Science AMA Series: We’re Silvia Martins and Julian Santaella-Tenorio, epidemiologists at Columbia University’s Mailman School of Public Health. Our latest study finds that U.S. states that legalized medical marijuana saw declines in fatal car accidents. Ask Us Anything!



Could you summarize the key reasons why you believe there is a reduction in drunk driving in states with legalized medical marijuana?


We did not examine the association between medical marijuana laws and traffic fatalities related to drunk driving, but we examined the association between these laws and overall traffic fatalities. Previous research from Anderson and colleagues suggested that reductions in traffic fatalities were mainly due to reductions in traffic fatalities in which at least 1 driver was positive for any alcohol in the blood and those with blood alcohol concentrations greater than or equal to 0.1 grams per deciliter, and also reductions in traffic fatality rates occurring on weekends, which are more likely to be alcohol-related than those on weekdays. We also found that the strongest effect was among those ages 24-44 a group usually overrepresented among licensed medical marijuana users and those driving under the influence of alcohol. One of the tricky things with this research is that states have different degrees of law enforcement and many states do not test for blood alcohol/marijuana levels in all of their fatally injured drivers, so it is hard to draw conclusions about the extent to which declines in DUI explain the overall reductions that we observed.

After you released your study, were you contacted by any third parties that were more than upset with your findings?


The overall response has been very positive, with a number of news reports about our research findings. However, yes, we got emails asking us why we had published our findings when this could increase the number of people driving under the influence of marijuana, and also others suggesting that our findings were in contrast with published reports indicating that fatalities with blood samples positive to marijuana have increased in some states.
We want to make clear that we did not present any results based on the percentage of marijuana related fatalities. As we explained in the limitations section of our manuscript “…we could not examine whether MMLs were associated with increments in the rates of traffic fatalities in which drivers tested positive for the presence of cannabis metabolites in blood. Testing procedures vary by state and our own exploration of FARS data showed that only a limited number of states tested 80% or more of their fatally injured drivers. In addition, the FARS coding system does not differentiate between active and inactive cannabinoid metabolites and, therefore, it is not possible to know whether the driver was driving under the influence of marijuana.” In this regard, a person dying in a car accident can be positive to cannabinoids because they smoked a few days before, but this is not a measure of them them driving under the influence of marijuana when killed. Half-time varies across different active and inactive cannabinoids metabolites. FARS data does not differentiate between these cannabinoids making difficult for research purposes, given that inactive metabolites may be present in blood for 30 hours or more beyond the acute intoxication period. Therefore is difficult to know if a person positive to cannabinoids was really under the influence of marijuana while driving.

Is there any difference between legalized medical marijuana and legalized marijuana (no restrictions other than age)? I know there was some research that suggested drug overdoses decrease significantly with medical marijuana legalization but the results are less clear with general recreational legalization. As more and more states start adding various legalization initiatives on their ballots I'm curious how framing and limitations impact these issues.


Although we adjusted by states legalizing marijuana for recreational purposes, we did not provide estimates for these laws given that it is too soon to know for sure the effects of recreational laws on traffic fatalities or other outcomes. There is not enough post-legislation data yet.

Was the number and severity of non-fatal injuries also reduced? Were there any other reductions in traffic incidents? (Low speed crashes etc)


We only looked at fatal injuries, but we are interested in looking into non-fatal injuries as well. There are some challenges accessing the data, because not all states provide hospital injury data that can be used for research.

Sorry for asking the obvious, but since your research we're trying to discuss is paywalled, I have no idea here.

What about crashes relative to fatalities?

How did different road users compare (ie occupant versus pedestrian versus cyclist versus motorcyclist) and how did they compare to the national trend?

What exactly did you correct for? I can think of a lot of factors off the top of my head. Increased vehicle crash safety, improvements in vehicle handling and tire technology, signage, road surfacing and design. How about versus gas prices? There's a known correlation between gas prices and traffic injuries.

Did you correct for states that don't submit data to FARS? For example, in Massachusetts, Boston Police have never contributed crash data to the state's registry of motor vehicles, skewing the state's statistics.


We made clear in the limitations section of our paper that “…we used data on traffic fatalities, the most extreme injury outcome; therefore, we cannot make any observations on the association between MMLs and nonfatal traffic injuries.” We believe further studies should examine the effects of MML on non-fatal traffic injuries, this would provide better evidence on how changes in legislation and/or marijuana use is related to non-fatal traffic events. We adjusted for 14 state level variables to rule out that other state factors were explaining our findings. Here are these variables: State-level covariates included unemployment rate and median household income, speed limits of 70 miles per hour or greater, primary seat belt laws enforcement, laws decriminalizing the possession of small amounts of marijuana, and whether states had enacted a recreational marijuana law, state-level graduated driver license laws, blood alcohol content laws (0.08 g/dL), drug per se laws, administrative license revocation laws, and laws banning cell phone use and texting while driving, separately targeting adolescents and adults. In addition, we included a measure of state annual expenditures for highway law enforcement and safety per capita (adjusted to 2000 dollars), and also a state measure of the annual vehicle miles driven per licensed driver (thousands of miles) from Highway Statistics, US Department of Transportation. Finally we included a measure of the state-level per capita ethanol sales, total ethanol of all beverages combined per population aged 21 years or older. We also tested whether same sex marriage state laws, a exposure that is expected to have no association with traffic fatalities, was associated with traffic fatality rates. Finding this, would tell you that others factors may be responsible for the association (confounding). We found that same sex marriage laws were not associated with traffic fatalities, supporting the hypothesis that the found association between medical marijuana laws and traffic fatalities was less likely due to confounding. I am not sure about the specific limitations using Boston data, but we will look into it.

How would you compare driving under the influence of weed vs driving drunk or while using your phone, or just sober? I guess it would depend on how stoned you are, but is it safer?


We believe that driving under the influence of any drug is risky and individuals should avoid driving under the influence of any drug, including prescription drugs that may impact neurocognitive and neuromotor skills. Some simulator and on-road experimental studies show a dose-dependent association between marijuana exposure and several indicators of driving impairment. Some studies show that marijuana exposure is associated with increased response time and lane weaving. Marijuana exposure has also been associated with reduced speed and greater headway, which indicates some degree of awareness of marijuana-related impairment and a tendency to compensate. Different studies have also shown that drunk-driving is associated with increased speed and risk taking behavior while driving. Texting and driving have been shown to increase the risk of traffic accidents in other studies.

Your paper is not open access. Universities pay hundreds of millions of USD every year for journal subscription. It is a waste of the taxpayers' money, and many universities cannot afford such high journal subscription fees, which means this hampers the work of many researchers.

Why do you publish there, and don't you think it is ironic to talk about public health in a venue that is not publicly available?


Most health journal are not open access, we are sorry, but if by any chance you have access to university library you can probably download the full article there. Also if you contact us via email and we will be glad to share a copy of it.

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