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.]