How Much Marijuana Is Too Much to Drive?

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Mike Lynn, CEO of Hound Labs in Oakland, which plans to sell a marijuana breathalyzer to law enforcement agencies next year. (Eli Wirtschafter/KQED)

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.”