The head of the Office of National Drug Control Policy, drug czar Gil Kerlikowske, is pushing a campaign targeting drugged driving and has singled out marijuana as a main problem. But if the latest research findings on stoned driving are any indication, the drug czar may want to shift his emphasis if he wants to (as he claims) let policy be driven by evidence.

According to clinical trial data published in the March issue of the Journal of Psychoactive Drugs, subjects tested both before and after smoking marijuana exhibited virtually identical driving skills in a battery of driving simulator tests. Researchers in the double-blind, placebo-controlled trial tested 85 subjects -- 50 men and 35 women -- on simulated driving performance. The subjects had to respond to simulations of various events associated with vehicle crash risk, such as deciding whether to stop or go through a changing traffic light, avoiding a driver entering an intersection illegally, and responding to the presence of emergency vehicles. Subjects were tested sober and again a half hour after having smoked a single medium-potency (2.9% THC) joint or a placebo.

The investigators found that the subjects' performance before and after getting stoned was virtually identical. "No differences were found during the baseline driving segment (and the) collision avoidance scenarios," the authors reported. Nor were there any differences between the way men and women responded.

Researchers did note one difference. "Participants receiving active marijuana decreased their speed more so than those receiving placebo cigarettes during (the) distracted section of the drive," they wrote. The authors speculated that the subjects may have slowed down to compensate for perceived impairment. "[N]o other changes in driving performance were found," researchers concluded.

Past research on marijuana use and driving has yielded similar results as well, including a 2008 driving simulator clinical trial conducted in Israel and published in Accident, Analysis, and Prevention. That trial compared the performance of drivers after they had ingested either alcohol or marijuana. "Average speed was the most sensitive driving performance variable affected by both THC and alcohol but with an opposite effect," the investigators reported. "Smoking THC cigarettes caused drivers to drive slower in a dose-dependent manner, while alcohol caused drivers to drive significantly faster than in 'control' conditions."

Something to keep in mind when lawmakers in your state start pushing for zero-tolerance "per se" Driving Under the Influence of Drugs laws that want to label people impaired drivers because of the presence of a few metabolites left over from last week.