Don’t Drive Stoned And Drunk, Ctd

Sullum says Washington’s blood-THC limit effectively prohibits medical marijuana users from driving at all:

Washington’s five-nanogram rule, modeled after the per se standard for alcohol, was meant to reassure voters worried about the threat posed by stoned drivers. But like all per se standards, it treats some people as unsafe to drive even when they’re not.

Last year experiments by KIRO, the CBS station in Seattle, and KDVR, the Fox affiliate in Denver, showed that regular cannabis consumers can perform competently on driving courses and simulators at THC levels far above five nanograms. The lack of correspondence between the new standard and impairment is especially unfair to medical marijuana users, some of whom may be above the five-nanogram limit all the time, meaning they are never legally allowed to drive in Washington. …

“The five-nanogram rule doesn’t make sense,” says Mark Kleiman, a University of California at Los Angeles drug policy expert who was hired to advise Washington’s cannabis regulators. “It doesn’t correspond to impairment, and for regular users, they’re always going to be over the limit. It would be absurd to say you can smoke pot but then you can never drive.”

Along with Kleiman, Amy Weiss-Meyer reviews research on driving under the influence:

In 2000, researchers at Maastricht University in the Netherlands gave driving tests to subjects who had consumed various amounts of alcohol and/or marijuana. While all subjects both drank and smoked in each round of the study, some were given placebos, so that the researchers were able to test the effects of each substance on its own as well as their combined effect. They measured drivers’ “standard deviation of lateral position” (SDLP), or the distance they drifted out of their lane, and also the time out of lane (TOL).

The study found that alcohol on its own increased SDLP by 2.2 centimeters (as compared to double-placebo conditions). Marijuana, depending on the dosage of THC (100 or 200 micrograms per kilogram of body weight), increased SDLP from placebo conditions by 2.7 and 3.5 centimeters respectively. In other words, drivers who had smoked pot were less able to drive in a straight line than drivers with an elevated BAC. (Most drivers’ BACs fluctuated around 0.04 grams per deciliter, below the legal limit of 0.08.)

The researchers concluded that the percentage of TOL was not significantly affected by either alcohol or marijuana alone, but that it was much higher when both substances were used together.

Abby Haglage joins the conversation:

The truth is, after decades of analysis, we still don’t have a firm grasp on how THC impairs driving. Laboratory studies have confirmed that THC (officially, Delta-9-tetrahydrocannabinol) impairs many motor skills necessary for driving. But actual driving simulation studies have not mimicked these results. One sound example is a 2004 study in which three researchers found THC to inhibit attention, reaction time, hand-eye coordination, short-term memory, time and distance perception, and concentration.

But when tested in actual driving simulation, the authors found the results did not “replicate” their laboratory evidence. In other words, researchers were able to prove that THC should, technically, impair driving, but not that it does. Their explanation for the discrepancy: Drivers with THC are likely cognizant of their impairment and are thus able to “compensate…by driving more slowly and avoiding risky driving maneuvers.”