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The police would probably have a case if they pulled you over like this

THC tests for people driving high don’t actually work

The current legal limit for driving after smoking weed isn’t backed by science, and a new U.S study says it’s leading to wrongful convictions

Catching drunk drivers is pretty simple, after maybe doing the walking in a straight line thing and reciting the alphabet backwards, you'll blow into a breathalyzer that tests your blood-alcohol concentration. Then, you're either on your merry way or heading towards a big fat DUI. With cannabis, it's a lot less simple, and a new study in the U.S found that the current testing methods are hugely unreliable.

The AAA Foundation for Traffic Safety has released findings on blood tests that measure THC – the psychoactive component in cannabis – levels, and they believe trying to calculate the amount of marijuana in someone’s system is pretty much useless for determining how impaired your driving is. Low amounts of THC in blood may make some people act really, really stoned out, and vice versa. Essentially there isn’t much correlation when it comes to these physical tests and how fit you are to drive.

States such as Colorado, Montana, Nevada, Ohio, Pennsylvania and Washington have legal limits for THC in place, which the AAA is requesting they revise. The report found the tests are “arbitrary and unsupported by science, which could result in unsafe motorists going free and others being wrongfully convicted for impaired driving”.

Researchers went through the arrest records of drivers charged with the drug driving offence, examining their toxicology reports and Drug Recognition Expert exams (like standing on one foot and walking in a line). They compared 602 driver’s DRE exam results, those of which had THC in their system, to 329 sober volunteers who took the test.

Initially, the under-the-influence drivers fared worse than the sober volunteers: 6 per cent of the drivers passed the physical test, whereas over half of the volunteers who succeeded.

However, researchers then found no credible correlation when it came to blood tests. With the drivers, THC levels bounced from 1 to 47 nanograms per mil of blood, and more THC didn’t automatically spell failure for many in the physical tests. A lot of subjects who reached above the legal limit for Colorado, for instance, passed the DRE exam.

The reasoning for this ranges from a smoker’s frequent or infrequent use of the drug, the level of potency and amount used. THC can be burnt quickly by different metabolisms, so it maintains a varied effect on everyone.

“Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported,” the study found.

The report concludes with advice for police to assess the behaviour and psychological state of a subject, until actual science-backed measures come into place to ascertain whether a driver is too high for the roads.