- NewJazz 10mo ago
  Legalization does not signify usage.
- ambicapter 10mo ago
  How does this rule anything out? It is totally possible actual usage of THC didn't increase after legalization. It wasn't one of the hardest things to find when it was illegal.
  - mattmaroon 10mo ago
    I think you’re misunderstanding. The paper says that the rates of dead drivers having THC in their system above the threshold did not change significantly after legalization. So legalizing isn’t the culprit, exactly for the reason you cite: people could get it just fine before.
avereveard 10mo ago
If that's the conclusion you'd also expect 40% of the population using it.
- advael 10mo ago
  That sounds about right to be honest
- JK-Swizzle 10mo ago
  I would expect there to be significant overlap between the demographics of those who more commonly get in accidents and those who use THC. Based on nsc.org, it seems like the majority of car accidents are with drivers 25-34 years old, and occur more frequently late at night on weekends. That generally matches the profile of the stereotypical THC user. It is hard to find good numbers of THC use. Remember that not all the population drives, nor are accidents randomly distributed in the population. https://injuryfacts.nsc.org/motor-vehicle/overview/age-of-driver/ https://injuryfacts.nsc.org/motor-vehicle/overview/age-of-dr... https://injuryfacts.nsc.org/motor-vehicle/overview/crashes-by-time-of-day-and-day-of-week/ https://injuryfacts.nsc.org/motor-vehicle/overview/crashes-b...
  - cluckindan 10mo ago
    So in other words, people with a less risk-averse personality are more likely to engage in risky behaviors ”That generally matches the profile of the stereotypical THC user” Got a source for that claim?
belorn 10mo ago
My preferred way to wade through a political research topic like this is to look at the aviation industry. If a pilot can not use a medical substance, then it is very likely that there is some thing there. Pilots are generally fairly high investment, and they are also fairly international in research and standards. All nations with an airforce tend also be interested in such research, regardless of current political flavor. From glancing at it, it seems that TCH impair pilots ability. Here is such study (done with flight sims). https://pubmed.ncbi.nlm.nih.gov/1849400/ https://pubmed.ncbi.nlm.nih.gov/1849400/ "The results support our preliminary study and suggest that very complex human/machine performance can be impaired as long as 24h after smoking a moderate social dose of marijuana, and that the user may be unaware of the drug's influence. "
- sokoloff 10mo ago
  I think that aviation has a lot of factors that bias its decisions conservatively. The “cost” of being conservative can be pretty easily borne in aviation. As an example, Zyrtec is grounding for 48 hours after taking. I think most Zyrtec users drive daily and likely quite safely, but aviation can afford to ground pilots at a lower level of risk than probably makes sense for drivers. (I have very little doubt that THC is impairing; that pilots can’t legally use it is only very loosely related to that likely linkage.)
- cogman10 10mo ago
  I think you miss the point of the comment you replied to. > The law's assumption that this level of THC is evidence of impairment seems to be invalid. I think most will recognize that THC causes impairment. The question that (AFAIK) is unanswered is if it can be measured simply by looking at the concentration of THC in the blood. In fact, if you look into the mechanisms for alcohol tolerance vs THC tolerance. What you'll find is that alcohol tolerance is a result of the body developing fast paths for breaking down ethanol. Meaning the same BAC will have the same intoxication level, the body just works harder to keep the BAC down. THC tolerance, on the other hand, appears to be the THC receptors becoming desensitized to THC. Which means the body doesn't appear to metabolize THC faster as tolerance builds. That's where a blood test might not be a good indicator of THC impairment.
tqi 10mo ago
- [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. I don't understand how this study can make that claim just looking at crash report data. The assumption that not at fault drivers are representative of people who aren't in accidents at all is pretty generous? It seems likely that folks who are unimpaired are also better at avoiding accidents / driving defensively

by Youden 10mo ago

seizethecheese 10mo ago

Sure, but in this study 40% of people had very high THC concentrations. Is it even remotely plausible this is the baseline population level?

bookofjoe 10mo ago
DEAD people
- graeme 10mo ago
  If the population of dead drivers with over the limit THC is 40%, and this dramatically exceeds the population average, that would strongly suggest the THC level IS an indicator of either: 1. Impairment from THC, or 2. Worse than average driving and risk management skills in those who use the drug
  - sfn42 10mo ago
    Right, so it might just mean that reckless drivers are more likely to smoke weed.
  - mindslight 10mo ago
    The second seems eminently plausible with the correlation between driving skills and drug use both being due to higher risk tolerance.
  - machinationu 10mo ago
    2. young people are worse drivers and more likely to use
  - Retric 10mo ago
    The expected number seems to be about 20+% (depending on assumptions) so this is higher than expected but not drastically so. Critically, people are more likely to get in accidents later in the day and after drinking both of which also correlate with relatively recent cannabis consumption.
  - hattmall 10mo ago
    It's seat belts. People who die in wrecks are overwhelmingly not wearing seat belts. I would think marijuana users as a group probably have average seat belt usage, but people who don't wear seat belts probably have much higher than average marijuanna usage. Roughy 92% of people wear seat belts. But that 8% of people that don't wear setbelts makes up 50% or more of all fatalities. From my personal experience it seems easy to me to assume that 90% of the people that don't wear seat belts also use marijuanna.

