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The police sergeant’s voice was quiet but firm. She told the college student exactly what he was going to do, and then he did it.

“You’re going to take a series of nine heel-to-toe steps,” she said. “You’re going to look at your feet, you’re going to count your steps out loud, you’re going to keep your hands by your side, and you’re not going to stop once you start. … Then you’re going to come back.”


He put one foot carefully in front of the other, like a tightrope walker who’d made the mistake of looking down.

That sobriety test might have taken place on a windswept roadside, where Sgt. Deborah Batista had just pulled the student over for swerving across lanes. But they were going through the motions in the relative comfort of a Massachusetts General Hospital office building, where researchers were testing a brain imaging device to see whether it could identify people driving under the influence of pot.

As more and more states legalize marijuana, scientists and entrepreneurs are rushing to devise a Breathalyzer for pot — something more objective than officers’ observations of people as they drive, and then as they attempt the walk-turn and the one-leg stand. At Mass. General, they’re taking a different tack: Instead of looking for the chemical ghosts of pot on your breath, researchers are using a sensor-studded cap to look for impairment in the brain.

They know it’s a long shot. “Whether or not it will be useful, practically, is anyone’s guess,” said Jodi Gilman, an assistant professor at Harvard Medical School and Mass. General’s Center for Addiction Medicine, who is one of the researchers running the study.


Even other approaches that are closer to market aren’t ready for the roadside yet.

“Marijuana is a tough one, we admit that,” said Charles Hayes, who helps run the Drug Evaluation and Classification program for the International Association of Chiefs of Police. “There is no test out there at this time that can give us some sort of numerical reading … that would tell us that person is impaired.”

Drug-testing experts tend to break the country down into two categories: blood states and urine states. Get pulled over looking like you’re driving under the influence of drugs, and that’s the bodily fluid that will be sampled and sent to the lab.

Both fluids can provide evidence that you’ve been using marijuana — so if possession or use is illegal, then the police might have a case. But if you’re in one of the 29 states that, along with Washington D.C., allow some of kind of pot use, then it’s impairment while driving they’re after.

Part of the problem is that the chemical signs of marijuana use last for different amounts of time in different bodily fluids, and they don’t necessarily mean you’re high just then. And unlike with alcohol, for THC, the active ingredient in pot, there is no agreed-upon level that would mean a person is legally considered stoned.

A handful of states have, according to the Governors Highway Safety Association, established a threshold THC level drivers can have in their blood. But many experts worry that frequent pot smokers might be well past these thresholds without showing signs of impairment, while police officers say that occasional users might fall below the limit but still be driving dangerously.

Yet eyelid flutters and body tremors and difficulties following walk-and-turn instructions cannot provide the kind of numerical evidence that an alcohol breath test can.

“Police officers have their own biases, and aren’t perfect … look at all the stuff going on with persecution of minorities,” said Gilman. “If you have an objective test, it’s not left up to any one person’s judgment.”

Meanwhile, Batista, the Middleborough Police Department sergeant, who is also the assistant coordinator for the Massachusetts Drug Recognition Experts program, worries that courts aren’t taking standard sobriety tests seriously, which means police are loath to make arrests.

“I know of cases where officers are saying, ‘I know you’ve been smoking weed, I’m seeing signs of impairment, but we’re just going to give you a ride and tow your car,’” she said. “That’s really not addressing the issue.”

Batista said she dreads the day — now set for July 1 — when it becomes legal to sell recreational pot in Massachusetts. And what she’s most worried about are the kinds of stories told by the student whose sobriety she was testing.

MGH Marijuana test
Batista administers a sobriety test to the student, John, at a Massachusetts General Hospital research office. Jonathan Wiggs/The Boston Globe

John, the student who requested his last name not be used, first saw the study advertised on his university’s quickie job board. Gilman and her team were looking for regular pot users in the Boston area and John knew he fit the bill. He smokes about six days a week, almost always before dinner, because that’s when he gets the most bang from his bong: It elevates his takeout burritos and microwaveable orange chicken from plain old sustenance to existential delight.

By his account, John almost never drives stoned. The few times it’s happened — “definitely … no more than between 12 and 24 times,” he said — have been over his winter break, when he’s steered his parents’ car back to their house after hanging and smoking up and listening to Jay-Z with a friend. He talks about the experience the way someone might describe a drive after hot yoga, his eyes more attuned to the glow of headlights and the brightly colored sign outside of his old middle school.

“It’s entirely different from drinking and driving,” he said. “I feel almost more aware of what’s around me when I’m high. I feel almost safer.”

