How Will Police Regulate Stoned Driving?

Now that recreational marijuana is legal in two states, lawmakers and law enforcement have to grapple with how best to deal with cannabis-impaired drivers.


When Colorado and Washington state each passed ballot measures legalizing marijuana for recreational use late last year, one legal challenge was resolved, but another was just beginning Before, marijuana was simply prohibited. Now it has to be regulated. With their new legal standards for possession and use, Colorado and Washington now have to draw hard lines on a rather hazy landscape, creating legal standards not just for for taxation and licensing, but also some far more nebulous questions, like how much marijuana is reasonable for a single person to possess, and even what constitutes legal intoxication. Meanwhile, forty-eight other states are watching closely to see exactly how they do it.

For Colorado, regulation of marijuana under the new state standards (which, by the way, could still be challenged by the federal government) came to a head last month at the state house as legislators hashed out just exactly how local and state authorities will handle these questions. They also tackled the thorniest issue of all, one that has been a sticking point for previous legalization efforts and one that is eventually bound to go ’round and ’round in courtrooms: what scientifically constitutes “under the influence” of marijuana, and how can clinicians and law enforcement determine if someone — most importantly, a driver — is too high for the public good?

Now the state of Colorado has offered up its answer. Under House Bill 1114, the answer is five nanograms. If a blood screen detects five or more nanograms of THC (that’s delta-9-tetrahydrocannabinol, the psychoactive ingredient in cannabis) per milliliter of blood in a person’s bloodstream, that individual is considered legally under the influence of drugs. Washington has also set its intoxication limit at five nanograms per milliliter.

But the question is not that simple. What is marijuana impairment — what constitutes being “too high” to drive — and how can we scientifically evaluate it, particularly in a law enforcement context? Moreover, how can police officers test for it conclusively at the roadside, where blood tests aren’t available? How lawmakers define and answer these questions will have a lot to do with marijuana policy in the U.S. going forward, and unfortunately the body of science describing marijuana’s effects on the brain and body — though vast — isn’t exactly bound by broad consensus. Five nanograms per milliliter is a place for policy to start, but it’s by no means the last word determining how high is too high.


“Smoking is a very efficient way to deliver drugs to the brain,” says Dr. Marilyn Huestis, a senior investigator at the National Institute on Drug Abuse’s Intramural Research Program, part of the National Institutes of Health. “It goes into the lungs, into the heart, and gets pumped directly to the brain.”

Huestis has spent a career studying the effects of marijuana on the brain and the psychomotor capabilities of individuals, both among acute users (those that use marijuana occasionally) and chronic users who partake of marijuana daily. Unlike alcohol, which requires at least a little bit of time to work its way into the bloodstream, marijuana has shown in Huestis’s own studies to manifest itself within the first minute after use. From there, one’s ability to responsibly operate heavy machinery begins to come into question.

Cannabinoid receptors (known as CB1 receptors) in the brain are found in many key regions, including the amygdala (responsible for processing memory and emotional reactions) as well as the basal ganglia and cerebellum (responsible for motor control, among other things). “We know that when people smoke marijuana the lose some of their peripheral vision,” Huestis says. “We know it affects the passage of time, or the idea of how rapidly time is passing. It affects balance. And one of the most interesting areas it affects is the prefrontal cortex.”

Driving is an exercise in timing, multitasking, and situational awareness — and not one well suited for the cannabinoid-impaired. The prefrontal cortex is what separates us from other animals, Huestis says. It’s home to our executive function, the place where we take in and process information and use it to make choices about various courses of action. Cannabis impacts our executive function, which can slow or alter decision-making abilities. Moreover, it makes our brains work harder, Huestis says, and not necessarily in a good way. In tests, an individual dosed with cannabis can often perform a task just as well as he or she would if sober. But brain imaging of dosed individuals shows that it requires much more brainpower to complete that task. That means that under the influence of cannabis the ability to handle multiple tasks simultaneously, or to divide attention effectively, dwindles significantly. Even more specific to driving, marijuana has been shown in various studies to affect what’s known as “standard deviation of lateral pursuit,” or that natural, somewhat innate ability to hold an automobile more or less right in the middle of a traffic lane.

None of this — reduced peripheral vision, slowed decision making, inability to multitask — enhances one’s ability to drive. Humans are more prone to distraction when dosed with cannabis, and in the context of a moving vehicle a misperception of the passage of time translates to a misperception of distance as well, at least in the sense of how quickly a car traveling at a given speed will reach some distant object.

