A rigorous federal research study by the National Institute on Drug Abuse offers new data on the effects of marijuana on driving performance.
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.
It is illegal in all states to drive under the influence of anything, but years of work went in to establishing the .08 breathe alcohol limit that exists in most states. The question is whether we can establish a similar threshold for pot.
To find out, the study recruited 18 occasional cannabis smokers, 13 of them men, between the ages 21 and 37. The participants took six 45-minute drives in a driving simulator—a 1996 Malibu sedan mounted in a 7.3 diameter dome—at the University of Iowa. Each drive tested a different combination of high or low concentration THC, alcohol, and placebos (to create a placebo, participants were given fruit juice with alcohol swabbed in the rim, topped of with 1ml alcohol, to mimic alcohol’s smell and taste).