Medical Marijuana Laws Decrease Traffic Deaths? Pull Out Your Critical Thinking Skills

By Veronique Greenwood | December 2, 2011 2:02 pm


If you remember anything from statistics class, it’s probably that correlation ain’t no causation. Just because two numbers happen to go up at the same time doesn’t mean that one is causing the other to rise (or fall, or hold steady, or whatever). If there isn’t a plausible explanation for how the two might be connected, and proof that that explanation is indeed the cause, all you have is a couple of lines on a chart.

So it’s a move in the right direction when people try to suss out connections between two variables they have a hunch are related. But seeking such a connection can lead to some pretty convoluted reasoning. A new paper—unpublished, but released for discussion—claims that the passage of states’ medical marijuana laws cause decreased traffic fatalities, following on the researchers’ intuition that people might smoke pot instead of drinking alcohol if marijuana were more readily obtainable, and that driving while high is less dangerous than driving drunk. Their logic goes like this: if medical marijuana laws make pot more easily available, and people smoke more pot after laws are passed, and they buy less alcohol because of that, then there would be fewer traffic deaths.

The connection between alcohol sales and traffic deaths, at least, sounds plausible, and traffic deaths did drop after medical marijuana laws were passed. But as they go through the statistics that they claim are behind each of these links, a thick cloud of question marks descends. There isn’t crystal-clear evidence that medical marijuana laws actually do get people to smoke more pot—the three states they discuss, Montana, Rhode Island, and Vermont, have widely varying numbers of people signed up for medical marijuana, and although there was an increase in reported marijuana use in two of the states for some age groups, it wasn’t huge. Huge isn’t necessarily required, if the change is statistically significant, but one has to assume that there is lots of fibbing and thus uncertainty in any count of illegal (even kinda-sorta-legal) drug use. The statement that people who smoke pot consume less alcohol is also tenuous—the authors admit that research has provided no clear answer on whether people drink more or less when they smoke pot, and simply note that after legalization, phone surveys showed a drop in binge drinking. That, of course, is a correlation.

There do seem to be convincing arguments that although pot, like alcohol, slows your reaction time, high drivers drive more carefully than drunk ones—read more on that at Slate. But that’s more of an interesting factoid than an important contribution to the argument, given all the fuzziness elsewhere.

Can you think of other routes by which medical marijuana laws might affect traffic fatalities? Or other statistics to chase down that might link those laws to other changes? Think of it as an exercise in building scientific character, choose-your-own-adventure style. The paper is available for free here—check out their arguments for yourself.

Image courtesy of Caveman Chuck Coter / flickr

CATEGORIZED UNDER: Health & Medicine, Top Posts
  • scott

    I have never considered Discover magazine that good at scientific reporting. Its amusing that they provide an explanation for causation vs correlation when it is so frequently absent the rest of their literature. They often jump on vaguely defined scientific theories without bringing up more thorough discussions. But I digress.

    Several of their background statements are incorrect. There is evidence that despite lower the speed with which stone drivers operate their vehicles, they are no more likely to result in crashes. This is apparent across age etc.

    Also there is considerable evidence mounting that marijuana use leads to reductions in harmful behavior such as alcohol consumption despite their erroneous presumption otherwise. I believe they are confusing their statistics. There is no evidence that marijuana use among certain demographics such as teens and otherwise healthy adults increases after medical marijuana laws are enacted. What is demonstrated is that sick patients and people that use marijuana as a part of a harm reduction strategy do find it easier to access after these laws and their consumption does go up.

    Also, exactly what else would the author attribute the drop in alcohol sales to? With so many people publicly stating that they would use one over the other, isn’t fair to assume they are substituting one of the other when they have a choice?

    Causation is proved by showing direct mechanisms and I think there is ample evidence to suggest at least some people prefer marijuana over alcohol and will use it when provided legal means to do so. That would be a valid mechanism. IMHO.


    Sex differences in the effects of marijuana on simulated driving performance.
    Anderson BM, Rizzo M, Block RI, Pearlson GD, O’Leary DS.