metadope 10mo ago

> 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 This. In just the short time since legalization, recreational use in my immediate vicinity (a small Ohio village, one stop light) has decidedly, undeniably increased. I offer only an anecdotal observation, but my evening walks around town are now accompanied by a dank potpourri of skunk scents, representing I-don't-know-how-many strains of Sativa... Indica to me that at least 30% of the population here is puffing.

rayiner 10mo ago
Same thing in DC. I thought the new development in DC where we lived had a major sewer gas problem. Turns out it was marijuana.
- boston_clone 10mo ago
  Genuinely asking - had you never smelled cannabis before living in DC? To claim the odor is mistakable for sewer gas is borderline funny, unless you’re slyly trying to name a new strain.
  - rayiner 10mo ago
    I had never smelled it before. It smells identical to sewer gas to me—I know what that smells like because our house was missing several drain traps.
    - boston_clone 10mo ago
      Fascinating. I wonder if this is a genetic thing similar to people who sense soap for cilantro, except with terpenes instead of aldehydes.
      - cj 10mo ago
        I mean weed really doesn't smell good. If you're not turned off by the smell, it's a learned pleasure. Similar to how nearly every child will dislike the taste of alcohol, yet after drinking for a while they'll learn to tolerate or enjoy it. It can be a very overpowering smell. When an odor overpowers, it's harder to discern one scent from another.

dang 10mo ago

Thanks! Macroexpanded: Nearly half of drivers killed in (Ohio County) crashes had THC in their blood - https://news.ycombinator.com/item?id=45494730 https://news.ycombinator.com/item?id=45494730 - Oct 2025 (125 comments)

mattmaroon 10mo ago

But this paper specifically rules out legalization as cause because the numbers didn’t significantly increase after legalization.

NewJazz 10mo ago
Legalization does not signify usage.
ambicapter 10mo ago
How does this rule anything out? It is totally possible actual usage of THC didn't increase after legalization. It wasn't one of the hardest things to find when it was illegal.
- mattmaroon 10mo ago
  I think you’re misunderstanding. The paper says that the rates of dead drivers having THC in their system above the threshold did not change significantly after legalization. So legalizing isn’t the culprit, exactly for the reason you cite: people could get it just fine before.

avereveard 10mo ago

If that's the conclusion you'd also expect 40% of the population using it.

advael 10mo ago
That sounds about right to be honest
JK-Swizzle 10mo ago
I would expect there to be significant overlap between the demographics of those who more commonly get in accidents and those who use THC. Based on nsc.org, it seems like the majority of car accidents are with drivers 25-34 years old, and occur more frequently late at night on weekends. That generally matches the profile of the stereotypical THC user. It is hard to find good numbers of THC use. Remember that not all the population drives, nor are accidents randomly distributed in the population. https://injuryfacts.nsc.org/motor-vehicle/overview/age-of-driver/ https://injuryfacts.nsc.org/motor-vehicle/overview/age-of-dr... https://injuryfacts.nsc.org/motor-vehicle/overview/crashes-by-time-of-day-and-day-of-week/ https://injuryfacts.nsc.org/motor-vehicle/overview/crashes-b...
- cluckindan 10mo ago
  So in other words, people with a less risk-averse personality are more likely to engage in risky behaviors ”That generally matches the profile of the stereotypical THC user” Got a source for that claim?

belorn 10mo ago

My preferred way to wade through a political research topic like this is to look at the aviation industry. If a pilot can not use a medical substance, then it is very likely that there is some thing there. Pilots are generally fairly high investment, and they are also fairly international in research and standards. All nations with an airforce tend also be interested in such research, regardless of current political flavor. From glancing at it, it seems that TCH impair pilots ability. Here is such study (done with flight sims). https://pubmed.ncbi.nlm.nih.gov/1849400/ https://pubmed.ncbi.nlm.nih.gov/1849400/ "The results support our preliminary study and suggest that very complex human/machine performance can be impaired as long as 24h after smoking a moderate social dose of marijuana, and that the user may be unaware of the drug's influence. "

sokoloff 10mo ago
I think that aviation has a lot of factors that bias its decisions conservatively. The “cost” of being conservative can be pretty easily borne in aviation. As an example, Zyrtec is grounding for 48 hours after taking. I think most Zyrtec users drive daily and likely quite safely, but aviation can afford to ground pilots at a lower level of risk than probably makes sense for drivers. (I have very little doubt that THC is impairing; that pilots can’t legally use it is only very loosely related to that likely linkage.)
cogman10 10mo ago
I think you miss the point of the comment you replied to. > The law's assumption that this level of THC is evidence of impairment seems to be invalid. I think most will recognize that THC causes impairment. The question that (AFAIK) is unanswered is if it can be measured simply by looking at the concentration of THC in the blood. In fact, if you look into the mechanisms for alcohol tolerance vs THC tolerance. What you'll find is that alcohol tolerance is a result of the body developing fast paths for breaking down ethanol. Meaning the same BAC will have the same intoxication level, the body just works harder to keep the BAC down. THC tolerance, on the other hand, appears to be the THC receptors becoming desensitized to THC. Which means the body doesn't appear to metabolize THC faster as tolerance builds. That's where a blood test might not be a good indicator of THC impairment.

tqi 10mo ago

- [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. I don't understand how this study can make that claim just looking at crash report data. The assumption that not at fault drivers are representative of people who aren't in accidents at all is pretty generous? It seems likely that folks who are unimpaired are also better at avoiding accidents / driving defensively