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Study Finds That Per Se Driving Limits For THC Is Ill Advised


driving marijuana dui duiiBy Paul Armentano, NORML Deputy Director

Per se driving limits for the presence of THC are arbitrary and may improperly classify motorists who are not behaviorally impaired, according to the findings of a study published today by the American Automobile Association (AAA) Foundation for Traffic Safety.

Per se driving limits criminalize the act of operating a motor vehicle if the driver possesses detectable amounts of specific drugs or drug metabolites above a set threshold. Under these laws, drivers are guilty per se of violating the traffic safety laws even absent evidence of demonstrable behavioral impairment.

Five states – Montana, Nevada, Ohio, Pennsylvania, and Washington – presently impose per se limits for the detection of specific amounts of THC in blood while eleven states (Arizona, Delaware, Georgia, Illinois, Indiana, Iowa, Michigan, Oklahoma, Rhode Island, Utah, and Wisconsin) impose zero tolerant per se standards. In Colorado, the presence of THC in blood above 5ng/ml “gives rise to permissible inference that the defendant was under the influence.” Legislation similar to Colorado’s law is presently pending in California.

However, the AAA report concludes, “[A] quantitative threshold for per se laws for THC following cannabis use cannot be scientifically reported.” This is because the body metabolizes THC in a manner that is significantly distinct from alcohol. In particular, acute effects of cannabinoids lag well behind the presence of maximum THC/blood levels. Additionally, residual levels of THC may be present in blood for extended periods of time, long after any psychomotor-related effects have ceased.

The Automobile Association’s finding is similar to that of the US National Highway Traffic Safety Administration, which acknowledges: “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. … It is inadvisable to try and predict effects based on blood THC concentrations alone.”

NORML has long articulated a similar opposition to the imposition of per se driving thresholds for THC and/or its metabolites, stating, “[R]ecently adopted statewide per se limits and zero tolerant per se thresholds in the United States criminally prohibiting the operation of a motor vehicle by persons with the trace presence of cannabinoids or cannabinoid metabolites in their blood or urine are not based upon scientific evidence or consensus. … [T]he enforcement of these strict liability standards risks inappropriately convicting unimpaired subjects of traffic safety violations, including those persons who are consuming cannabis legally in accordance with other state statutes.”

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Johnny Green


  1. I think this particular talking-point is being floated among those who don’t know to check the actual data. Basically, a scare-tactic in a soundbite for the gullible. Research by Anderson and Rees (2013) showed that states with comprehensive medical cannabis laws on the books showed two striking statistical trends: a mean reduction in DUI-fatalities by 9% accompanied by a mean reduction of beer sales by 5%. That trend is reflected in the yearly road fatalities in Washington state.

    The talking-point only popped up on my radar because the Georgia State House Majority Leader is facing a primary challenge from a guy who actually supports legalization, which is why the current Majority Leader used this technically accurate but wholly misleading factoid against his challenger. The way it’s phrased, it gives the reader/listener the impression that fatalities on the road have doubled when in reality, they’re still significantly lower now than they were before legalization. It bothers me that the primary challenger didn’t know about this talking-point in advance so that he could have addressed it on the spot (instead of letting it stand).

  2. saynotohypocrisy on

    If there was a problem with people high on weed causing serious crashes, why isn’t anyone being charged with crimes over it? Fatal crashes get investigated thoroughly and the many cops who are down on cannabis would love to be able to charge cannabis users with DUI killings. So where are all the charges of cannabis use causing wrecks?

    Traffic fatalities are up strongly nationwide in 2015 compared to 2014, largely because of more traffic due to lower gasoline prices. Prohibs are likely to try to demagogue around this as well, talking about increases in the free states without mentioning the increases in the alcohol supremacist states, or the reason for the increases.

  3. saynotohypocrisy on

    10ng is a low level, but I’m still guessing you’re good. Hope so.

  4. This is good news, but what we’re ignoring is the OTHER study from AAA claiming that the fatalities involving drivers who have used cannabis “have doubled” since cannabis legalization.


    What they mean by “doubled” is an increase from 8% to 17%. It makes perfect sense that, in a state where “per se” limits are imposed that the number of people testing positive would increase. So what about total fatalities on the road? Has there been a jump in traffic fatalities in Washington State? HARDLY! This is data from the FARS Encyclopedia about traffic fatalities in Washington State by year:

    2005: 649 fatalities
    2006: 633 fatalities
    2007: 571 fatalities
    2008: 521 fatalities
    2009: 492 fatalities
    2010: 460 fatalities
    2011: 454 fatalities
    2012: 438 fatalities
    2013: 436 fatalities
    2014: 462 fatalities

    Washington State is still WELL below their historic average of 511 traffic fatalities per year over the last ten years. HOWEVER, while cannabis advocates run around patting themselves on the back-side congratulating each other over the AAA study in this article, the “doubled fatalities” talking-point is being used across the nation as a reason to NOT LEGALIZE. We cannot ignore this talking-point. It’s being repeated by state legislators around the country. We must be keenly aware of this ridiculous talking point or else we’ll be at the mercy of those touting it as “proof” cannabis is dangerous.

  5. I was clean for 2 months. I took one bong hitt , that’s it. Took a ten nanogram drug screen a week later, u think I passed??

  6. If authorities actually wanted to improve traffic safety, they would test for impairment, not some arbitrary and meaningless number. Drivers using alcohol and prescription opioid drugs pose a much greater risk than smokers.

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