Government Study Casts Doubt on Legal Definitions of Stoned Driving

Simulator tests also confirm that marijuana impairs drivers less than alcohol.


National Advanced Driving Simulator

A new study by researchers at the National Institute on Drug Abuse (NIDA) confirms that marijuana has a less dramatic effect on driving than alcohol does and casts further doubt on the standard that Colorado and Washington use for determining when someone is too stoned to drive. In the double-blind study, 18 occasional cannabis consumers—defined by marijuana use at least once in the previous month, but no more often than three days a week—took tests at the National Advanced Driving Simulator in Iowa City after vaping marijuana, drinking alcohol, or taking a placebo. The researchers, led by NIDA senior investigator Marilyn Huestis, found that a THC level of 13.1 nanograms per milliliter of blood had an impact on weaving within the lane (a.k.a. standard deviations of lateral position, or SDLP) similar to a blood alcohol concentration (BAC) of 0.08 percent, the cutoff used for most drivers in every state.

Notably, that THC level is more than twice as high as the five-nanogram standard established by Colorado and Washington when they legalized marijuana for recreational use. This result is especially striking because the subjects were occasional users. People who use cannabis every day, including patients who use it for medical reasons, may develop enough tolerance that they can drive safely at THC levels even higher than 13 nanograms. Yet according to Time, "Huestis believes that the 5 ug/L limit is not strict enough, particularly when you take into account those with low tolerance." That's another way of looking at it, I suppose, but the five-nanogram cutoff in practice means that many regular cannabis consumers can never legally drive, even when they're not impaired, which hardly seems sensible or fair.

Huestis and her colleagues found that alcohol and marijuana together had an additive effect on SDLP, with five nanograms of THC per milliliter plus a 0.05 percent BAC having an impact similar to a 0.08 percent BAC. In addition to SDLP, Huestis and her colleagues measured lateral acceleration (weaving speed) and lane departures per minute. Both were affected by alcohol but not by marijuana. This particular study did not consider any of the many other indicators used to measure driving impairment, although Huestis et al. plan to publish more on this subject to help illuminate the question of how best to define stoned driving.

Another result of this study that is relevant to enforcing such limits: Saliva tests accurately confirmed exposure to cannabis, but "with greater THC concentration variability than paired blood samples," which "poses challenges in concentration-based effects interpretation." In other words, saliva tests may be easier and less invasive, but they are less reliable in determining whether someone has exceeded whatever the legal threshold might be.

Also interesting, especially for people who have heard the frequent warnings from anti-pot prohibitionists about the dangers posed by increased potency: Most of the subjects, who were allowed to vape at will for 10 minutes, achieved about the same blood THC levels whether they were given weak cannabis (with a THC content of about 3 percent) or stronger cannabis (with a THC content of about 7 percent). As the researchers put it, "smokers can self-titrate cannabis dose to achieve desired pharmacological response."

Although this study provides additional evidence that marijuana poses less of a road hazard than drug warriors often claimTime health reporter Eliza Gray manages to obscure that point. "While drunk driving is on the decline in the U.S.," she writes, "driving after having smoked or otherwise consumer marijuana has become more common. According to the most recent national roadside survey from the National Highway Traffic Safety Administration of weekend nighttime drivers, 8.3 percent had some alcohol in their system and 12.6 percent tested positive for THC—up from 8.6 percent in 2007." Gray neglects to mention that a landmark NHTSA crash risk study released the same day found that, once the data were adjusted for confounding variables, cannabis consumption was not associated with an increased probability of getting into an accident.

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  1. but the five-nanogram cutoff in practice means that many regular cannabis consumers can never legally drive

    That's the point.

  2. "smokers can self-titrate cannabis dose to achieve desired pharmacological response."

    Unsurprising, considering the long standing evidence that tobacco smokers adjust to "low tar / low nicotine" cigarettes by changing their smoking habits. I remember when anti-smoking activists were skeptical of such cigarettes for exactly that reason; it seems an equal reason to assume that high-THC varieties won't make a difference.

  3. the five-nanogram cutoff in practice means that many regular cannabis consumers can never legally drive, even when they're not impaired

    Feature, not a bug. I have to worry about this at all times. Luckily I never, ever drive actually stoned, so I just have to hope that if I get pulled over the cop isn't so venal that they try to fuck me anyway. Here's...hoping?