    In the United States, one in six teenagers has driven under the influence of marijuana. Driving under the influence of marijuana and alcohol is equally prevalent, despite the fact that marijuana use is less common than alcohol use. Much of the research examining the effects of marijuana on driving performance was conducted in the 1970s and led to equivocal findings. During that time, few studies included women and driving simulators were rudimentary. Further, the potency of marijuana commonly used recreationally has increased. This study examined sex differences in the acute effects of marijuana on driving performance using a realistic, validated driving simulator. Eighty-five subjects (n = 50 males, 35 females) participated in this between-subjects, double-blind, placebo controlled study. In addition to an uneventful, baseline segment of driving, participants were challenged with collision avoidance and distracted driving scenarios. Under the influence of marijuana, participants decreased their speed and failed to show expected practice effects during a distracted drive. No differences were found during the baseline driving segment or collision avoidance scenarios. No differences attributable to sex were observed. This study enhances the current literature by identifying distracted driving and the integration of prior experience as particularly problematic under the influence of marijuana.

  • O

    I live in Colorado and being able to use Marijuana without fear of prosecution is one of the main components of my ability to quit drinking alcohol. To this day I still abstain from alcohol, as a result of making the safer choice to smoke Marijuana.

    People who smoke Marijuana are very good drivers, this is “the most comprehensive study ever” on this issue and one again they have come to this conclusion.

    Bill Hicks said it best, people should be forced to get high before they drive, it would make for less reckless behavior, road rage, tailgating, etc…

    Much Love,

    Orph L. Messia

  • Sean

    Yea good point. Nothing good will ever be allowed to come of the devil weed Marijuana no matter how much data proves otherwise. Not one confirmed death from Marijuana(Cannabis) in the history of its use. Hmm… WAKE UP PEOPLE!

  • Micah

    Maybe people are just getting high and are too lazy to drive anymore. Not the most well thought out argument, but maybe if we did some research to see if there was a correlation between people getting high and the amount of time they drive….

  • Amanda

    I was going to say the same thing about the use of marijuana and the amount of drivers on the road. Perhaps there are just less people on the road. This could be a reason that less accidents happen. Although, did anyone look into other factors for why the accidents decreased? It could be traffic speed cameras or something else that did it.

  • Ben

    And that caused a gigantic 9% drop in traffic deaths? Stoners bought more groceries while at the store so they could drive less?

    So beer sales go down 5% after medical marijuana is legalized, drinking among young people also goes down, and traffic deaths dramatically drop. I wonder what sort of crazy, wacko conclusion we could possibly freaking draw from this.

    Also this article failed to mention that there was a larger, negative correlation between medical marijuana and traffic deaths involving high BAC levels (which are very easy to track since they almost always involve police) and deaths during the weekend and at night but there is NO correlation between medical marijuana and traffic deaths not involving alcohol.

    Just a bunch of strange correlations.

  • Ug

    Hey Scott,
    If Discover isn’t good at scientific reporting then why are you reading it?

  • willie watson

    Like Winnie T. Pooh examining the honey pot, I guess we’re just going to have to do some more research.

  • John Lerch

    You’ve used the British slang “suss” twice in 2 different articles today–please don’t do it again, primarily since you seem to be using it worng :-). “Suss” is short for suspect so it wouldn’t precede “out” as you used it both times.
    To the question: It’s well known that people self-medicate; and presumably it’s this use that causes people to drive drunk–i.e. the lowering of anxiety and such in social situations. OTOH I don’t approve of blanket approval of medical marijuana without provisos for no psychological medical dispensation.
    Scott wrote “There is evidence that despite lower the speed with which stone drivers operate their vehicles, they are no more likely to result in crashes”. This makes no sense–have you been using the weed yourself with your multiple grammatical errors that muddy the meaning?

  • Mareka

    How many people are killed by drunk drivers each year? How many people are killed by high drivers each year?

  • maureen.brian

    Suss out is a perfectly legitimate usage. Check any dictionary and many will tell you it is the more common form.

  • Tiktaalik

    John Lerch:

    Don’t get your panties in an uproar.

    Common usage includes the “out”. It’s not necessarily “worng”.

