Jacob Sullum summarizes a recent experiment (seen above) by a Seattle TV station:
The station enlisted three volunteers: Addy Norton, “a 27-year-old medical marijuana patient and heavy daily marijuana user who smoked pot before arriving at the test site”; Dylan Evans, a 34-year-old weekend pot smoker; and Jeff Underberg, 56-year-old who smokes pot occasionally. All three of them satisfactorily completed a driving course at THC levels far above the legal limit.
Norton arrived with a THC level of 16 nanograms, more than three times the DUID cutoff, but nevertheless drove fine, according to the driving instructor who accompanied her with his foot hovering over a second brake and his hand ready to take the wheel. After Norton smoked three-tenths of a gram, she tested at 36.7 nanograms, more than seven times the legal limit, but still drove OK. Even after she consumed nine-tenths of a gram, a “drug recognition expert” from the Thurston County Sheriff’s Office said her driving was merely “borderline.” Only after consuming a total of 1.4 grams of pot and achieving a THC level of 58.8 nanograms, almost 12 times the legal limit, was Norton clearly too stoned to drive.
Josh Harkinson looks at more research on the subject:
While booze can make people drive faster and more aggressively, marijuana has the opposite effect: Pot smokers, studies show, tend to compensate for their impairment by slowing down and leaving larger gaps between themselves and other cars. Still, Ramaekers cautions against thinking that stoners acting like Sunday drivers are safer. Marijuana users may “try to create their own box of safety, and within that world they can operate fine,” he says. “But there’s a lot of other information outside of that box that they can’t process, and that is a problem.”
Road tests and driving simulator studies have found that the more weed drivers inhale, the worse they do at essentials such as staying in their lanes, responding to sudden hazards (like a dog running into the street), and multitasking—for example, reading street signs on a twisty road while avoiding oncoming traffic. On average, drivers with blood THC levels equal to or in excess of 5 ng/ml cause crashes at 2.7 to 6.6 times the rate of sober drivers, and getting into the driver’s seat less than an hour after smoking a joint nearly doubles your risk of getting into a crash.