That being said, Huestis notes, individuals under the influence of cannabis — unlike those under the influence of alcohol — tend to be aware of their impairment. Some studies have shown that stoned drivers are more cautious behind the wheel and tend to drive more slowly. But that’s not really any kind of compensation. Driving is an exercise in timing, multitasking, and situational awareness — and not one well suited for the cannabinoid-impaired.


“The properties of marijuana are not going to liken themselves very much to a ‘breathalyzer’ type test,” says Dr. Christina Hantsch, a toxicologist working within the Loyola University Health System. “I think it’s going to have to be a different bodily fluid if you’re looking for more immediate testing.”

Why? For one, THC is fat-soluble, which means it can be absorbed by the body’s fat cells and remain within the metabolism for extended periods of time. For heavy users, THC can remain within the body for days, making it difficult to connect the presence of THC in a person’s bloodstream with that person’t current state of impairment. For chronic users the picture is even murkier. Regular marijuana users who stop using cannabis can still have detectable amounts of THC in the bloodstream even 30 days after they cease using. There are even documented cases of former chronic users that haven’t had a dose of cannabis in years testing positive for THC while undergoing rapid weight loss, Hantsch says. THC is really good at tucking itself away in the body’s fat cells, and it can remain there for a really long time.

Complicating things further still: an emerging body of scientific evidence suggests that this residual THC in the bloodstream of chronic users might still cause impairment. Though the effects of these trace amounts of THC in the bloodstream don’t manifest themselves with nearly the intensity that a fresh blast of THC to the CB1 receptors does, both Huestis and Hantsch note that there is research out there suggesting that just because these levels of THC are relatively low doesn’t mean they aren’t having some impairing effects on the psychomotor skills of both acute and chronic users for the duration that THC remains in the bloodstream.

All that is to say that detecting the presence of THC in the bloodstream doesn’t necessarily correlate to impairment, and there’s certainly no overwhelming body of hard science that can draw connections between a specific amount of THC in the blood (like, say, five nanograms per milliliter) and a specific degree of impairment. Things grow more dubious still at the place where government really needs certainty the most: at the roadside.

The most promising solution for the problem of roadside THC testing in recent years has been oral specimen testing (read: saliva sampling). Several academic and government labs as well as commercial companies have developed various tests claiming they can detect THC in the bloodstream via handheld devices that analyze a swab taken from inside a subject’s mouth. But the results have been mixed — mostly mixed degrees of disappointment.

“Oral fluid testing actually went into effect in Australia in 2004,” Huestis says. “The reason it didn’t get going in Europe or the U.S. is because the roadside devices were, frankly, terrible.”

You can get really good results from THC testing via oral sample in the lab, Huestis says, but the problems with collecting and analyzing samples at the roadside became immediately apparent to early adopters of the portable oral specimen technology. First, THC is so lipophilic that it had a tendency to stick to the collection devices themselves, which dulled the sensitivity of the analysis from the point of that samples were collected. Saliva is also loaded with enzymes that break molecules down, so in the period between collecting the oral sample and getting it to a lab for analysis the samples would continue to degrade themselves, further skewing the results.

But assuming there was an oral specimen test that was effective for accurately measuring THC in the bloodstream at the roadside, there’s still the problem of correlating it to impairment, which is ultimately what law enforcement officers are concerned with, especially in a context where possession and use are no longer strictly prohibited.

“There’s still a lot of work to be done to really tie in all those connections, to say that if you do pick up this level of a marijuana metabolite in a oral fluid specimen there is some solid scientific evidence that also indicates some degree of impairment or effects on the behavior of the individual,” says Dr. Stephen Kahn, a professor of pathology and toxicologist at Loyola University’s Stritch School of Medicine. “And that’s harder to do than with blood ethanol.”

Nonetheless, the state of both the science and the technology is improving. The tools for oral specimen detection and analysis improve each and every year, Huestis says, and her own lab recently folded trials of a new portable oral specimen diagnostic into experiments there. Under controlled conditions in which the THC levels of dosed subjects were being tested independently in the lab this new portable device showed impressive efficacy, Huestis says, with very low incidence of false negatives or false positives.

Huestis thinks we’ll see these kinds of tests used by law enforcement in the U.S. within 3-5 years. Kahn is less willing to put a firm projection on the adoption of such technologies, but he does believe that the science will eventually become good enough to gain the confidence of the courts and law enforcement.

“I think it’s absolutely going to happen,” Kahn says. “But I’m just not sure how long it will take.”

