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An article by Quinton James and Johnkeria Kinglocke titled "The Truth About Marijuana" published on the website of the Hanley Foundation (an anti-drug abuse organization in Florida) claims (my emphasis):

Would you allow someone to drive you while they are high? According to the Department of Children and Families (DCF), marijuana-impaired driving fatalities have more than doubled since marijuana use has become legalized. Did you know one in five drivers are under the influence of marijuana?

It is unclear if the authors intended to source their claim that 20% of drivers are under the influence of cannabis on research from DCF (or rather was sourcing DCF only to support the doubled fatalities claim), but, in any event, no DCF resources are cited in the article's bibliography.

Are 20% of automobile drivers under the influence of marijuana?

Discussion:

I would consider a claim that 20% of drivers have some trace of cannabis metabolites or other evidence of past drug exposure in their system that could be detected with a sufficiently sophisticated medical laboratory to be plausible, but that is not what the claim is. Rather, I find the idea that 20% of drivers one encounters on the road on any random day, time, or place are literally so whacked out on weed that they cannot exercise ordinary safe driving practices to be absurdly high (pun intended).

I did find a 2019 article "Driving Under the Influence of Marijuana and Illicit Drugs Among Persons Aged ≥16 Years — United States, 2018" from the US Centers for Disease Control (CDC) that reports that it found that 4.7% of those 16 or above reported driving under the influence of marijuana in the past year. 4.7% is much lower than 20%, and still even isn't the same claim. The CDC is reporting on any instance of stoned driving in the past year even if the person drove sober (or not at all) on the other 364 days of the year, while the Hanley Foundation appears to claim that 20% of drivers on the road right now (or on average) are high.

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    I suspect that any article that starts with 'The Truth About ...' is likely to be deliberately false. As for being able to detect drugs - there was a famous episode of Blue Peter (a British Children's programme) where one of the presenters ate a breadroll with poppy seeds on, and they could detect opium in her urine afterwards. I doubt anybody would claim she was under the influence.
    – j4nd3r53n
    Nov 22 at 10:24
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    I wonder whether it's 1 in 5 drivers, or 1 in 5 drivers who are stopped by a cop.
    – Kyralessa
    Nov 22 at 15:15
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    Separate fallacy: A doubling of a tiny number is still tiny. If there were three marijuana-related fatalities a year before (number pulled out of thin air), and six the year after, that's a doubling - but compared to 20% of drivers, it's so tiny as to be a rounding error. If they don't cite a specific number, then a relative change is meaningless.
    – Bobson
    Nov 22 at 15:52
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    Or maybe the intended quote was supposed to be "1 in 5 has driven under the influence". The claim as quoted is superbly unbelievable, since 1 in 5 drivers are bad enough already that I don't think the weed would hurt.
    – fredsbend
    Nov 22 at 19:29
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    I actually ran into the same problem after listening to a presentation by Mothers Against Drunk Driving, which made the claim "1 in 3 drivers on the road with you when you drive to work in the morning are under the influence of alcohol." The source for this data? I think you could be generous and call it a misunderstanding of statistics. But more accurately you could call it manipulation to try and win a point. Nov 22 at 22:46

1 Answer 1

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In short: No

I first tried to find the article's own source, however the Florida Department of Families and Children did not have any such information that I could find. There are other Departments of Families and Children, but the article mentions Florida specifically several times so that seems like the most relevant one.

So I went looking for other sources. The best data I can find for driving under the influence of marijuana in the US comes from the NHTSA's National Roadside Survey. The most recent NRS was conducted across 2013-2014, and the results can be accessed here (the relevant portions are under Volume III: Drug Results). That survey found national numbers for drivers with a positive test for THC at 8.7% during the day and 12.6% at night. Certainly not "one in five", but probably a lot higher than people would expect. It may be worth noting that the category of "Nighttime drivers in the Midwest" did reach 20.5% according to that survey, but that seems like an extremely cherry-picked statistic for an article written about Florida if that's their basis.

However, the NRS doesn't tell the whole story. A 2017 report to Congress on the issue of Marijuana-impaired driving mentions several key factors complicating the collection of data on the subject including references to that same NRS report. In particular, THC remains detectable in blood tests long after it stops impairing their driving; a study they cite even showed a chance for detection 30 days after use. Additionally, further studies cited in the report show that THC concentration in the blood is not closely related to the level of impairment reported subjectively or measured in tests.

Finally, taking a broader look the Substance Abuse and Mental Health Services Administration puts out reports about drug use in general. The 2019 report shows that responses indicating "used marijuana at least once in the last year" were at 17.5%. Considering that number, "20% of drivers" seems at least nearly impossible.

In conclusion, according to the national data I could find, it's beyond unlikely that 20% of drivers are under the influence of marijuana. Using some blatant cherry-picking and extremely loose definitions (e.g. "anybody who uses marijuana regularly is 'influenced' by it even if they aren't currently impaired by it") they might be able to come up with a "justification" for the claim, but that wouldn't make it true (and they haven't done so in the article).

It's possible that the authors are referencing some Florida-specific data which I haven't found, but all their actual citations are from national sources which do not support this specific claim.

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    Background question on the NHTSA survey: Do they only test people for drugs and alcohol whose driving already looked suspicious enough to pull them over (meaning most people tested will test positive on either alcohol or some drug) or is that a sample over all drivers so that the vast majority of drivers should test as clean and fit to drive?
    – quarague
    Nov 22 at 8:03
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    note that such numbers exclusive reflect the results of tests done in highly specific locations, at specific times. And those times and locations are selected to coincide with areas and times when high rates of abuse are known to happen. E.g. on the exit roads of festivals and bar/pub areas towards residential and hotel areas. So the percentage of impaired drivers will be significantly higher than that of the general population.
    – jwenting
    Nov 22 at 8:33
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    @quarague The methodology section of the report indicates that they set up survey stations at randomly selected locations, and pulled drivers out of traffic randomly. I haven't read everything to check whether they have additional mechanisms setup to avoid biased data collection, but one thing that could impact results is that surveys were only performed in locations where local law enforcement was willing to cooperate. I doubt it's perfect data, but they at least claim that the sampling is random. Nov 22 at 16:24
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    @Stewart My source is the one linked in that paragraph, specifically this sentence from the bottom of page 4: "Thus, while THC can be detected in the blood long after ingestion, the acute psychoactive effects of marijuana ingestion last for mere hours, not days or weeks" Nov 22 at 16:43
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    @quarague There's a huge built in bias. 1) The daytime samples were taken when a lot of people would be at work (and thus not driving). 2) Also, it looks like the samples were done as "paid voluntary survey" with roadside signs advertising (so you wouldn't get anyone who's actually driving as part of their work). So you have a bias towards people driving around in the middle of the day who have time/inclination to pull over to do a survey. I'm not sure how that impacts the results, but I'm pretty sure that's not an average group of drivers. Nov 22 at 18:36

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