Drug War

Do Medical Marijuana Laws Reduce Traffic Fatalities?


A new study published by the Institute for the Study of Labor, a German think tank, finds that medical marijuana laws in the United States are associated with a 9 percent decline in traffic fatalities. That result is based on data from the Fatality Analysis Reporting System for 13 states that legalized medical use of cannabis between 1990 and 2009. The study's authors, Montana State University economist D. Mark Anderson and University of Colorado at Denver economist Daniel Rees, argue that the most plausible explanation is the substitution of marijuana for alcohol, since laboratory research suggests that smoking pot impairs driving ability substantially less than drinking does (citations omitted):

Laboratory studies have shown that cannabis use impairs driving-related functions such as distance perception, reaction time, and hand-eye coordination. However, neither simulator nor driving-course studies provide consistent evidence that these impairments to driving-related functions lead to an increased risk of collision. Drivers under the influence of marijuana reduce their velocity, avoid risky maneuvers, and increase their "following distances," suggesting compensatory behavior. In addition, there appears to be an important learning-by-doing component to driving under the influence of marijuana: experienced users show substantially less functional impairment than infrequent users.

Like marijuana, alcohol impairs driving-related functions such as reaction time and hand-eye coordination. Moreover, there is unequivocal evidence from simulator and driving-course studies that alcohol consumption leads to an increased risk of collision. Even at low doses, drivers under the influence of alcohol tend to underestimate the degree to which they are impaired, drive at faster speeds, and take more risks.

In support of their hypothesis that these differences explain why states with medical marijuana laws have seen especially big drops in fatal car crashes, Anderson and Rees cite data from the National Survey on Drug Use and Health for three states that enacted such laws during the last decade: Montana, Vermont, and Rhode Island. Marijuana use by adults rose more in those states after they adopted their laws than it did in neighboring states, a difference that was statistically significant in Montana and Rhode Island. (At the same time, the survey data do not indicate that medical marijuana laws boosted consumption among minors—a charge commonly made by drug warriors.) Anderson and Rees also note that the drop in fatal car accidents associated with medical marijuana laws was statistically significant for crashes involving alcohol but not for other crashes, and they present some evidence that alcohol consumption has declined in medical marijuana states.

Assuming that people are substituting marijuana for alcohol, Anderson and Rees note, the impact on traffic fatalities might be due to shifting locations of consumption rather than the difference between the two drugs' effects on driving ability:

Alcohol is often consumed in restaurants and bars, while many states prohibit the use of medical marijuana in public. Even where it is not explicitly prohibited, anecdotal evidence suggests that the public use of medical marijuana can be controversial. If marijuana consumption typically takes place at home, then designating a driver for the trip back from a restaurant or bar becomes unnecessary, and legalization could reduce traffic fatalities even if driving under the influence of marijuana is every bit as dangerous as driving under the influence of alcohol.  

Either way, this benign effect of medical marijuana laws hinges on a tradeoff between a recreational drug and a purportedly medicinal one:

Because it is prohibitively expensive for the government to ensure that all marijuana ostensibly grown for the medicinal market ends up in the hands of registered patients (especially in states that permit home cultivation), diversion to the illegal market likely occurs. Moreover, the majority of MMLs allow patients to register based on medical conditions that cannot be objectively confirmed (e.g. chronic pain and nausea).

So while the study undermines one complaint about medical marijuana laws—that they encourage minors to smoke pot—it reinforces another: that they allow people to get high under the cover of treating unverifiable illnesses. Then again, those who believe adults should not need an officially recognized excuse to consume marijuana might view that aspect of the laws as a feature rather than a bug. 

You can read the whole report here (PDF). I discuss stoned vs. drunk drivers here and here. I discuss pot-smoking malingerers here.

[Thanks to Eric Sterling for the tip.]

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19 responses to “Do Medical Marijuana Laws Reduce Traffic Fatalities?

  1. Drivers under the influence of marijuana reduce their velocity, avoid risky maneuvers, and increase their “following distances,” suggesting compensatory behavior.

    IOW, they are too zoned to drive aggressively.

    1. Now the cops have a reason to pull over safe drivers.

      1. +5 mph under the speed limit

        1. -5? I dropped my default from +20 to +15 about 3 years ago and have only been pulled over once since. -5 is for when you’re plastered and but desperately need some White Castle.

          1. IIRC, that one time you were driving in reverse.

      2. “You were driving the speed limit and keepin a safe distance from the cars ahead of you… Have anything to smoke today? You wouldn’t mind if I searched your car and gave you a sobriety test? Ya don’t have anything to hide, correct?”

