Most Cannabis Consumers Are Confident to Drive When They’re Most Intoxicated, Study Finds

Many cannabis-high drivers consider themselves safe to drive when they’re actually at their most impaired, according to a new study.

Published in JAMA Psychiatry, the study involved 191 regular cannabis consumers, who were asked to smoke cannabis or a placebo and then complete a driving simulation.

The researchers found that the cannabis-high participants were consistently worse at the simulation than the control group, particularly 90 minutes after smoking – just as most of the high participants claimed they were safe to drive.

A high-quality simulation

To test how THC affects a person’s ability to drive, the research team from the University of California, San Diego, recruited 191 regular cannabis consumers who could drive.

The recruits were asked to smoke seemingly identical cigarettes, which actually contained either 13.4 percent THC, 5.9 percent THC, or 0.02 percent THC (the placebo).

They were then asked to complete a 25-minute-long digital driving simulation.

“When people drove [in the simulation], it emulated city/country driving,” Thomas Marcotte, a professor of psychiatry at the University of California, San Diego, and lead author of the study, said in an interview with JN Learning.

“So they would have to drive through residential zones [and] business districts. They’d have to respond to what we call the amber-light dilemma – the lights turning yellow, ‘Should I go? Should I stop?’ They had to merge onto the freeway.”

The recruits also gave blood samples and were questioned (“would you drive in your current state?” etc.) before and after the simulation.

Compared to the placebo group, the high participants were significantly worse drivers and drifted out of their virtual lanes much more frequently.

Interestingly, though, there was little to no difference between the results of the high drivers that had smoked the stronger 13.4 percent THC cannabis and the drivers who smoked the weaker 5.9 percent THC cannabis.

But, according to the researchers, this is likely because the participants titrated their cannabis to match its strength (the recruits given the strong cannabis smoked less than the recruits given the weaker joints).

As for the high drivers’ opinions on their own performances, 47.5 percent said they’d be able to drive safely 30 minutes after smoking. This proportion increased to 68.6 percent 90 minutes after smoking.

However, at this same point in time (90 minutes after smoking), most of the high drivers were actually at their most impaired, according to their performance scores in the simulation.

The participants did start to sober up and improve their driving three hours and 30 minutes after first smoking, but many were still driving more poorly than their placebo-counterparts at the four hour and 30-minute mark.

“It’s important that people know that impairment is present, even in regular users,” Marcotte remarked. “They should be aware of the perception-performance disconnect following smoking, understand that reduced performance in some people lasts many hours, and that experienced users shouldn’t necessarily assume that they’re less impaired just because they’re familiar with the product.”

“And on the law enforcement end,” he added, “it’s important that law enforcement officers understand that not everyone becomes significantly impaired when using. And you can’t necessarily assume impairment based upon the THC content of the product.”

Marcotte and his team also found that the blood-THC levels taken from the participants had little correlation with the recruits’ driving scores. As such, they have urged that driving-sobriety tests move away from measuring blood-THC levels and take a more holistic approach.

“Really what we need are measures like the field-sobriety test,” Marcotte said, “other measurements of impairment beyond just looking at biological measurements such as blood concentrations.”

Cannabis behind the wheel

Several previous studies have also found a link between THC use and poorer driving.

In one study published in 2020, most of the participants who vaped 13.75 milligrams of THC became significantly impaired and had poor performance scores when asked to drive a car on an actual road. Three drives were even cancelled before they began because the participants, who had all vaped THC, said they were worried about their ability to drive.

Interestingly, though, several participants in the same study were asked to vape 13.75 milligrams of CBD, and these recruits drove perfectly fine.

However, as THC-heavy cannabis products remain popular among many cannabis consumers, the risk of cannabis-impaired drivers on the roads remains in many areas with legal access to the drug.

Several studies have found that such jurisdictions tend to experience a “small but significant increase” in fatal motor vehicle accidents.

Published in 2021, one study estimated that Canadian cannabis legalization could have contributed to 308 additional deaths due to cannabis-impaired driving.

Perhaps unsurprisingly, perceptions about the risks of driving under the influence of cannabis also appear to be colored by a person’s affinity for the drug.

One study published in 2019 found that more than half of American cannabis consumers would feel safe if they were in a car with a high driver. On the other hand, three-quarters of respondents who didn’t consume cannabis said that they would feel somewhat-to-very unsafe.