Marijuana Law, Policy & Reform

Editor: Douglas A. Berman
Moritz College of Law

Tuesday, June 23, 2015

"How Much Does Marijuana Impact Your Driving?"

The question in the title of this post is the headline of this new Time article, which includes these excerpts:

The exact impact of marijuana on driving ability is a controversial subject—and it’s become more important states continue to loosen their drug laws. And, while drunk driving is on the decline in the U.S., driving after having smoked or otherwise consumer marijuana has become more common. According to the most recent national roadside survey from the National Highway Traffic Safety Administration of weekend nighttime drivers, 8.3 percent had some alcohol in their system and 12.6 percent tested positive for THC—up from 8.6 percent in 2007....

[In a recent federal study], researchers looked at 250 parameters of driving ability, but this paper focused on three in particular: weaving within the lane, the number of times the car left the lane, and the speed of the weaving. While alcohol had an effect on the number of times the car left the lane and the speed of the weaving, marijuana did not. Marijuana did show an increase in weaving. Drivers with blood concentrations of 13.1 ug/L THC, the psychoactive ingredient in cannabis, showed increase weaving that was similar to those with a .08 breath alcohol concentration, the legal limit in most states. For reference, 13.1 ug/L THC is more than twice the 5 ug/L numeric limit in Washington and Colorado....

The study also found that pot and alcohol have more of an impact on driving when used together. Drivers who used both weaved within lanes, even if their blood THC and alcohol concentrations were below the threshold for impairment taken on their own.... Smoking pot while drinking a little alcohol also increased THC’s absorption, making the high more intense. Similarly, THC delayed the peak of alcohol impairment, meaning that it tended to take longer for someone using both to feel drunk. Such data is important to educate the public about pot’s effects before they get on the road.

“I think this has added really good knowledge from a well-designed study to add to the current debate,” on marijuana’s effects on road safety, says Dr. Marilyn Huestis, the principal investigator in the study, which was conducted by researchers at the National Institute on Drug Abuse.

https://lawprofessors.typepad.com/marijuana_law/2015/06/how-much-does-marijuana-impact-your-driving.html

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