Researchers at the University of Iowa’s (UI’s) National Advanced Driving Simulator (NADS) published a new study that examined how inhaling cannabis affects driving. The results may help shape the rules of how law enforcement handles drivers under the influence of cannabis.
“Alcohol is the most common drug present in the system in roadside stops by police; cannabis is the next most common, and cannabis is often paired with alcohol below the legal limits.
We know alcohol is an issue, but is cannabis an issue or is cannabis an issue when paired with alcohol? We tried to find out.” ~ Tim Brown, associate research scientist at NADS and co-author of the study.
The new study, conducted by Gary Gaffney, Tim Brown and Gary Milavetz, put 18 participants through a 35 to 45 minute simulated driving test, with one group having consumed alcohol, another having vaporized cannabis, and a third group under the influence of both alcohol and cannabis. The effort was sponsored by the National Highway Traffic Safety Administration, National Institute of Drug Abuse, and the Office of National Drug Control Policy.
Four important findings were reported:
- Drivers under the influence of only cannabis showed little driving impairment when compared to drivers under the influence of alcohol or both substances.
- Drivers with blood concentrations of 13.1 ug/L THC (delta-9-tetrahydrocannabinol, which is the active ingredient in cannabis) showed similar impairment to drivers with a .08 breath alcohol concentration, the legal limit in most states. The current legal limit for THC in Washington and Colorado is 5 ug/L.
- Drivers who use alcohol and cannabis together weave more on a virtual roadway than ones that used either substance independently, although consuming both does not double the impairment.
- Analyzing a driver’s oral fluids can detect recent use of cannabis although it should not be considered a reliable measure of impairment.
For the study, researchers selected 13 men and five women between the ages of 21 and 37 who reported drinking alcohol and using marijuana no more than three times a week. After spending the night at the UI’s facilities to ensure sobriety, participants were taken to NADS for “dosing” followed by a simulated drive in a 1996 Malibu sedan mounted in a 24-feet diameter dome.
Before the simulation test, each participant had 10 minutes to drink an alcoholic beverage, or a juice with flavoring that mimicked alcohol, and then another 10 minutes to inhale a placebo or vaporized cannabis. The goal was to have some participants’ blood alcohol level at about 0.065 percent, and some participants’ blood concentrations at about 13.1 ug/L THC, and some under the influence of both.
Once in the simulator, drivers were assessed on: weaving within the lane; how often the car left the lane; and the speed of weaving. The researchers reported that drivers with only alcohol in their systems showed impairment in all three areas. They reported that participants only under the influence of cannabis showed impairment only with weaving within the lane.
Andrew Spurgin, a postdoctoral research fellow with the UI College of Pharmacy, shared another important fact as part of the study:
“Everyone wants a Breathalyzer which works for alcohol because alcohol is metabolized in the lungs. But for cannabis this isn’t as simple due to THC’s metabolic and chemical properties.”
The study’s finding are not likely to have any immediate effect on the current legal limits for THC, but hopefully it will slow the attempts to deploy devices for instant roadside THC testing before further research can be conducted.
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