Medical marijuana


Of course, all of this hinges on having a legal standard to measure against. Some states in the U.S. where medicinal marijuana is legal have already established zero-tolerance policies for driving under the influence of cannabis; get caught with THC in your bloodstream while driving and it’s an immediate conviction regardless of how much THC that is (or whether it is the cause of your impairment). But with Washington state and Colorado opening the door to legal, recreational use of cannabis their legislatures are choosing — like several other medicinal marijuana states — to treat cannabis more like alcohol. That is, you can toke a responsible amount and still get behind the wheel, but should you cross a certain threshold you are in serious legal trouble.

So where does the five-nanogram-per-milliliter rule established by both Washington and Colorado come from? Not from Washington or Colorado. It’s an administrative decision that might seem somewhat arbitrary, though it’s no more arbitrary than decreeing that somewhere between .07 BAC and .08 BAC a person transforms from capable to dangerously drunk. The five-nanogram rule is rooted in several studies and for several scientific reasons, the first of which actually sides with the regular marijuana user. In the first studies that emerged showing that blood tests could detect residual THC in the bodies of chronic cannabis users even days after they last dosed, none of those chronic users registered higher than five nanograms per milliliter at 24 hours after their last dose, Huestis says. So the rule is in part designed to reduce the likelihood that chronic users will get slapped with D.U.I.D. convictions when in fact they haven’t consumed cannabis in more than a day.

The other reason states tend to gravitate toward the five-nanogram rule is far more nebulous, but there’s some scientific evidence, borne out by data, that when THC counts in the bloodstream of a driver are at five nanograms per milliliter or higher, that driver’s chance of being involved in a fatal accident begin to climb steeply. One Australian study found that with any measurable THC in the bloodstream a driver is twice as likely to be involved in a fatal accident, but at five nanograms per millimeter of THC that number jumps to 6.6. times more likely, Huestis says. Five nanograms is the point where the chances of something bad happening seem to start climbing steeply.

In the eyes of the law no one really cares how impaired you are, only that you are impaired.But that’s not really so clear. Some German studies have shown significant impairment in subjects testing in the area between two and five nanograms per milliliter (Germany is considering adopting the five-nanogram standard as well) and in Sweden, where standards for impaired driving are among the most rigorous and enforced by stringent legal penalties (the legal BAC limit is 0.02, or a quarter that of the U.S.), one laboratory found that 90 percent of that country’s cannabis impairment cases had a level of one nanogram of THC per milliliter. So where impairment is concerned there’s a lot of gray area — and a lot of scientific debate — between zero nanograms and five nanograms per milliliter, not to mention a lot of varying opinion on what constitutes “impaired.”

But ultimately the long-sought portable roadside THC test for law enforcement may be less important than many have made it out to be. After all, in the eyes of the law no one really cares how impaired you are, only that you are impaired. That’s the way it works for alcohol impairment, and the way it has worked for years. Though easier to measure and evaluate at the roadside, blood alcohol concentration really has no quantifiable correlation to how impaired a person is. Alcohol affects different people in different ways, but regardless you still go to jail for driving in the U.S. with a blood alcohol concentration that tops 0.08 (celebrities and the politically well-connected notwithstanding). How capable you really are of driving is beside the point.

Like the five-nanogram standard, the 0.08 blood alcohol concentration limit was an administrative decision. And as with the 0.08 rule, governments will likely simply set THC standards wherever the existing body of science makes them feel comfortable. Creating such a threshold not only establishes a firm legal standard that can hold up in court, but it somewhat obviates the need for precision roadside testing — a simple field sobriety test for THC impairment testing for time and depth perception, coordination, and other psychomotor abilities tied to cannabis impairment will do, and officers already have those kinds of tests in their collective toolbox.

Of course, a reliable ‘breathalyzer’ for marijuana — something easily administered at the roadside that’s capable of returning a number that, like blood alcohol concentration, correlates roughly to a degree of impairment — isn’t completely out of reach. If the new recreational marijuana laws in Colorado and Washington state have created something of a regulatory headache, it’s a headache that recreational pot laws might also be able to cure. If necessity is the mother of invention, these new laws have certainly created a need among the legal community that is helping to focus the science and technology community on potential solutions.

“This is a hot area right now, and there really is a lot of attention being paid in my field to oral fluid testing for drugs of abuse in general,” Kahn says of the potential for a portable THC testing device. “The kinds of issues we’re talking about are exactly why. This is where it’s headed. It may not happen in exactly the way that we think, but in one way or another I think it will happen sooner or later.”