A researcher from the University of Sydney wheeled a driving simulator into Royal Prince Alfred Hospital. Picture an arcade racing game built for science: a real car seat, steering wheel and pedals, wraparound screens showing the road and sensors recording everything the driver did. How fast they braked. How smoothly they steered. How much the car drifted within its lane.
In the study, volunteers vaped a measured dose of cannabis. Then they drove while the computer logged their every move. An hour later, they drove the same route again. Then again after that and again, right throughout the afternoon. Why keep repeating it? Because they wanted to know whether cannabis affected driving and how the effect changed as time passed. By testing the same people over and over throughout the day, they could watch any impairment arrive, reach its peak and then slowly wear off.
That is exactly what they saw. Soon after vaping there was a mild effect on driving. As the hours ticked by, it faded. Before the day was out, the volunteers were driving normally again. The impairment was real, it was modest and it did not last! Next the team needed people driving a real car at real speed in real traffic. You cannot legally do that in Australia, so they took the experiment to the Netherlands, one of the few places in the world where this kind of study is allowed.
The car had a second steering wheel and pedals on the passenger side, so a driving instructor could take over instantly. Volunteers vaped just under 14 milligrams of THC—an amount that’s within the range a doctor might prescribe—then drove 100 kilometres on a live motorway at highway speed while instruments measured how steadily they held their lane.
Each person did the driving test twice. Once in the first hour after vaping and once again four to five hours later. In that first hour there was a mild wobble, the kind of small effect you would see in a driver with a blood alcohol reading somewhere between 0.02 and 0.05, at or below the legal alcohol limit. By the second test, four to five hours on, it had gone.
Something else stood out. Unlike drivers impaired by alcohol, these drivers drove carefully. They eased off the speed and left bigger gaps to the car in front, as if they sensed they needed a wider margin and gave themselves one. The study was published in 2020 in JAMA, one of the most respected medical journals in the world.
By 2021, the same team of Sydney researchers, now with Danielle McCartney leading, stepped back to look at the whole field. Researchers around the world had run dozens of studies like theirs. McCartney’s team gathered every study that met a strict quality bar, pooled the raw results and modelled how long it took for driving skills to recover after people consumed cannabis. The pattern seen in the earlier research held.
Here the work crossed from interesting findings into settled science: the impairment window has a clear, predictable shape. Almost everyone in those early studies, however, used cannabis only occasionally. Patients differ from other cannabis consumers: they tend to be older and often use the same strong product every single day.
So researchers at Swinburne University tested actual patients on their own prescribed medicine. The results sat in a surprising range: from slightly impaired at one end to unchanged, or even slightly improved, at the other, with a minority showing no impairment at all. Even soon after a dose, many patients retain most driving skills. So the people who use their medication most often, turn out to be the people with the least impairment.
Population level risk has been assessed too. Step back from the lab and look at the whole road, every driver, every crash, across years of pooled data and the picture holds. Among all the things that can affect driving, cannabis sits low on the risk register.
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Evidence reviews put a cannabis-positive driver at around 1.1 to 1.4 times the crash risk of a sober one. This is in the same ballpark as a blood alcohol reading at or below the legal limit (.05) and is less impairment than drivers experience with common prescription medicines like opioids and benzodiazepines. Across the whole population, the presence of THC barely shifts crash risk and cannabis contributes to only a small share of total crashes. The real risk sits with one small group: people driving soon after a heavy dose—exactly what a good recent-use test would catch. |
We already know what happens to the road stats when patients are allowed to use their medicine and drive, because plenty of other places have already done it. In US states that legalised medical cannabis researchers have tracked road deaths over several years. Those states saw fewer fatal crashes, for both men and women. Part of the reason traces to patients shifting off riskier medicines like opioids. Letting sick people use their medicine left those roads safer.
The US is just one example. Many countries run medicinal cannabis schemes, among them the UK, Germany, Ireland, Norway and New Zealand, and they let prescribed patients drive whenever they remain unimpaired. Every one of them treats a trace of THC more sensibly than our law does. If protecting patients on the road carried the danger the critics claim, those countries would show it first. Years on, their roads look fine.
That’s the evidence. It’s a decade deep, it’s peer-reviewed and it’s clear.