    1. You and me both, baby. Just pray no one else causes an accident that might make them decide to test you.

      1. Does IL have a ridiculous, enshrined-in-law nanogram cutoff like this state where it is technically legal? I'm still waiting for someone who gets busted under this to challenge it, but nothing yet.

        1. IL has been a zero-tolerance state since before WA and CO legalized.

      2. Yeah. Someone could cause an accident by running a red light and hitting you, and they'd find you at fault.

        1. Pointing to how the alcohol laws are bullshit as well.

          You are not at fault just because you are drunk/stoned.

    2. You're more responsible than I am. I got pulled over (for going over the speed limit by 7 mph) once after I had just finished smoking not five minutes before. Terrifying twenty minutes, I'm telling you.

      But everything turned out better than expected--I didn't even get a ticket, just a warning.*

      *Disclaimer: I did not flash him my tits.


        Women get away with shit because they HAVE tits.

      2. I drove high once as a teen. I very slowly drove up into my friends yard, missing his driveway completely. We laughed and laughed and then I lost my keys in some shrubs.

        1. I used to drive home from college on some two lane highways before getting onto a long 5-6 hours of interstate. I had to stay straight on the two lanes or I would space out and miss the turns. Then smoke a jay once I was on the interstate. That worked reasonably well.

  4. found that a THC level of 13.1 nanograms per milliliter of blood had an impact on weaving within the lane

    Did they test the effects of driving a prius on how it relates to weaving within the lane?

    1. Paul, this may shock you, but I was on 99 the other day and there was this Prius driver who wasn't one of the worst drivers I'd ever seen. I was stunned.

      1. Careful, Epi. You'll get pulled over for stunned driving.

      2. This morning I got stuck in a long line of cars behind some dipshit in a Prius driving 5mph under the limit. Grrrrr.....

        1. Gotta hypermile bro

          1. Prius drivers tend to cruise through stop signs and run redlights....full stops kill the mileage donchaknow

            1. In my experience, most people tend to do those things. But the "hypermilers" are particularly bad.

  5. many regular cannabis consumers can never legally drive

    Good thing MJ is "legal", huh?

  6. This will be the down fall to the greatest country on the planet. too many leaches think they are entitled to other peoples earnings. ?????

  7. Again, facts don't matter. Because they want to tell you what you can and cannot put in your body. Let's see, highway that sets limits based upon real world testing, or gov't that sets limits based upon feelings, and desires to ticket or jail people. If rather choose the private road and reward them with my money. I don't smoke, nor do I even get in the car if I've had a sip of alcohol. But u value choice and rewarding good economic actors.

    A company that harassed or imprisoned, its customers, or that escalated non issues into violence wouldn't last long.

  8. The major difference (or one of them) between alcohol and weed when it comes to driving, is that when you are drunk you think you are driving better than you really are and when you are stoned, you think you are driving worse than you really are. This is based on my observations and experience, but I think it holds pretty well.

    Driving drunk is pretty fun sometimes. Which is one of several reasons I don't do that anymore. Being stoned often makes driving less fun.
    No one should drive intoxicated, but I honestly have zero worries about stoned drivers.

  9. Just a nit to pick: "Huestis believes that the 5 ug/L..." you may want to enter [sic] here since ug can stand for microgram which is 1000 nanograms.

    1. 5 ng/ml ==5 ug/l. It's correct, just confusing unit changing.

      1. d'oh!

  10. So NIDA spins their study conclusions to conform to their vested interest? Gee, didn't see that one coming ... !
    Another example of why we'd all be better off without them, and their fellow Drug War parasites at the ONDCP and DEA.

  11. Why not come up with a good test of IMPAIRMENT, not just an arbitrary level of a substance in the blood.
    No two people react the same to the same level of content in the bloodstream, whether it be alcohol, THC, or any other impairing substance.
    Develop a fool-proof roadside test, to be recorded on video, to show that the person really shouldn't be driving.

  12. I've seen this article quoted over the internet in quite a few places. I would appreciate it if the blood measurements are corrected. I'm not sure if you mean nanograms or not. Nanograms are ng, the notation in the article is micrograms ug.

  13. Top shelf medical marijuana donation text me for more info about strains, available pain relief and anxiety meds,delivery and shipping.:(415) 658-5976

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