How Much Does Marijuana Use Impair Driving?


We’ll soon find out… at least according the federal government. The National Highway Traffic Safety Administration and the National Institute on Drug Abuse are teaming up to conduct the first federally funded study on the effects of marijuana on driving.

It goes without saying that alcohol impairs driving ability. There are countless studies demonstrating this. But the evidence on how marijuana use impairs driving ability is less established. While there are numerous studies on the effects of marijuana on the brain, there are few studies on how the effects might influence driving skills.

A 2009 National Institutes of Health report stated, “Surprisingly, given the alarming result of cognitive studies, most marijuana-intoxicated drivers show only modest impairments on actual road tests.” Some suggest that reason for this is that the effects of marijuana cause the user to be more cautious when driving. This, in turn, results in less risk-taking, slower driving, and avoidance of difficult driving tasks. In fact, one study from the Institute for Human Psychopharmacology in the Netherlands noted, “Evidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments. Another way THC seems to differ qualitatively from many other drugs is that the former’s users seem better able to compensate for its adverse effects while driving under the influence.”

The current study to be conducted by the National Highway Traffic Safety Administration and the National Institute on Drug Abuse will take place at the University of Iowa’s National Advanced Driving Simulator, which is said to be the most advanced driving simulator in the world. Twenty volunteers will be “dosed” cannabis coming from the only federally funded laboratory allowed to grow marijuana for medical and research purposes at the University of Mississippi.

Scientists will then monitor the participants’ driving ability in different driving scenarios such as when a driver is confronted with an unexpected event that may require them to abruptly apply the brakes or swerve out of the way of something. Because cannabis also affects a person’s sustained attention, the researchers will also monitor the participants’ ability to drive for long periods of time though otherwise boring and uninteresting stretches of road.

If this study confirms previous studies, it will be interesting to see how law makers in zero-tolerance states address the contradiction of not allowing you to have any THC in your system when driving with having a legal alcohol limit when driving.

The study is set to end on November 14, 2012 and you can be sure that I’ll be posting about the results as soon as they’re made available.

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