The first test simulated an "uneventful" drive. The second test had the subjects answering math problems while driving. The third test introduced cars coming out of intersections, a changing traffic light and a dog running into the road. Researchers found "no significant difference" between the subjects who smoked marijuana and those who smoked a placebo in the collision avoidance simulation. Researchers noted that those who smoked marijuana drove at a slower speed that those who didn't during the distracted driving simulation.
It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects. Concentrations of parent drug and metabolite are very dependent on pattern of use as well as dose...It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH concentrations. -- NHTSA Fact SheetResearchers were quick to point out that they did not believe driving while high was safe because drivers who smoked marijuana may also have consumed alcohol and would be distracted by the radio and cell phones. Of course most of us are distracted while we drive because we are listening to the radio or to music or talking or texting on cell phones or interacting with the people in our cars.
The question, as always, is whether or not the driver lost the normal use of his mental or physical faculties because he was smoking marijuana -- and that is something that looks increasingly hard to prove.
NHTSA Drugs and Human Performance Fact Sheets: Marijuana