  2. The only “medical” potheads I know are from Los Angeles, and apparently the deal there is you’re guided by someone who’s in the weed-know to the office of a fraud-friendly doctor who for a few hundred dollars and a few seconds of your time, during which you’re obliged to claim you’re an insomniac (and not “creep him out”), will hook you up with a card and direct you to use it at whatever dispensary he’s informally attached to and/or bribed by.

    And yeah, the L.A. potheads “view that aspect of the law as a feature rather than a bug”?the part where you don’t have to have a medical need but you have to know a guy and you have to have a spare G to throw down?because it keeps pot illegal for “losers” and darkies.

    “You don’t have a card?”

    1. Im glad you prefaced your statement with a background of “what you know.” Since it appears to be very little.

  3. allow people to get high under the cover of treating unverifiable illnesses.

    We are talking alcohol here, are we not?

  4. “Yeah man, am I drivin’ OK?”

    “I think we’re parked, man.”

  5. The truth is that those smoking pot aren’t even on road. Who has ever smoked and said, “Hey, let’s go for a drive”?

    1. you are joking right? what %age of people in jack in the box drive thrus at 3 am are high?


      fwiw, NHTSA (and common sense) show that people high on pot are MUCH less dangerous than people impaired by alcohol. it aint even close.

      there are a host of reasons for this, but in brief, alcohol tends to increase recklessnes, iow it decreases aversion to risk, tends to make people grossly overestimate their coordination/timing/skill, etc.

      people should not drive while high on pot OR alcohol.

      but if every person high on alcohol driving were magically replaced by somebody high on pot, the roadways WOULD be safer

      of course, the roadways are orders of magnitude safer than they were 30+ yrs ago due to much stricter DUI enforcement, improved attitudes about the wrongness of impaired driving, improved vehicle design, trauma medicine, etc.

      1. As someone who used to do the drive thru thing after getting high, another thing to consider is how long the “high” lasts. About an hour after taking your last toke, the intoxicating effects drop pretty quickly. You can use that lull to get your food, get back home, and get high again.

        That’s much less possible with alcohol in my experience.

      2. a friend who worked @ nhtsa told me their testing of passing skill, using gummint-grown pot (& requiring naive subjects to smoke an entire joint!-) was thwarted by the subjects lack of desire to pass: they were happy just to follow the car in front;-)

        seems to me agressive driving would be reduced greatly if drivers were required to mellow out;-)

  6. Yeah, but is the pot as good as alcohol medicinally? Presumably they’re substituting medical marijuana for medical alcohol, and I just don’t see that working in most cases, such as dextromethorphan in terpene hydrate elixir.

  7. These guys really do know exactly what time it is. Wow.


  8. Medical marijuana – is there anything it can’t do?

  9. Support H.R. 2306! If you are for ending federal prohibition it should be in ALL your posts. If your post doesn’t contain “Support H.R. 2306!” then the effectiveness of all your cogent arguments is substantially diminished. Include it in all your posts. SUPPORT H.R. 2306!

  10. Google MARIJUANA DRIVING STUDY. You’ll see two common findings:

    1. Drivers under the influence of marijuana are VERY SLIGHTLY impaired.

    2. Unlike those under the influence of alcohol, marijuana consumers are aware they are VERY SLIGHTLY impaired and they CONSISTENTLY ADEQUATELY COMPENSATE by slowing down a little and being a little more cautious. That doesn’t mean they get in the fast lane on the interstate and drive 15 miles per hour. Marijuana makes you cautious, not crazy! Those Cheech and Chong movies were comedies, NOT documentaries!

    * Fact: The Journal of Psychoactive Drugs, Hartford Hospital in Connecticut and the University of Iowa Carver College of Medicine document states that MARIJUANA DOES NOT CAUSE DANGEROUS DRIVING:

    * Fact: When combined 2002 to 2005 data are compared with combined 2006 to 2009 data, the Nation as a whole experienced a statistically significant reduction in the rate of past year drugged driving (from 4.8 to 4.3 percent), as did seven States: Alaska, California, Florida, Hawaii, Iowa, Michigan, and Pennsylvania. Four of these seven States have legalized medicinal marijuana, Alaska, Hawaii, Michigan and California.

    * Fact: California led the US to a nationwide, statistically significant reduction in the incidence of “drugged” driving during a time period when the number of patients claiming the protection of the California Compassionate Use Act and SB-420 increased by a factor of 10.

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