Marijuana

AAA Finds No Basis for Equating THC Blood Levels With Driver Impairment

Traffic safety trends in Washington after marijuana legalization are ambiguous.

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AAA Foundation for Traffic Safety

According to a new report from the AAA Foundation for Traffic Safety, the percentage of drivers in fatal collisions who tested positive for THC doubled in Washington after that state legalized marijuana. But other reports published the same day by the same organization cast doubt on the significance of that finding, underlining the perils of equating THC in the blood with impairment.

In 2014, the first year that marijuana was legally sold for recreational use in Washington, 17 percent of drivers in fatal crashes tested positive for THC, up from 8.3 percent in 2013, when recreational marijuana was legal to possess but not to grow or sell. "The proportion of drivers positive for THC was generally flat before and immediately after Initiative 502 [Washington's legalization measure] took effect," the AAA report says, "but began increasing significantly…approximately 9 months after Initiative 502 took effect. It was not clear whether this increasing trend was attributable to Initiative 502 or to other factors that were beyond the scope of the study."

The report also notes that "results of this study do not indicate that drivers with detectable THC in their blood at the time of the crash were necessarily impaired by THC or that they were at fault for the crash," since "the data available cannot be used to assess whether a given driver was actually impaired, and examination of fault in individual crashes was beyond the scope of this study." The increase in drivers testing positive for THC may reflect an increase in marijuana use, but it does not necessarily indicate an increase in the number of dangerously stoned drivers on the road.

Another AAA study further muddies the picture. It found that the share of drivers involved in accidents (both fatal and nonfatal) or arrested on suspicion of driving under the influence (DUI) who tested positive for THC rose from 20 percent in 2005 to 30 percent in 2014. But this upward trend, which may be related to an increase in the number of Washington State Patrol troopers trained to recognize drug-impaired drivers, slowed after passage of I-502, contrary to what you would expect if legalization led to more stoned driving. In both this study and the one focusing on fatal accidents, drivers who tested positive for THC typically also tested positive for alcohol or other drugs. That was true for 66 percent of the THC-positive drivers in fatal crashes and 73 percent of the THC-positive drivers who were arrested or involved in any sort of collision.

The report notes that the median time between a DUI arrest or collision and obtaining a blood sample was more than two and a half hours. That lag, which in some cases might have been long enough for THC to fall below detectable levels, suggests that the prevalence of "THC-involved driving" may be underestimated. The researchers say "evaluating the impact of protracted time until blood testing is complicated by the lack of available standardized law enforcement data on the time of testing."

A third AAA study focuses on the distinction between "THC-involved driving" and THC-impaired driving, finding no clear relationship between THC blood levels in DUI arrestees and performance on roadside sobriety tests (the walk-and-turn test, one-leg-stand test, and finger-to-nose test). THC-positive drivers were much more likely to fail the tests than a group of drug-free controls (although even the latter group had substantial failure rates, ranging from 33 percent to 51 percent, which makes you wonder how accurate these tests are as measures of impairment). But the amount of THC in drivers' blood was not correlated with their test performance. "There was no correlation between blood THC concentration and scores on the individual indicators," the report says, "and performance on the indicators could not reliably assign a subject to the high or low blood THC categories." In short, "there is no evidence from the data collected…that any objective threshold exists that established impairment."

The implications for states contemplating a per se DUI standard similar to Washington's (which equates a THC blood concentration of five nanograms per milliliter with impairment) are clear: "Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported." That conclusion is similar to the position taken by the National Highway Traffic Safety Administration (NHTSA). "Whereas the impairment effects for various concentration levels of alcohol in the blood or breath are well understood," NHTSA says, "there is little evidence available to link concentrations of other drugs to driver performance."

The lack of a scientific foundation for defining impairment based on THC blood levels has not stopped legislators from adopting or considering such standards as a way to create the impression that they are doing something about the threat posed by stoned driving (which is real but pales beside the threat posed by drunk driving). Such rules are both underinclusive, since some people may be dangerously impaired at THC levels below the cutoff, and overinclusive, since some regular cannabis consumers are perfectly capable of driving safely at THC levels above the cutoff.

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  1. The lack of a scientific foundation for defining impairment based on THC blood levels has not stopped legislators from adopting or considering such standards as a way to create the impression that they are doing something…

    An information vacuum isn’t required for lawmakers to craft their wares badly, but it doesn’t help.

  2. Dude, that’s totally chill.

  3. OT: word on the street is that the Kochs are contemplating endorsing Hillary.

    The Hillarity! Can you imagine the teeth gnashing & head assploding that’s gonna take place among progs (who are already lukewarm on her)???

    Popcorn’s a-poppin’.

    1. Are you kidding?!?

      They will be welcomed into the fold as one welcomes the prodigal son!

      1. I doubt that. They are still EVUL POLLUTERS WHO DESTROY MOTHER GAIA!

        1. They should endorse Bernie…

      2. She already ran to the nearest microphone to announce she wouldn’t accept their endorsement.

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  5. AAA is just one of those crazy special interest groups.

