Defining Drugged Driving by THC Levels Makes Little Sense, Heritage Foundation Concedes

Paul Larkin suggests states should reduce BAC levels instead.



A new Heritage Foundation paper warns that state laws allowing the use of marijuana for medical or recreational purposes "pose the risk of halting and in some instances reversing gains that society has made over the past 40 years in reducing the morbidity and mortality caused by alcohol-impaired driving." So far there is not much evidence of that. Traffic fatalities in Colorado and Washington—where medical use has been legal since 2001 and 1999, respectively—for the most part continued to decline as they made access to marijuana easier. Furthermore, while experiments show that marijuana can impair driving ability, it does not have nearly as dramatic an effect as alcohol does, and its role in crashes has proven difficult to measure in the real world. Still, it's possible that increased cannabis consumption could result in more fatal accidents, especially if it is not accompanied by less drinking. But as the author of the paper, Heritage Senior Legal Research Fellow Paul Larkin, concedes, it is not obvious how the law should deal with that possibility.

Larkin notes that defining drugged driving based on specific levels of THC in the blood, as Colorado and Washington have done, is highly problematic:

Alcohol has been the subject of extensive testing over decades, and science has found that (1) a strong relationship exists between BAC level and impairment or crash risk and (2) a person's BAC level changes slowly over time. By contrast, a host of factors affect how a given drug concentration affects someone. Individuals differ in their body weight and composition, absorption, distribution, metabolism, and accumulation of a drug, as well as the effect it may have due to the rate at which it is absorbed, the frequency by which it has been used, and whether the blood-concentration level was obtained when the amount of the drug consumed was rising or falling.

The effect of cannabis on an individual also hinges on what is known as "the set and setting" in which he uses marijuana—that is, an individual's prior experience with marijuana, his attitude toward its effect, his current mood, and the social setting in which it is used. Moreover, there is a poor correlation between the level of a drug in a driver's blood and the effect that drug currently may have on his psychomotor or executive functions, because there may be detectable levels of illicit drugs in a driver's system long after the drug's impairing effect has worn off. Also, some parties who repeatedly use certain drugs develop a tolerance to their neurocognitive effects, requiring users to increase their dose over time in order to obtain the same pleasurable effect, which means that the effect a drug may have on a driver's motor skills will vary from driver to driver.

The upshot, as NHTSA concluded in 2009, is twofold: First, testing for the presence of marijuana in a driver's system has not yet reached the same state of scientific knowledge that we possess today for BAC testing. Second, specific drug-concentration levels cannot be reliably equated with effects on a driver's performance. The result is that any particular level could be overinclusive or underinclusive.

Since there is little scientific basis for a per se definition of driving under the influence of marijuana, Larkin says, perhaps it makes more sense to reduce the legally tolerated concentration of alcohol in the blood of any driver "who is a registered marijuana user" from 0.08 percent to 0.05 percent "or lower, even zero." In other words, rather than charge cannabis consumers with drugged driving when they are not actually drugged, police should charge them with drunk driving when they are not actually drunk. "The justification for such a revision is straightforward," Larkin says. "Medical marijuana users are likely to use that drug, and the combination of marijuana and alcohol impairs a driver more seriously than does the use of either substance alone."

The justification does not seem all that straightforward to me, since Larkin's proposed policy, like the THC standards he questions, would treat many people who are perfectly capable of driving safely as public menaces, which hardly seems fair or sensible. Furthermore, enforcement of this special alcohol limit relies on the registration of marijuana users, something no jurisdiction has done (or plans to do) with recreational consumers. Patients therefore would be subject to special alcohol limits that would not apply to recreational users, which seems doubly unjust and unreasonable.

[Thanks to Paul Armentano for the tip.]

NEXT: A.M. Links: Obama Plans Executive Action on Overtime, Rand Paul Courts Marijuana Donors, Chris Christie Launches White House Bid

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  1. Traffic fatalities in Colorado and Washington for the most part continued to decline as they made access to marijuana easier (see charts).

    The charts do not seem to show what you claim they show.

    1. Larkin’s warning applies to medical as well as recreational marijuana. Medical use has been legal in Colorado since 2001 and in Washington since 1999, so you need to look beyond the last two years.

      1. I notice that the charts seem to have disappeared.

  2. To be fair, the effect of the combination of the two substances is more than the sum of the parts.

    1. But if you never drink, why does that matter?
      Where did they get the idea every pot smoker has a couple beers whenever he smokes? It’s pretty rare in my experience

      The truth is that pot intoxication and impairment drop about 75% in the 1-2 hours after use, while alcohol impairment may still be increasing an hour after use. Booze and pills are the killers on the highway, not pot. Make it a law that smoking while driving equals
      DUI, but otherwise base it on BAC.

  3. “who is a registered marijuana user”

    Is that like being a registered sex offender? Or holding a Mexican passport?

  4. So, if you’re a marijuana user who has a beer, you’re not impaired by anything at all, but you’re DUI? How is that better than the current standard? It just adds another layer of retardation. Normal people would consider that worse.

    1. How do we handle people impaired in pills like ambien and Valium?

  5. God forfuckingbid the government didn’t have a method to throw you in a rape cage.

    How many units of tired do you need to be before you’re “impaired”?

  6. Simple solution: punish people who harm others, not people that drink or smoke.

    1. But how would the state get it’s pound of flesh (ie, fine money).In Ohio you can be charged with DUI and test well below the limit,even 0.00.Then you have to spend money to fight it and that’s not cheap.

    2. Not much of a revenue-driver there. Also missing fear factor with which to control a populace. Who are you trying to appeal to here, redneck terrorists or people who count?

      1. Will someone blame the children?

  7. What level of distracted is legal to drive?

    1. When a cop eats or talks on the phone,that’s cool.

      1. Or types on his little in car computer.

  8. You’re either driving recklessly or you aren’t. Cause is immaterial.

    1. Either you hurt someone or you didn’t. How you were driving is immaterial.

  9. Lowering the BAC just means that everyone should just refuse the breathalyzer.

    Speaking of why do we accept a metering device with no reported error as a legal standard to screw over millions of people? Show me another measuring device used in any trial that does not have a documented error.

  10. Just as Lucille said I didnt know that anyone able to get paid $7158 in four weeks on the computer .You can look here?????????????

  11. Drunk (or stoned) driving should be legal. As long as you cause no property damage,injury, or death, the contents of your blood stream are not criminal. However, if you do cause property damage, injury or death, being drunk or stoned should automatically bump you into intentional categories of criminal charges, i.e if you run down a kid while drunk, it’s never an accident or involuntary manslaughter.

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