Recreational cannabis use is on the rise around the world – and so is the number of car accidents involving people who have taken the drug. In the US, for instance, a report published last year found that the from 9 per cent in 2000 to 21.5 per cent in 2018. The International Council on Alcohol, Drugs & Traffic Safety, meanwhile, noted in 2020 that using cannabis .
Such figures suggest there is an urgent need to define legal limits for driving under the influence of cannabis – equivalent to those associated with alcohol. In line with this, 12 US states have introduced a zero-tolerance policy, with drivers penalised if their blood is found to carry any tetrahydrocannabinol (THC) – the main psychoactive component of cannabis.  and several European countries, including the UK, can expect a fine if the amount of THC in their blood exceeds certain limits, usually between 1 and 5 nanograms per millilitre.
But the science is more complicated. “Many studies have shown that the level of THC in blood is really no indication of anything,” says at the Colorado School of Public Health. “[It] does not indicate impairment.”
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Assessing THC levels using other techniques comes with its own problems. Even using a driver’s behaviour to evaluate whether or not they have consumed cannabis and pose a danger to other road users is not straightforward. So why is it such a challenge to identify when someone is too high to drive?
After alcohol, cannabis is the most likely drug to be detected in a driver’s body, and millions of people drive after they have taken it. For instance, in 2018 the US Centers for Disease Control and Prevention found that approximately – about 4.7 per cent of the adult population. In the same year, 5 randomly selected at traffic stops in Spain tested positive for THC.
There are some indications that these drivers pose a danger to other road users and can be spotted by the way they drive.
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“Individuals who are high slow down, but have poorer control over their vehicle,” says at the University of British Columbia in Canada. Simmons was part of a research team that analysed 57 studies involving a total of 1725 participants to determine how cannabis use influences driving. The results, published last year, suggested that  to raise the blood-alcohol concentration to around 0.05 per cent – which typically occurs . Taking alcohol and cannabis together, which is common, leads to even worse driving.
Results like this would seem to suggest spotting stoned drivers should be fairly easy but other research suggests it is more complicated.
In the US, specially trained law enforcement officers – known as drug recognition experts (DREs) – may stop drivers they suspect are impaired to conduct field sobriety tests at the roadside. They measure drivers’ vital signs and evaluate their behaviour to judge whether or not they have recently consumed cannabis and are a danger to other drivers. But to do so accurately is a huge challenge, says at the University of California, San Diego.
“Give me 3 or 4 hours in a room doing neuropsychological tests and I am still not sure I can be great at predicting how someone who consumed cannabis is going to drive,” he says. At the roadside “you can only get one brief snapshot of how a person is doing”. In research published in August, Marcotte and his colleagues recruited 184 drivers and 11 DREs to investigate the accuracy of field sobriety evaluations. The drivers all smoked cannabis cigarettes prior to the tests, but only some of them contained THC.
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The DREs classified 81 per cent of the drivers who had consumed THC as showing signs of a drug-induced cognitive impairment. But they also made the same determination for 49.2 per cent of those who had smoked the placebo cigarettes. Marcotte and his colleagues concluded that field sobriety tests could reasonably safely.
He says that DREs could avoid some of these false positives if they followed up their behaviour-based field sobriety evaluations with a chemical test for recent cannabis use. But here, again, the question of interpreting the results of such tests becomes relevant.
In 2021, at Harvard University and his colleagues that measures THC levels in saliva to determine whether someone used cannabis in the past 12 hours, with results in just 5 minutes. Called EPOCH, the device looks like a high-tech version of an at-home covid test, with a swab for collecting saliva and a liquid the saliva is mixed into before being dripped onto a detector. Results are then read out by a smartphone. Lee says the chemistry of THC molecules makes them harder to pick out than the ethanol molecules that alcohol breathalysers detect, so EPOCH uses special antibodies that react to THC.
However, the biggest challenge his team has had to grapple with is figuring out how to interpret EPOCH’s readings. Lee says there is no definitive answer on what result means someone is too impaired to get behind the wheel. So, EPOCH’s best use may be for collecting data on how THC levels in saliva correlate with cognitive impairment or with THC levels in blood, he says.
Blood levels of THC
But even here there are problems. While it is generally accepted that measuring the level of alcohol in the blood gives a good indication of someone’s ability to drive, the same may not be true when it comes to measuring blood levels of THC. For instance, in a study published last year, Brooks-Russell and her colleagues found that .
The study involved 85 people who used cannabis either daily, occasionally or not at all. The participants completed short driving scenarios in a car-based simulator, first in a baseline test and then after a 15-minute period where they were asked to smoke or vape as much legally self-procured cannabis as they wanted. The researchers measured participants’ THC blood levels before and after the smoking session.
“Daily users took a lot more inhalations, they used more of the minutes available to them to smoke, more of their products burned and their THC blood levels were way higher,” says Brooks-Russell. Unsurprisingly, this had an effect on driving performance: after smoking cannabis, the daily users drove at lower speeds and had slower reaction times.
“But the occasional-use group actually showed more declines in [performance] – despite using less product and having lower THC blood levels,” she says.
The findings are particularly interesting in light of a study published last year by and at the University of Connecticut. The two researchers surveyed drivers and found that after taking the drug than infrequent cannabis users.
Research like this suggests that measuring THC levels in the body isn’t a surefire way identify people who are too high to drive. Nevertheless, some companies are now commercialising technology to measure drivers’ THC levels. Proprietary technologies produced by firms including in Canada and the California-based use mass spectrometry to detect THC in breath samples.
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Hound Labs’s device was tested in a clinical study published in 2019, which involved 20 people. This showed – which the researchers presume is the period where driving performance is most impaired. In a statement to żěè¶ĚĘÓƵ, Hound Labs said that it is collaborating with researchers at the University of California, San Francisco, and third-party laboratories on testing the device further. Cannabix Technologies did not respond to żěè¶ĚĘÓƵ’s request for comment.
While most of Hound Labs’s marketing is aimed at employers who want to deter workplace cannabis use, the company stated that some of the early tests of the breathalyser were done with law enforcement groups in California. Some of these groups wanted to use the technology to corroborate drivers’ claims that they haven’t recently used cannabis.
Marcotte says breathalyser data seems promising for that specific use, but that a breathalyser reading still wouldn’t necessarily indicate anything about a driver’s impairment.
Brooks-Russell says that combining saliva or breath tests with cognitive assessments measuring memory and reaction time could be more accurate than current roadside practices. Ultimately, both researchers say that the most pressing issue is a dearth of data on cannabis use among drivers in the real world, outside of controlled laboratory settings.
Auguste says that more research into how cannabis impacts a person’s ability to drive could help better educate drivers on how to stay safe – similar to advice already available on counting their drinks before driving.
