Adults can now legally buy recreational cannabis in California, but there’s no standard for how much marijuana you can have in your system and get behind the wheel. The law simply says you can’t drive while impaired. So is it possible to test if someone is too high to drive?

Meg Schwarzman used to bike through the intersection of Bancroft and Fulton in Berkeley almost every day. Then last February, she was in a crash that almost killed her.

She had an 11-month-old baby at home, and she was being an especially careful biker. She was wearing a dayglo green jacket and a new helmet over her curly hair.

Schwarzman says she reached the intersection, when a driver “ran over me from behind, trapping me under the car, and dragging me 60 feet across the intersection.”

The driver was a medical cannabis user. He told police he had smoked shortly before getting in the car.

“He actually was high enough he didn’t know he’d hit me,” says Schwarzman.

Meg Schwarzman stands at the Berkeley intersection where she was hit by a medical cannabis user, which now features more protections for bikers. (Eli Wirtschafter/KQED)

Firefighters pulled her out from under the car just in time to save her life. She had 20 broken ribs, two collapsed lungs, a lacerated liver, and fractures in her pelvis and her skull.

“They’d called in the fatal accident investigation team,” she recounts. “They didn’t expect me to survive.”

Schwarzman recovered completely, apart from some serious scars. But she’s worried that as marijuana is legalized, there will be more stoned drivers on the road.

“We have some deterrence with alcohol in that we have breathalyzer tests and legal consequences,” says Schwarzman. “There are legal consequences for driving high also, but there aren’t good tests for it.”

Why a Blood Test for Marijuana Isn’t so Simple

In California, the consequences for driving drunk and driving high are the same. But while there’s a clear blood limit for alcohol, there’s no such standard for marijuana.

“Marijuana is very different than alcohol,” says Jolene Forman, a lawyer with the Drug Policy Alliance. With alcohol, impairment links strongly with the amount you’ve had to drink, depending on your weight and gender.

When you drink a glass of wine, the alcohol shows up in your blood and your brain at the same time. As its effects wear off, alcohol disappears from your brain, and your blood, too.

“Marijuana works totally differently, where it can stay in your blood for days, and even weeks, after you’ve consumed it,” says Forman.

In addition, THC, the psychoactive chemical in marijuana, can affect people very differently. So measuring THC has not been proved to be a reliable measurement of someone’s driving ability.

California’s Subjective DUI Laws

On average, marijuana does impair driving skills for most people, especially 20 to 40 minutes after smoking. But measuring THC isn’t a good way to tell how much worse.

The lack of scientific evidence didn’t stop Washington and Colorado from setting a THC limit for drivers. But California went the other way. Here, there’s no numerical limit on THC.

Instead, the law says you can’t drive under the influence of any drug — period. It’s also illegal to consume cannabis while driving or riding in a vehicle.

In California, a marijuana DUI conviction depends on things an officer observes, such as the smell of marijuana in the car, or a physical sobriety test like those given to drivers who may be drunk.

Some officers are trained to identify specific signs of marijuana use, like dilated pupils. Sgt. Jennifer Tate, a drug recognition expert with the Berkeley Police Department, says the physical tests have gotten better over the years.

“We don’t need a better test,” says Tate. “If people are impaired, we can tell.”

But Andrea Roth, a law professor at UC Berkeley, says those physical tests are relatively subjective, relying on the judgment of the officer. Because of that, she says there’s potential for racial bias in how they’re enforced. Cops already pull over black and Latino drivers at higher rates. Roth says an objective test could help prevent innocent drivers from getting DUIs.

“There’s less risk of racial bias at the point of determining if someone is intoxicated if there’s a machine measurable result,” says Roth.

As marijuana becomes legal, there’s growing demand for an objective test that really works.

“The holy grail will be to find … some scientifically robust way to determine level of intoxication,” says Roth.

An Oakland-Grown Marijuana Breathalyzer

Several companies are chasing after that holy grail. One of them, called Hound Labs, is headquartered in downtown Oakland, right in the heart of the new cannabis economy.

Mike Lynn is a trained ER doctor turned entrepreneur. But his company, unlike some of its neighbors, doesn’t make cannabis chocolates or THC lollipops. Lynn is co-founder and CEO of Hound Labs, which makes a breathalyzer for marijuana.

Lynn says the breathalyzer actually measures both THC and alcohol. The two substances together have a stronger effect on drivers than either one alone.

The device looks a bit like a black plastic tape dispenser, with a tube sticking out. If you blow through it at the right rate, it beeps.

Although THC lingers in blood for days or weeks, Lynn says THC disappears from breath after a few hours. Even in those hours right after smoking, THC in breath comes in tiny concentrations. It’s hard to detect, and there isn’t much research on it published yet. But Lynn says the Hound Labs device can measure it.

Lynn says “nobody really knows” why THC stays in breath only a few hours. But because of this short time span, users with THC in their breath “have used pot very, very recently and [are] much more likely to be impaired.”

So although the Hound Labs test doesn’t prove someone is truly impaired, Lynn says it could serve as evidence someone used cannabis recently.

Hound Labs hopes to sell the breathalyzer to police departments starting this year, including to some in the Bay Area and elsewhere in California. The company would not disclose which departments, citing confidentiality agreements. Lynn says his company is contacted by agencies from all over the world.

“Everybody’s dealing with the same issue,” he says.

A marijuana breathalyzer could mean big business.

Hound Labs says it has raised about $14 million, including $8.1 million from the elite Silicon Valley fund Benchmark.

