Medical marijuana prevents road traffic deaths [indirectly].

A friend who is a Justice of the Peace tells me that in 20 years sitting in court he has never once had to deal with a driver who has caused an accident under the influence of marijuana. He sentences several drunk drivers a week. Of course, this is just anecdotal evidence that drugged drivers aren’t as dangerous as drunk drivers but now research has shown compelling if indirect statistical proof of this fact. A recently published study of road traffic fatalities in US has found that Medical #Marijuana saves lives by reducing drunk driving fatalities.

The study was conducted by two economists,  D. Mark Anderson (Montana State University) and Daniel Rees (University of Colorado at Denver) both from states that have medical marijuana legislation. They studied data on traffic fatalities in 2010 provided by the Center for Disease Control and Prevention. They found that road traffic fatalities declined by an average of 9 percent in the 13 states that had previously legalised medical marijuana. A substantial effect comparable in size to raising the drinking age to 21. They thought that the reasons for this were severalfold but all related to a likely substitution of marijuana for alcohol among a substantial proportion of recreational users combined with the fact that drivers under the influence of  marijuana are less dangerous:

Laboratory studies have shown that cannabis use impairs driving-related functions such as distance perception, reaction time, and hand-eye coordination (Kelly et al. 2004; Sewell et al. 2009). However, neither simulator nor driving-course studies provide consistent evidence that these impairments to driving-related functions lead to an increased risk of collision (Kelly et al. 2004; Sewell et al. 2009). Drivers under the influence of marijuana reduce their velocity, avoid  risky maneuvers, and increase their “following distances,” suggesting compensatory behavior (Kelly et al. 2004; Sewell et al. 2009). In addition, there appears to be an important learning-bydoing component to driving under the influence of marijuana: experienced users show  substantially less functional impairment than infrequent users (Sutton 1983).
Like marijuana, alcohol impairs driving-related functions such as reaction time and handeye coordination (Kelly et al. 2004; Sewell et al. 2009). Moreover, there is unequivocal evidence from simulator and driving-course studies that alcohol consumption leads to and increased risk of collision (Kelly et al. 2004; Sewell et al. 2009). Even at low doses, drivers  under the influence of alcohol tend to underestimate the degree to which they are impaired (MacDonald et al. 2008; Marczinski et al. 2008; Robbe and O’Hanlon 1993; Sewell et al. 2009), drive at faster speeds, and take more risks (Burian et al. 2002; Ronen et al. 2008; Sewell et al. 2009).

The data are inconclusive and the authors suggest that an alternative explanation could to be that marijuana is used more often in the home and not used so much in public. So potentially, there is less occasion for driving under the influence.

In light of this research, it seems like the US drug tsar’s pursuit  of drugged driving is at best misdirected. Moreover to insist on prosecutions based on trace-level concentrations of drugs is even more irrational and unjust. A case going through the courts at the moment illustrates the point, where a woman with minute traces of marijuana from four days prior is facing 28 years in prison for aggravated DUI. Pete Guither at describes the cases:

An Aurora woman who was reaching for her sunglasses when she set off a chain reaction crash that killed a St. Charles couple riding a motorcycle pleaded guilty Friday to aggravated DUI.
Alia Bernard, 27, admitted having marijuana in her system on May 29, 2009, when she rear-ended a stopped car on Illinois Route 47 in Kane County between Sugar Grove and Elburn.
Although tests showed Bernard had marijuana in her system, she hadn’t used the drug in the three or four days before the crash and wasn’t impaired, said her lawyer, Bruce Brandwein.
Illinois has a per se law for marijuana and driving (what the Drug Czar is pushing for the entire country). This means that any amount in the blood counts as driving impaired for enhanced sentencing.
She could get as much as 28 years.

The injustice based on government’s unscientific propaganda on drugged driving « Drug WarRant. (

In the UK, the government is currently undertaking a review of drugged driving laws. While there is no question that driving under the influence is potentially dangerous, the law needs to be very clear about exactly what it means by “influence”.  Fortunately, it seems that the British panel is going to consider that the absolute level of a drug in a person’s system is more important than mere presence or absence.

The panel will look at how such an offence could be defined as well as considering whether it is possible to set levels for the impairing effects of specific drugs. When it starts work this spring it is likely that the panel will consider whether it is possible to identify, for average members of the adult population, the levels of drugs that have an impairing effect broadly equivalent to the current blood-alcohol level.

Panel members will consider this effect for a number of drugs including cocaine, MDMA (commonly known as ecstasy), cannabis, and opiates. In cases where such levels can be identified the panel may then look at how these would vary across the population, including for habitual users of these substances.

Drug driving could become new offence (Huffpost UK)

Meanwhile, in tangentially related news, 2010 saw  “Prescription drugs surpass motor vehicle accidents as No 1 cause of accidental death in almost half US states” (Guardian)


ResearchBlogging.orgD. Mark Anderson, & Daniel I. Rees (2011). Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption IZA Discussion Paper No. 6112


ResearchBlogging.orgCenters for Disease Control and Prevention (2011). Vital signs: alcohol-impaired driving among adults–United States, 2010. MMWR. Morbidity and mortality weekly report, 60 (39), 1351-6 PMID: 21976118

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About Caspar

Caspar Addyman has a BA in mathematics, a BSc in psychology and PhD in developmental psychology. He works at the CBCD at Birkbeck, University of London. Before becoming an infantologist he spent eight years writing trading systems in the City. He lives in Brixton, Berlin and Dijon. He never drinks the same drink twice in a night and dances without spilling a drop. Twitter: @BrainStraining
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