  • thesmartone

    I personally believe that marijuana is the safer “drug” and that alcohol is one of the most dangerous. with that being said i do beleive that if marijuana was made available in the same way that alcohol is it would greatly benifit our society, and yes lead to less traffic deaths/accidents. This is what upsets me about this topic and im sure all of you have seen it. Some dumb****s will get together and decide to test a persons driving skills while sober and then under the influence of marijuana. these tests are almost always bias due to the fact that the “driver” is always someone who has never smoked in thier entire life. I know for a fact that this leads to incorrect results. think about it, your allowed to drive to a bar at the age of 21 drink 2 or 3 beers and drive home legaly so long as your bal is under .08. now if a man is 45 years old and has been drinking since he was 18 im sure he can go to a bar spend 3 hours there and pound 4 to 5 beers and get home safely each time. now replace that 45 year old with another 45 year old who has never so much as taken a sip of wine at thanksgiving dinner. have him pour those 5 beers down his throat and i gaurentee he’s hitting three or four cars before he gets out of the parking lot. basicly its not fair to judge someones ability to perform while on a drug if that person has never had any experience with it. regular “pot smokers” have no trouble driving infact they drive more careful. this has been proven through one study where they actually tested regular pot smokers and thier abilities to drive while “impaired” guess what all of them did better while high. Im just dumbfounded at this country’s ignorance torwards this.

  • Veronique Greenwood

    @ John, see definition 1: 1. ( often followed by “out”) to attempt to work out (a situation, person’s character, etc), esp using one’s intuition. I’m not familiar with the British usage you describe, but thanks for commenting.

  • Meg

    Does anyone know why they chose Montana, Rhode Island, and Vermont, of all places? There are 16 states where medical marijuana is legal, and the three they chose to study legalized it relatively recently, and have small populations. Why not California, where it’s been legal since 1996? That would provide a large statistical sample and a nice, long baseline. Why not also add states that have decriminalized possessing small amounts of pot, like Massachusetts? This seems like cherry-picking the data to me.

  • Veronique Greenwood

    @Meg, nice observation!

    They write that they chose those states because the structure of one of the databases they were using changed in 1999, so for consistency, they had to go with states that legalized it after that point. However, there are certainly more than three states in that camp, you’re right.

    The paper is actually available for free (check out the link at the end of post) so you can look through it yourself.

  • Chris the Canadian

    Good point Meg, never even thought of it. Montana, Rhode Island, and Vermont are not exactly hotbeds of pot use or have huge populations of drivers. Pretty sketchy to use those states as opposed to Massachusetts, California, or British Columbia Canada.

    For those trying to argue ‘compare the number of drunk driving deaths last year to the number of stoned driving deaths’ give it a rest. You can’t compare the two. The only way one could compare those two is to take into account the ratio of people drinking versus those who use pot. There are WAYYYYYY more people who drive drunk than there are who get stoned and drive.

    The stereoptype of someone who is stoned is someone who is relaxed, calm, at ease, but in control of all their senses. I would argue that being stoned and driving is as dangerous as being drunk and driving. While stoned a persons ability to react and make split second decisions is impaired. Add to that the fact that while stoned many people become drowsy and sleepy. To me, that doesn’t paint the picture of safety either. Simple deductive reasoning: Driving drunk is bad because your senses are impaired. Smoking pot to the point of being stoned impairs the senses and a person’s ability to react and make life saving decisions while also causing drowsiness. Therefore, both drinking while driving and being stoned while driving is dangerous.

  • Geack

    @ John Lerch,

    To “suss out” is an expression I’ve heard here in the American midwest since my earliest childhood. Maybe it’s used differently in the UK, but the use in this article is perfectly acceptable colloquial American English.

  • RudiKelle

    … Drunk drivers dont care … about speed, road rules, bumps and crashes, people, cops, or consequences. Drug drivers do care – to the point of paranoia sometimes. They are more aware due to heightened awareness – as opposed to drunk, slurrey, blathering, staggering, arguing, fighting, crying, barfing, unfocussing, falling asleeping, not fit to be driving – morons. All stoners know if they have had too much to drive and they will have a lie-down for a couple of hours. Not so the boozers. Stats say stoners have less prangs than drunks … Really? … Why?? – Back to top


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