When the ACT introduced zero-tolerance drug-driving laws in 2011, there were immediate concerns that motorists who had accidentally or innocently ingested cannabis might find themselves falling foul of the law. So it has proved. There have been at least two cases of Canberra motorists who, despite claims they had neither smoked nor ingested the drug, tested positive for low levels of tetrahydrocannabinol or THC, the chemical responsible for cannabis's psychoactive properties. Both escaped conviction after persuading magistrates that the THC in their blood was the result of eating muesli which contained hemp seeds.
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There have been other unintended consequences, too. A motorcycle rider badly injured after hitting a car which pulled out in front of him was found to have traces of THC in his blood and subsequently charged, even though he had not been impaired by cannabis at the time of the crash. The man, who was not an habitual drug user, had allegedly smoked a small amount of cannabis some two days before the accident. Though he was not at fault, a mandatory drug and alcohol test after the crash revealed traces of THC and he was charged by police. Like the two other motorists, the rider convinced the magistrate that the THC present in his blood was not enough to impair his driving. He received a non-conviction order, a hollow consolation given his injuries.
Attorney-general Simon Corbell has defended the zero-tolerance approach to THC by arguing that since cannabis use is illegal in the territory, drivers should have no quantity whatsoever in their blood. Given cannabis slows people's reaction times, and impairs their ability to make the split-second decisions required of everyday driving, that proposition is correct and warranted. But what of the body's tendency to retain marijuana traces long after the harmful effects have dissipated? Or of THC traces originating from the ingestion of hemp seeds bred for nutrition rather than their ability to impart a high? The rate of THC metabolism varies, but average occasional cannabis users test positive for up to four days after smoking. To state that an individual's driving ability remains impaired up to and including the time when the last of the THC in his or her bloodstream is metabolized is drawing a somewhat long bow.
The police's reasoning that cannabis users ought to know the law and not drive while under the influence (and for some days afterwards) is a reasonably compelling one – but for the argument (apparently accepted by ACT magistrates) that minute quantities of THC do not significantly impair driving ability. A regime where the administration of drug-testing was determined primarily by suspicion of driving impairment would be the commonsense approach to solving this conundrum. However, police would argue that drivers impaired by cannabis do not always present as such, just as drink-drivers frequently maintain a semblance of orderly and law-abiding progress on the road.
Nonetheless, the anomalies revealed thus far suggest a legislative review of the zero-tolerance approach to drug-driving (particularly as they pertain to cannabis) may be in order.
