Currently, laws in states where marijuana is legal rely on legal blood limits of THC to test for impaired driving. A new study reminds us that this method is, for lack of a better word, dumb. Released by the AAA’s Foundation for Traffic Safety, the report finds that the amount of THC in a human’s bloodstream is not an accurate barometer of their performance behind the wheel. The correlation for alcohol remains clear, but marijuana’s effects on the body are a lot less straightforward.

Simply put, the body metabolizes marijuana differently than it breaks down alcohol. Some marijuana users can have soaring THC blood levels and function relatively normally while the others will pull a Maureen Dowd. As if this didn’t already make it difficult to settle on an objective standard for measuring how stoned a person is, many of the drivers assessed by the study had also been drinking, making it even harder to tease out the actual effects of marijuana use.

When arresting individuals for driving under the influence, officials measure weed impairment using two types of tests. There’s the Standardized Field Sobriety Test, where arrestees walk and turn, stand on one leg, and try to touch their nose, and the objective drug test, which measures blood THC concentration. Using data collected from these arrests, AAA found that a substantial number of the drivers that tested positive for THC did not show impairment in the physical tests.

The legal blood THC concentration threshold in Colorado, Washington, and Montana, is currently 5 nanograms per milliliter of blood; in other states, such as Illinois, it’s forbidden outright. But the AAA report suggests all of those measures are total BS — and could be leading to the prosecution of innocent people.

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

Until scientists figure out a better way to objectively assess marijuana impairment, AAA urges state officials to rely on human assessments, not tests, to gauge whether a person is too stoned to drive.