So can we test impairment? If science can predict impairment, could police simply measure impairment directly at the roadside, instead of testing saliva for a trace? The practical answer is that there is no quick, objective roadside test for cannabis impairment. Roadside impairment assessments, where an officer watches someone balance and turn, are slow, subjective and easy to argue about in court. They do not scale to the side of a highway at two in the morning.
That leaves us with the real question. Can a saliva test be made to sort people fairly, even though it cannot read impairment directly?
It can, once you set a sensible threshold grounded in the science and stop treating every trace as a crime. There’s two ways to test for THC in saliva, and a lot of the confusion in this debate comes from mixing them up. One cannot measure the concentration, it just tests if there is any presence at all, even a trace amount is enough to register a positive test.
The other method is measuring the actual amount of THC in the saliva: the ‘concentration number’. These tests can be set to different cut off levels, known as ‘per se threshold tests’. Under the current Queensland zero-tolerance law any presence above zero is an offence. But presence tells us nothing about impairment, it can’t tell how long ago the patient consumed.
Multiple clinical studies have been conducted using the higher quality tests that measure concentration. These studies have also found a clear time based pattern. Researchers asked frequent and occasional cannabis users to smoke under controlled conditions, then measured the THC in their saliva again and again over the next thirty hours.
The typical timeline for THC concentration spikes quickly after inhalation, often reaching several hundred nanograms per millilitre. Then it falls off a cliff, both for occasional and frequent users. Within hours it’s a small fraction of that and by the next day, in occasional users, it drops to zero. However, for the average frequent user, the concentration does obviously drop, but not to zero. Given patients are often prescribed daily use, if the threshold is set too low, for example, a cut-off below 10ng/ml, the average frequent user would test positive at 20 hours.
This is just the average, this does not take into account any outliers. The great majority of regular users would still register a positive reading, long after all effects have cleared. The low threshold cut-off sets the bar so close to zero that the trace trips it. So looking closely at what a zero tolerance regime actually does, it checks for one thing only: whether you have metabolites of THC in your system at all, from anytime in the past few days or more.
A patient who dosed the night before and drives perfectly safely the next morning tests positive just the same as someone who used cannabis twenty minutes ago. Thus the law is actually checking for trace residues and convicting the sober.
The referenced studies (see below) have measured how THC affects driving and how that differs between regular and occasional users and found the impairment real but modest and that regular users, with their built-up tolerance, feel it least of all. The studies measured how long impairment lasts and found a predictable clock: a few hours after vaping, up to around eight hours after edibles.
The roadside test threshold needs to sit high enough to pass a patient who has waited the right time, including a daily user who carries a high personal baseline of THC residue, while still catching anyone who used too recently. Get the level right and the test stops chasing yesterday’s medicine and starts measuring what matters: recent use.
References:
- Arkell, T. R., McCartney, D., & McGregor, I. S. (2021). Medical cannabis and driving. Australian Journal of General Practice, 50(6), 357–362.
- Arkell, T. R., Vinckenbosch, F., Kevin, R. C., Theunissen, E. L., McGregor, I. S., & Ramaekers, J. G. (2020). Effect of cannabidiol and Δ9-tetrahydrocannabinol on driving performance: A randomized clinical trial. JAMA, 324(21), 2177–2186.
- Manning, B., Arkell, T. R., Hayley, A. C., & Downey, L. A. (2024). A semi-naturalistic open-label study examining the effect of prescribed medical cannabis use on simulated driving performance. Journal of Psychopharmacology, 38(3), 247–257.
- McCartney, D., Arkell, T. R., Irwin, C., & McGregor, I. S. (2021). Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review. Neuroscience & Biobehavioral Reviews, 126, 175–193.
- Newmeyer, M. N., Desrosiers, N. A., Lee, D., Mendu, D. R., Barnes, A. J., Gorelick, D. A., & Huestis, M. A. (2014). Cannabinoid disposition in oral fluid after controlled cannabis smoking in frequent and occasional smokers. Drug Testing and Analysis, 6(10), 1002–1010.
Adapted from, They say the science on cannabis and impairment doesn't exist. Here it is by Unharm
21 September, 2026