That’s exactly what worries Batista: People don’t just think they’re able to accomplish the multitasking and quick-thinking of driving when they’re high. They think they’re even better at it.

They’re not. “We’ve done a couple of driving studies where we get people stoned, get them behind the wheel in an obstacle course,” said Dr. Mike Lynn, the CEO and co-founder of Hound Labs, one of the companies now working on a Breathalyzer for marijuana. Part of the circuit involved a simulated cyclist popping out of nowhere, as cyclists sometimes do, and, Lynn reported, “all of them hit that cyclist when they were stoned, and most of the time, they didn’t even know they’d hit him.”

John’s trip into the lab — for which he’d get $100 and a chicken avocado wrap — didn’t involve anything as exciting as driving a closed course while blitzed. But if he were lucky, he’d still get a handful of large red pills that contained a strong dose of free THC.

“This is where the magic happens, this tiny windowless room,” said research coordinator Eve Manghis, leading him in.

He wasn’t getting the drug just yet. First, the researchers needed to look at his brain unbaked. Their device of choice was a cloth cap — a kind of wig wired up with emitters and receivers of infrared light — held in place by a chin strap.

MGH Marijuana test
Researchers set up a functional near infrared spectroscopy device on a student as part of the study to see whether it might help identify marijuana impairment in drivers. Jonathan Wiggs/The Boston Globe

The device uses a technique known as functional near infrared spectroscopy, or fNIRS for short. By scattering certain wavelengths of light through John’s skull and into the tissue underneath, the researchers could detect changes in blood flow to assess the extent of communication between different parts of the brain.

Before starting, Manghis and her colleagues fiddled with the cap to get it to work properly. They stuck a long cotton swab through the holes in the cap to twist John’s hair out of the way. They tried another aviator-like cap over the first, to see whether that helped. It didn’t. Then they smeared a bit of ultrasound gel between the sensors and John’s scalp. “I think we’re cruising,” said Manghis, after looking at the data coming in to the computer.

John sat still, his eyes closed, as the blood flow in his brain was translated into red and blue squiggles on a screen.

By looking at these data before and after participants got any THC — and by giving some of them a placebo — the researchers hoped to see whether being high produced any distinctive patterns of brain activity. They also wanted to know how that correlated with a police officer’s assessment of impairment.

At this early stage, it’s a bit hard to imagine — in these beige-carpeted offices where researchers fiddle with wires and cotton swabs, calibrating and re-calibrating, administering psychological tests and surveys — that an officer would be able to rig up one of these caps and look at blood flow in the brain on the side of the road.

After all, the researchers would need to make sure that the signals they are seeing are in fact pot impairment and not something else. As Gilman explained, she wants to avoid false positives at all costs, and that means making sure that marijuana impairment couldn’t be confused with sleep deprivation or a medication the driver is taking.

The side-of-the-road scenario is also challenging because no one would know what the driver’s blood flow in the brain looked like normally. “You don’t have the baseline, so it can’t be a within-person comparison, and that’s problematic,” said Heather Bortfeld, a University of California, Merced, psychologist who sits on the board of directors of the Society for functional Near Infrared Spectroscopy.

Meanwhile, Hound Labs is saying it will start taking pre-orders for its marijuana breath testing device from law enforcement agencies in the spring. The company said in an email that it has done hundreds of tests on human subjects and that there is an ongoing clinical trial at the University of California, San Francisco, to validate the machine’s performance.

John was disappointed not to get the warm “full-body high” he’d felt the last time he’d swallowed a handful of red capsules from the Mass. General pharmacy. “Last time, I was feeling effects at this point, so I think it’s a placebo,” he said. But when Batista arrived, he got up, ready to stand on one leg, touch his nose, and have his pupils peered into by a police sergeant. After that, he’d slip back into the windowless room to have his brain scanned yet again.

  • Typical crap reporting you expect from the media these days. The author clearly did not take the time research the topic. It’s clearly just pandering to his audience. I have attached an actual study

    Surprisingly, given the alarming results of cognitive studies, most marijuana-intoxicated drivers show only modest impairments on actual road tests.37, 38 Experienced smokers who drive on a set course show almost no functional impairment under the influence of marijuana, except when it is combined with alcohol.39

    Although cognitive studies suggest that cannabis use may lead to unsafe driving, experimental studies have suggested that it can have the opposite effect.

  • A penitentiary for your thoughts?

    Not content with drug dogs, facial scans, x rated full body scanners at the airport and the like, big brother’s little helpers now want to look inside your head for forbidden blood flows!
    May I make a suggestion?

