Driving Under The Influence Of Marijuana And Alcohol Impairs Driving Significantly, But Blame The Alcohol

Marijuana
Hoping to enlighten policymakers, researchers studied how marijuana and alcohol affect driving when used together and individually. Photo courtesy of Shutterstock

Among the many concerns opponents of marijuana legalization have, one of them is whether more people, under the influence of marijuana, will be involved in fatal car crashes. Just as concerning is whether more people will think it’s OK to drive under the influence of both marijuana and alcohol together. What effect do these substances have on driving when used together and on their own? A new study sought to find out.

The study comes out of the University of Iowa, which houses the National Advanced Driving Simulator (NADS) — once the world’s largest driving simulator, it comes complete with a full-sized 1996 Chevy Malibu sedan. Inside the simulator, researchers tested how well a small group of people drove under the influence of alcohol and marijuana, both together and individually. “What we saw was an additive effect… when we put them together,” said Tim Brown, associate research scientist at NADS and corresponding author of the study. “You get what you expect if you take alcohol and cannabis, and merge them together.”  

The study involved 18 participants who drove in the simulator under the influence. When the two drugs were combined, the researchers found participants' abilities to stay in their lanes dropped — they weaved more. Although, as Brown said, the effects were additive. When used individually, the researchers found marijuana use only led to increased weaving within the drivers’ lanes, which was comparable to the weaving of someone with a 0.08 blood-alcohol content. By comparison, after drinking alcohol, the drivers were not only weaving within their lanes but also leaving their lanes. The speed in which they wove was also more erratic.

Each of the participants was given either a mixed drink with alcohol or juice with an alcohol-rimmed glass and topped with alcohol to mimic the taste and odor. They used a vaporizer to consume the marijuana or a placebo. These methods ensured the participants had a BAC around 0.65 and THC (the psychoactive chemical in marijuana) blood concentrations between 8.2 and 13.1 micrograms per liter.  

The current study, though small, builds on others that have suggested drunk driving could be more dangerous than driving while high — though neither is safe — because it tests driving skills while participants were surely under the influence. With the other studies, drivers were tested for drugs following crashes. However, as the authors noted, while a person who is drunk will test positive for alcohol in their system, a person who tests positive for marijuana may not be high at the exact moment. THC can be found in the body for weeks after its effects have worn off.

These are especially important points to make as policymakers craft laws to regulate marijuana use in states where it’s already legal, as well as states that are considering legalization. While states that have legalized medical marijuana have reported increases in fatal crashes involving drivers who tested positive for marijuana — Colorado has too — it’s possible that the drug was still in their system from recent use rather than from using it immediately before getting behind the wheel. After all, traffic fatalities were nearly identical in 2013 and 2014, the years before and after recreational pot began being sold. 

Source: Hartman R, Brown T, Milavetz G, et al. Cannabis Effects on Driving Lateral Control With and Without Alcohol. Drug and Alcohol Dependence. 2015. 

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