Yes, There’s an App For That

Not everyone buys into the breathalyzer idea.

Dale Gieringer, director of California NORML, says a breath test “will be no more effective than the ineffectual blood tests and urine tests and oral swab tests that they’ve got now.”

Gieringer says the important thing in setting a DUI law is how risky someone’s behavior is. And marijuana, he says, just isn’t that big an increased risk — “less than the risk of having two other people in the car with you, not to mention the risk of being a driver under 25 or being a male driver.”

Exactly how marijuana stacks up against those other risks is disputed by different studies. But most research shows marijuana having only a small effect on driving.

Gieringer says for marijuana — or any drug — we need a way to measure if you’re really safe to drive, not just if there’s a chemical in your system.

Gieringer has an app in his pocket designed to do just that. It’s called “My Canary.” It asks him to remember a series of numbers, and then do a balancing test.

Dale Gieringer, the head of California NORML, uses a sobriety-testing app called My Canary. (Eli Wirtschafter/KQED)

Gieringer is on a statewide Highway Patrol task force that will suggest policies for drug DUIs. He says apps like this one could give a more fair assessment of whether someone is impaired, no matter what they’ve used. Apps could help people measure their own impairment before they get on the road.

Meg Schwarzman, who was hit by a stoned driver, says we need not just better testing, but also better public awareness.

She says there has been such a “cultural shift” because of our growing acceptance of the benefits of medical marijuana, that we now “may be losing sight of some of the places that we actually have to be really cautious about it.”

A version of this story originally aired on KALW’s “Crosscurrents.”


Ask Bay Curious a question ...

How Much Marijuana Is Too Much to Drive? 2 January,2018Eli Wirtschafter

  • Michael Milburn

    Working memory, what the My Canary app measures, is not related to driving impairment. Instead, 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.

    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 week 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

  • Jamie

    There is no causal relationship established between cannabis and the incidents described in this piece.

  • lovingc

    The Drug and Alcohol Crash Risk report, produced by the Department of Transportation’s National Highway Traffic Safety Administration, found that while drunken driving dramatically increased the risk of getting into an accident, there was no evidence that using marijuana heightened that risk. In fact, after adjusting for age, gender, race and alcohol use, the report found that stoned drivers were no more likely to crash than drivers who were not intoxicated at all.
    It’s worth taking a closer look at that 2015 NHTSA study, because federal officials put a lot of stock in it as “the first large-scale [case control crash risk] study in the United States to include drugs other than alcohol.” Data was collected from more than 3,000 crash-involved drivers and 6,000 control drivers (not involved in crashes) over a 20-month period in Virginia Beach, Virginia. The data was fresh and solid: Research teams responded to crashes 24 hours a day, 7 days a week. Drivers were considered THC-positive if they tested for active THC, not for non-impairing metabolites still in their blood days or weeks after consumption.
    While THC-positive drivers were 5% more likely to be involved in a crash, the researchers found that drivers who’d taken an opioid painkiller had a 14% greater risk of crashing. Here’s a chart from that NHTSA study comparing THC (marijuana) with opioids (narcotic analgesics) and other drugs:
    Those levels of increased risk were tiny, however, compared to the risk involved with alcohol. Drivers within the legal range of blood alcohol level as registered by a breathalyzer (BrAC) were found to be 20% to 222% more likely to be involved in a crash. At .08 BrAC, the legal limit, the risk increased to 293%. At 0.15 BrAC, drivers were more than 12 times (+1118%) more likely to be involved in a crash than a sober person. Here’s a chart from that same study, calculating the increased risk of crashing at rising blood alcohol levels:
    By comparison, a driver who has taken penicillin is 25% more likely to be involved in a crash. Drivers carrying two or more passengers are 120% more likely to crash. Drivers using mobile phones to talk or text are 310% more likely to crash.
    A separate NHSTA study (“Marijuana And Actual Driving Performance”) further conceded it’s “difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects … Drivers with high concentrations showed substantial [impairment], but also no impairment, or even some improvement.” In other words, cannabis affects different drivers in different ways, depending on a number of factors.

  • Suzanne Cerny

    I’m totally against marijuana. What causes the high? It slows down the way a thought moves from one nerve synapse to another. So if you’re high when driving and you see a person walking in a crosswalk you might hit him before you get the realization that you really should slow down and stop. Cocaine is the next fun drug after pot so you really are not going to stop because now you’re on speed. Educationally we are going to have a next generation of monsters. The KQED station today said Opiods is the greatest cause of deaths now, and the age limit of healthy people living is dropping because of it.

    • Duncan20903

      Utter fiction Suzanne.

    • Dave C

      Totally BS

    • drbobmelamde

      You have no idea what you’re talking about, psychoactive cannabinoids are in mothers milk are you against breast-feeding as well?

    • Tom Kelley

      You just mentioned two other drugs and linked them with cannabis use. Thought process is much more complex than a single synapse firing, and bio electric speed would be a constant like the speed of light.

  • Tom Kelley

    As much as I sympathize with this woman for her injuries, I have to wonder if there were other factors involved in the driver’s impairment. Was alchohol a factor ? Fatigue? Distraction?


Eli Wirtschafter

Eli Wirtschafter is a freelance reporter and producer focusing on transportation, art, and activism. He previously interned with The California Report and KQED Local News. Eli is also KALW’s transportation reporter, and a graduate of KALW’s Audio Academy. Eli’s work has been heard on NPR, Here & Now, and BackStory. Find him on Twitter at @RadioEli.

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