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Driving Stoned: San Diego Scientists Try To Find DUI Limit For Marijuana

Tom Marcotte, co-director of UC San Diego's Center for Cannabis Research, sits at the wheel of the center's  driving simulator, April 17, 2017.
Matt Bowler
Tom Marcotte, co-director of UC San Diego's Center for Cannabis Research, sits at the wheel of the center's driving simulator, April 17, 2017.

This is the final story in a three part series. Click here for Part 1 and Part 2.

The Problem

On Friday and Saturday nights, according to roadside surveys conducted by the National Highway Traffic Safety Administration, one out of five drivers has a drug other than alcohol in their system.


The agency says the drug that showed the greatest increase between 2007 and 2014 was marijuana.

The National Institute on Drug Abuse says after alcohol, marijuana is the drug most commonly involved in crashes.

Despite this, some argue marijuana does not necessarily make a driver unsafe.

Like one San Diego woman, who asked us not to use her name because she does not want her boss to know she smokes pot. She said marijuana makes her a better driver.

“I think it’s just because the weed makes me feel much more relaxed, and I don’t feel rushed, because most of the time I feel rushed just because the nature of my job," she said. "But when I’m smoking weed, I can go whatever pace I want to.”


But others could smoke the same amount of pot as this woman and feel totally out of it.

So when does someone cross the line and become impaired? Researchers at UC San Diego are trying to find out.

Driving Stoned: Part 3
Driving Stoned: San Diego Scientists Try To Find DUI Limit For Marijuana
California has a legal limit for alcohol intoxication, but it does not have one for marijuana. Researchers at UC San Diego are working on trying to determine how much pot leads to impairment.

The Study

On a recent weekday afternoon, Tom Marcotte showed a visitor a driving simulator, located in a building just across the street from UC San Diego Medical Center in Hillcrest.

“So this is the room where we do the driving simulations," he said. "We have our participants sit here in this seat, and go through a fully-interactive driving simulation.”

Marcotte is the co-director of UCSD’s Center for Cannabis Research. He is also the co-principal investigator of a state-funded study that aims to establish when a person is too stoned to drive.

People in the study are given different amounts of pot of varying strengths, and then asked to take a spin in the simulator.

“We have things where people need to make left-hand turns across oncoming traffic, which requires judgement of time and distance and all of that," Marcotte explained, as he sat behind the wheel of the simulator. "And if you go too fast, you’ll squeal tires, and if you go too slow, you may be in an accident.”

Researchers will be testing participants immediately after they smoke, and then hours later, to see how impairment changes over time.

Dr. Igor Grant, director of the Center for Cannabis Research, said to make sure the results aren't biased in any way, it's a double-blind study. That means neither participants nor researchers will know if "drivers" are smoking a real marijuana cigarette, or one in which the THC has been removed.

THC is the psychoactive compound that gives marijuana users a high.

“We’re monitoring blood levels, saliva levels, expired breath, all those kinds of things," Grant said, "and using not only driving, but other kinds of cognitive tests to see how impaired a person really is. I think based on that, maybe we can develop a better indicator of impairment.”

The study is just getting underway, so it will be some time before any results are announced.

Grant said previous research suggests in modest doses, marijuana does not greatly increase the risk of accidents. But that is not to say he is encouraging people to drive stoned.

“My point is that we don’t know the exact criteria to detect intoxication, how long after a person has taken the marijuana are they unsafe to drive, and these are the things we need to answer," he explained.

A lot is riding on it.

Marijuana is now legal for recreational use in California, and retail sales are set to begin next year. But the state has not established a legal limit for marijuana intoxication.

Is Any Amount Safe?

But Scripps Mercy Hospital’s chief of staff, Dr. Michael Sise, believes any amount of marijuana makes someone unsafe to drive.

In his decades of experience as a trauma surgeon, Sise has seen too much carnage caused by drug-impaired drivers.

“Basically, buzzed driving is dangerous driving," he said. "So, arguing over amounts right now is probably not the way to think about it. And I think all of us should be responsible drivers. Bottom line is, if you’re buzzed, from whatever, you should not drive.”

KPBS has created a public safety coverage policy to guide decisions on what stories we prioritize, as well as whose narratives we need to include to tell complete stories that best serve our audiences. This policy was shaped through months of training with the Poynter Institute and feedback from the community. You can read the full policy here.