    Wouldn’t it make more sense to create a universal device and related standards that measure actual realtime driving impairment.

    After all isn’t that is why all these substances are prohibited. Instead of strapping on a roadside polygraph that probes your brain for illegal activity, you are simply handed an iPad, tilting right and left to steer, forward and back to move and stop. A driving simulation program provides streets and various tests of reaction time, motor control and fitness to drive.

    States could devise universal tests and standards of impairment, insurance rates could be related directly to driving skill and associated risk.
    Or people could simply wisely use it to determine if it is a good time for them to drive, and avoid trouble in the first place.

    A man walks out of a bar to his car, Siri says “I’m sorry Dave, I can’t let you do that” in her best HAL voice.
    “Based on how you have been shaking me for the last hour you are too: drunk, stoned, and /or emotionally upset,…….. To drive”.

    While the article suggests that stoned drivers would more likely run over cyclists, there are no statistics nor parameters given. Only a very self serving story.
    What has been shown is that states with legal cannabis have reductions in the consumption of alcohol, opiates and a wide variety of other legal and illegal substances. The future will show whether these secondary effects are helping reduce accidents and criminal behavior, but these are the trends.

    Sadly the most hysterical anti cannabis crowd conflates its effects with alcohol, with its proven effects of loss of motor control and inhibitions. THC has an entirely different effect on driving. Their own lab rat tried to explain that he would be much more cautious driving stoned, and just wouldn’t do it, but rather than test out his explanation, they simply contradicted it in their own minds. After all, they wouldn’t want to prove their beliefs wrong with actual science.
    That’s not the kind of science that drug and alcohol companies pay for you to see.

    Nature has given us a wonderful plant, love it, grow it share it with your friends. And ask why you have been lied to about this for so long? and what else you are being lied to about?

  • No one should drive impaired, but actual impairment should be measured, and the level of impairment from cannabis that is criminalized should be the same as the level of impairment for the .08 blood alcohol level. How to measure impairment? Read on!

    I have developed a new public health app that measures actual impairment–it is called DRUID (an acronym for “DRiving Under the Influence of Drugs”) available now in the App Store and in Google Play. DRUID measures reaction time, decision making, hand-eye coordination, time estimation and balance, and then statistically integrates hundreds of data points into an overall impairment score. DRUID takes just 2 minutes.

    NORML of California is promoting DRUID on their website and is encouraging cannabis users to download it.
    Our website is

    DRUID allows cannabis users (or others who drink alcohol, use prescription drugs, etc.) to self-assess their own level of impairment and (hopefully) decide against driving if they are impaired. Prior to DRUID, there was no way for an individual to accurately assess their own level of impairment. DRUID also demonstrates that it is feasible to measure impairment reliably by the roadside, not just exposure to a drug. It could also be a way for cannabis users who have developed tolerance to show they are unimpaired.

    DRUID was featured on NPR’s All Things Considered:

    Also on television:

    And this past December on Spokane Public Radio:

    After obtaining my Ph.D. at Harvard, I have been a professor of psychology at UMass/Boston for the past 40 years, specializing in research methods, measurement and statistics.

    Michael Milburn, Professor
    Department of Psychology

    • Wow, the future is already here! Good luck with distributing it.
      After all why force people into a mental straight jacket when you can easily get them to play a fun, free game, even if it means incriminating themselves, or more likely saving lives.

      Seriously, I think this is a great idea, and I would love to see its use in all arenas where equipment is operated to ensure piblic safety, and also required for all congressmen prior to any votes.

      Thanks for bringing a reasoned voice into the fray.

  • In states allowing recreational marijuana use I foresee a significant auto insurance increase. If someone hits me and I suspect they are high I will insist that they be tested. If positive I would sue.

    • There is no test to determine the state of being high. That is the whole point of the article.

      Your opinion about someone being high carries no legal authority and has no weight in determining if a person who hit you needs to be tested for anything.

      I am in a cannabis legal state (Oregon) and my car insurance rates have not changed in the 3 years it has been legal.

      If a person suspects the driver that hit them is high, they will not be the one calling the shots and requiring testing. The officer at the scene will be the one to determine what is or is not tested according to very specific protocol and law.

      This article is about determining which protocol works and which does not. Currently, there are no reliable protocols.

    • Joe B is correct that the Standard Field Sobriety Test does not reliably assess cannabis impairment. That’s why I created the DRUID app, so people can self-assess their impairment and decide not to drive if they are impaired. Or if they are a passenger, ask the driver to use it.

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