In 2016, the latest year for which data was available, the devices were used to prosecute more than 10,000 cannabis users for drug driving in NSW. Drivers who are roadside drug tested first have their saliva tested using a "Securetec DrugWipe" a device that is dragged over the tongue for the detection of an illicit substance. In NSW, ecstasy, cannabis, cocaine and methamphetamine are tested at the roadside. If a positive reading is detected, drivers are taken into an MDT van, where a second saliva sample is reviewed by the Draeger DrugTest 5000. The study found the Securetec DrugWipe gave a false positive reading when saliva THC concentrations were very low or negligible 5 per cent of the time and gave a false negative result 16 per cent of the time. The Draeger DrugTest 5000 gave a false positive reading 10 per cent of the time and a false negative 9 per cent of the time.

The tests are reviewed for a third and final time before prosecution at a forensic facility in Lidcombe which is said to be extremely "sensitive" to any amounts of THC. "If you've been pinged by the first two, your chances of escaping the third are very, very low," Professor McGregor said. "We need better science and better guidance around when cannabis users are safe to drive. "You can't just say to someone using medicinal cannabis that you can't drive ever again, and that's kind of what they're being told." Whether a cannabis user tests positive roadside also greatly depends on the make-up of their saliva, Professor McGregor said. "You've got this real problem where some people who have 50 times more THC [in their] saliva may not be pinged, compared to someone who has a different type of saliva," he said.

Loading A user is also able to avoid a positive THC reading easily by the way in which they use the drugs, Professor McGregor said. "If you were to take THC capsules, then no THC goes into your saliva; it just goes straight into your tummy and into your bloodstream," he said. "You could take 10 of these and be completely intoxicated with THC and it wouldn't come up in your saliva." Professor McGregor said police were "not forthcoming" in how low THC levels had to be to produce a positive reading but recommended a 12-hour waiting period after taking cannabis so as to be able to drive without impairment. The study, therefore, attempted to replicate the researchers' "best guess" of what the cut-off would be for a driver to test positive roadside.