Impairment tests used by authorities in U.S. states where marijuana use is legal in some form have no basis in science, and their results essentially mean nothing, a recent study concludes.

Commissioned by the American Automobile Association’s safety foundation, the study found that no blood test for THC, the active ingredient in marijuana, can accurately determine a driver’s level of impairment, the Associated Press reports.

The finding blows law enforcement’s main method of convicting high drivers into the weeds.

In five of the six states that recently legalized marijuana, a motorist is deemed guilty of impaired driving if their blood level of THC passes a certain threshold.

Because impairment levels are highly variable and depend on the individual, rather than blood THC levels, the tests lead to some motorists being wrongfully charged, with others going free, despite being high as a kite at the time of the test.

Regular users, be it stoners or medicinal users, often show lower levels of impairment than intermittent users. Their blood, however, often contains THC for longer periods than a once-in-a-while weed smoker. TCH tests that minic the blood alcohol tests administered to drunk drivers won’t recognize these distinctions.

AAA wants the tests scrapped and replaced with an observation-based system, where trained officers analyze a number of physiological and behavioral indicators to determine a person’s level of impairment.

“There is understandably a strong desire by both lawmakers and the public to create legal limits for marijuana impairment in the same manner we do alcohol,” said Marshall Doney, AAA’s president and CEO. “In the case of marijuana, this approach is flawed and not supported by scientific research.”

The six states that use the blood test — Ohio, Nevada, Washington, Colorado, Montana and Pennsylvania — could be joined by more states this fall. Their ranks could double, in fact, as lawmakers seek a way of detecting high drivers.

The study is a buzzkill for law enforcement members and state-level bureaucrats, but if the sole purpose of the THC blood test is to make roads safer from impaired drivers, its findings beg for acknowledgment.

North of the border, Cannabis — erm, Canada — plans to legalize marijuana use in the near future. The study will be of interest to their authorities, too.

A New York University professor quoted by the Associated Press said there’s a number of marijuana-related laws in need of fixing. Mark A. R. Kleiman, who specializes in drug and criminal policy, said several states have laws against motorists having any THC in their bloodstream, despite the fact that trace amounts can linger weeks after the individual became sober as a judge.

A noisy child in the backseat of a vehicle is as big a risk factor as using marijuana, Kleiman said.

[Image: Check Grimmett/Flickr (CC BY-SA 2.0)]