  6. which makes you wonder how accurate these tests are as measures of impairment

    I’m not capable of reciting the alphabet in reverse without significant effort while sober. Can anyone? Who hasn’t made it some weird goal in their life to practice it? The alphabet song goes one way, god dammit.

    1. I can, while drunk or stoned or both, recite the alphabet forwards, backwards, phonetically, and phonetically backwards. That is a skill I mastered while in the military, as I was always drunk (or stoned) and wished to avoid captain’s mast or court-martial.

    2. People do. It’s not that hard.

  7. A stolen car was going 80mph down the street in front of my house this weekend. Losing control, the vehicle wrecked into a wall. I watched one of the occupants running down the street, dodging traffic with two cops in pursuit. I didn’t witness the actual crash, but came upon it moments after. Talking to witnesses, when the vehicle doors swung open, a cloud of marijuana smoke was seen (and smelled) billowing from the vehicle.

    FACT: Marijuana use causes mayhem on our once peaceful streets.

    1. Wait a minute! Was this post copied over from the script of “Reefer Madness”?

    2. Impaired driving should be condemned and punished, but I doubt this accident was mainly due to pot impairment. Quite often, these people have extremely high Blood Alcohol that range from 2-3 times the legal limits, meaning the alcohol alone increases the risk of a fatal crash between 4 times(.08 BAC) and 25 times(.20 BAC), and the pot has little or no extra risk.

      That’s why the Virginia Beach study by National Highway Traffic Safety Administration showed “No statistically significant interaction effect” for extra risk of accident from Cannabis.

      The effects of THC dissipate with a half-life of about 30 minutes, so 75% of the effects of THC can diminish in the hour spent looking for keys and sampling munchies before getting behind the wheel of a car.

      I’ll not be one who claims Cannabis is harmless. Certainly, pot can cause impairment, but its normally much less risky than alcohol, or talking on the cell phone for that matter.

  8. The dumbest article you’ll read this week.

    Long considered nothing more than an unpleasant part of growing up, bullying is now attracting attention as a major public-health problem with long-term effects ? on the same scale as youth concussions.

    http://www.seattletimes.com/ed…..anel-says/

  9. Smoking a joint should be mandatory before entering the Dan Ryan at rush hour.

    Eliminates road rage.

    Everyone is a little more cautious.

  10. I’m going to take a guess and say that probably something close to 1 in 6 of all drivers in WA have recently used marijuana.

    1. Fun fact, there are now people in WA claiming they have “deadly allergies” to marijuana and therefore cannot even come in contact with people who have used it (in one example, a hair salon). This is logical because before marijuana was legalized, they never came in contact with anyone who ever used it. Ever. Not once.

      1. Hmm. Hair stylists using marijuana can kill their customers. Licenses must be denied to those that test for marijuana usage. And licensing must now required testing. And testing must be paid for. And the cost of testing should really come from licensing fees…plus a bit of a markup to handle the cost of government processing (of course).

  11. No one seems to know what time it is out there.

    http://www.Complete-Privacy.tk

  12. The report notes that it took an average of about two hours for police to obtain blood samples after an accident or a DUI arrest. That lag, which in some cases might have been long enough for THC to fall below detectable levels, suggests that the prevalence of “THC-involved driving” may be underestimated

    If the THC in your system was low enough to have become undetectable in the two hours between arrest and testing, you haven’t smoked pot in about a month, and are thus unlikely to still be impaired by it.

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  16. There is no scientific test that measures impaired judgement, but the accident by the driver demands that his judgement be questioned.

  17. The statement that lag time between LE “contact” and the blood draw for testing is a factor when it is in the range of a few hours is uttelry false. THC can remain, in measurable levels, in one’s blood for a week or more. Thus I can be at a friend’s house Friday night, do a doobie, stay over, spend the day there on Sat, then be in a wreck on Monday, not haiving used any more since Fri night, and have a positive observable level after the crash on Monday. The immediate effects of the THC ingestion Friday night were gone well before morning Saturday, and certainly gone two days later.

    The Washington Legislature head tesitmony concerning this, but made their stupid and unjust law anyway.

    And no, I do not use any form of marijuana, so I’m not stumping for personal gain. Only to limit governmenty tyranny as so clearly practiced with these stupid laws.

    1. I think that actual THC levels do drop off pretty quickly, which is why most of the effect drops within an hour or two.
      THC is metabolized with a biological half life of under an hour, so within 4 hours THC levels drop to about 1/16 of original level, which is why one joint wont keep you high for a day, much less a week.

      Its the inactive Metabolites formed when THC is metabolized that can linger for days and weeks. These metabolites are slowly eliminated with only a half-life of 2-3 days, so if you smoke more than 2-3 times a week, levels can build up in your fatty tissues, and cause you to test positive weeks after you last enough there was enough THC to feel any effects.

      With a 1/2 hour half life for THC, the actual impairment drops by 75% in an hour, and by almost 90% within 90 minutes.

      So…. if you have a few munchies after a smoke, and take 30 minutes to find the car keys in your pockets, its likely that most of the limited cannabis impairment has worn off.

  18. This could only mean that the AAA is desperately trying to retain the membership of hippies.

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