This makes it clear: stoned driving is indeed dangerous
May 4, 2014, 3:36 pm
This investigative report from KIRO-7 Eyewitness News in Seattle is a surprisingly good illustration of the effects of marijuana smoking on driving skills. It would have been even better if the piece was more clear on the concept of a “legal limit.”
There are some people on the internet and in print arguing that driving under the influence of THC is no big deal, and that the effects on driving skills are minimal at most. This is just wrong, and goes against both common sense and medical evidence. A recent literature review in the BMJ showed that driving under the influence of cannabis significantly increased the risk of non-fatal and — especially — fatal motor vehicle collisions. Cannabis impairs many key driving skills, including reaction time, divided-attention tasks, and cognition. This impairment is clearly seen in the KIRO report.
As the reporter points out, the subjects in his report are driving on a closed course, apparently with no surprise hazards requiring quick reaction. I’ve seen people who are stoned on THC who have trouble sussing out their relationship to their own hand, let alone two-ton piles of metal bearing down on them with a relative speed of 130 mph.
As for the term “legal limit,” it is a legal and political concept rather than a medical one. In most states, this is set at a blood alcohol level of 0.08 gm/dl (80 mg/dl). This does not mean that at levels below this a person is perfectly okay to drive or can’t be charged by the state with driving under the influence. It merely means that at levels above .08 a person is presumed to be intoxicated, and that the prosecution does not have to present additional evidence demonstrating specific impairment.
In fact, many of us think the legal limit for alcohol is set way too high. The medical literature that even at levels of .03 or even lower, alcohol affects vision and reaction time, as well as other functions crucial for safe driving. However, the legal limit is set by legislators and voters, not scientists or physicians.
Part V of the Washington Initiative 502 on marijuana reform set the legal limit for cannabis at 5 ng/ml Δ-9 THC. Some argued that this limit was too low and would turn legitimate medical marijuana users into criminals. However, as pointed out in a recent review, even a negative blood test for THC does not rule-out cannabis-induced impairment. It’s possible that further scientific evidence will prompt revision one way or the other of the 5 ng/ml level.