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.

Related post:

Effects of marijuana on driving ability



  1. Justin Says:

    Like Nader’s infamous “unsafe at any speed”, why can’t people think of alcohol and drugs as “unsafe in any amount” with regards to driving?

    FWIW, the 0.08 was set because it was the concentration in which everyone is impaired, based on science, but definitely outdated. It needs to be lowered.

    I’m more concerned with prescription drugs, especially at the rate they are prescribed. Knowing what drugs are in deceased drivers makes me a little nervous while driving, as everyone should be clear headed while driving a 2-ton deathmobile.

    Thanks for writing about this.

  2. Leon Says:


    Thanks for the comment. I think that as long as alcohol and (in some states) cannabis is legal, it would be hard to push for making it illegal with any amount of the detectable drug in the blood. For example, I think it would hard to prosecute a driver and take away his or her driving privileges just because of a blood alcohol level of, say, 20 mg/dl, even if this level were associated with some impairment.

    This is a legal and political matter and not really a scientific one, although hopefully it will be based on good science.

  3. Rob Says:

    I’m only 28 years old right now, and at 18 I was smart enough to know that any drug that impairs your ability to think will inevitably impair your ability to drive. It doesn’t matter what the drug is–marijuana, alcohol, pain medication–if it changes the way your mind works and how you feel, you should stay off the roadways ESPECIALLY if you’re an inexperienced driver to begin with.

  4. Leon Says:


    I couldn’t agree with you more. Those who maintain that a person can drive safely while stoned are dangerously wrong.

  5. MJ Says:

    I would point out that all subjects were aware that they should not be driving and that they would rather not drive after they had smoked 900mg of THC. I would also point out that most studies in this arena test subjects at 100-300mg THC. 900-1400mg was too much, and despite being stoned, they were aware that it was too much. On the other hand, if states are going to suggest through legislation that it is okay to treat medical symptoms with smoked marijuana, they should have an idea of what dose is going to be required to achieve symptom relief, and if that dose is upwards of 900mg, maybe there should be a recommendation on driving. Just saying’.

  6. Leon Says:


    Excellent point! Unfortunately, some folks are arguing that driving stoned is not a hazard, given that THC seems to be safer than alcohol and to have the effect of making drivers slow down. Of course, it is virtually impossible to titrate the actual dose of THC with any precision when smoking marijuana, even it obtained through a medical prescription. The big question will be where to set the “legal limit” for presumed intoxication, a term some toxicologist have problems with. I think it will end up being an essential construct.