State laws currently criminalize driving a vehicle while "under the influence" of marijuana, just as they do for alcohol. And as I’ve commented in past posts, the evidence that marijuana usage impairs the ability to safely operate a motor vehicle — that is, "drunk driving" — is essentially non-existent. See Does Marijuana Affect Driving Ability?
A recent study, in fact, has confirmed this.
First of Its Kind Study Finds Virtually No Driving Impairment Under the Influence of Marijuana
The first study to analyze the effects of cannabis on driving performance found that it caused almost no impairment. The impairment that it did cause was similar to that observed under the influence of a legal alcohol limit.
Researchers at the University of Iowa’s National Advanced Driving Simulator carried out the study, sponsored by National Highway Traffic Safety Administration, National Institute of Drug Abuse, and the Office of National Drug Control Policy.
“Once in the simulator—a 1996 Malibu sedan mounted in a 24-feet diameter dome—the drivers were assessed on weaving within the lane, how often the car left the lane, and the speed of the weaving. Drivers with only alcohol in their systems showed impairment in all three areas while those strictly under the influence of vaporized cannabis only demonstrated problems weaving within the lane. Drivers with blood concentrations of 13.1 ug/L THC, or delta-9-tetrahydrocannabinol, the active ingredient in marijuana, showed increased weaving that was similar to those with a .08 breath alcohol concentration, the legal limit in most states."
The article went on to point out that there is no accurate way of determining impairment by chemical testing (as I’ve pointed out in previous posts, such as California Proposes New Law to Allow Roadside Marijuana Test).
Another important finding should deter any attempts to deploy instant roadside tests for THC-blood levels.
The study also found that analyzing a driver’s oral fluids can detect recent use of marijuana but is not a reliable measure of impairment.
“Everyone wants a Breathalyzer which works for alcohol because alcohol is metabolized in the lungs,” says Andrew Spurgin, a postdoctoral research fellow with the UI College of Pharmacy. “But for cannabis this isn’t as simple due to THC’s metabolic and chemical properties.”