Researchers from the University of Iowa’s (UI’s) National Advanced Driving Simulator (NADS) have published their new study that analyzed the effects that inhaling cannabis has on driving. The results may be able to help law enforcement shape rules on how to handle drivers who are 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.

Gary Gaffney, Gary Milavetz, and Tim Brown conducted the study by putting 18 participants through a simulated driving test that lasted 35-45 minutes. One group was under the influence of alcohol while another under vaporized cannabis, and a third under the influence of both. The National Highway Traffic Safety Administration, the Office of National Drug Control Policy, and the National Institute of Drug Abuse all sponsored this study.

There were 4 important findings that were reported.

When compared to those under the influence of alcohol or both substances, those who were only under the influence of cannabis showed little driving impairment.

A blood concentration of 13.1 ug/L THC (delta-9-tetrahydrocannabinol, the active ingredient in cannabis) in drivers showed a similar impairment to those with a .08 breath alcohol concentration. The current legal limit for THC in Colorado and Washington is 5 ug/L.
For those drivers who used cannabis and alcohol together tended to weave more, on the virtual roadway, than those who used either substance independently. Consuming both, however, does not double the impairment.

Though it should not be considered a viable measure of impairment, it is possible to analyze a driver’s oral fluids to detect the recent use of cannabis.

Researchers selected 5 women and 13 men, between the ages of 21 and 37 who reported using marijuana and drinking alcohol no more than 3 times a week. They were made to spend the night at the UI’s facilities so that sobriety could be ensured, they were then taken to NADS for their ‘dosing’. The simulated drive followed in a 1996 Malibu sedan that was mounted in a 24-foot diameter dome.

Each participant had 10 minutes to drink an alcoholic beverage or a juice that had a flavoring that of alcohol, and another 10 minutes in which to inhale a vaporized cannabis or placebo. Their goal was to have participants with blood concentrations around 13.1 ug/L THC, and some with a blood alcohol level around .065%, and other participants under the influence of both. While in the simulator drivers were assessed on: weaving within the lane, the speed at which they weaved, and how often the car weaved out of the lane. It was reported that drivers under the influence of alcohol only revealed impairment in all three areas while those under the influence of cannabis only showed impairment with weaving within the lane.

Andrew Spurgin, a postdoctoral researcher with the UI College of Pharmacy, offered another important fact paired with 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.”

Though these findings will most likely not have an immediate effect on current legal limits for THC, they may stem the attempts of deploying devices for instant roadside THC testing before any further research may be conducted.