When a driver registers a 0.08 or higher on a blood-alcohol test, drawing an OUI and all the expensive, life-altering consequences that come with it, there is little doubt that they were impaired while driving.

In fact, many people are at least slightly impaired well before they reach that level, but the law gives those people some leeway by setting the strict standard high enough so that unimpaired drivers are not mistakenly charged and prosecuted.

There is a temptation to apply the same logic to those driving under the influence of marijuana, by establishing a measurable limit to how much of the drug one can have in their system before it becomes to unsafe to drive. Go over it, and you get charged with operating under the influence, no questions asked.

But the science just isn’t there yet, and any attempt by Maine to force such a legal threshold will result in bad convictions, and not make roads any more safe.

Unfortunately, that’s the recommendation of the majority of a working group charged by the Legislature with studying the issue.

In its report released last week, most of the group agreed that it should be a crime to operate a vehicle with a level of THC, the primary psychoactive ingredient in marijuana, of 5 nanograms or more per milliliter of blood.

That’s the law in Colorado, Washington and Montana, where it is used in the same way as blood-alcohol content to determine an OUI charge.

Except that it is nothing like a blood-alcohol test.

Alcohol enters the body and is metabolized quickly. Its presence in the bloodstream and its impairing effects occur at the same time. As a result blood-alcohol level corresponds well with impairment.

Marijuana metabolites, however, remain in the body longer, after the impairing effects have passed, particularly for medical marijuana patients and frequent recreational users.

The blood test, then, can only tell for sure that someone has used marijuana in the recent past, maybe a day or two before. It says nothing definite about impairment.

Yet if the study group’s recommendation is followed, a positive test would bring a charge, loss of license, increased insurance rates, and possible jail time just the same.

And there is no evidence it would make Maine roads any more safe.

In the first full year after marijuana was legalized in Washington state, the number of drivers testing positive increased by nearly 25 percent, but there were no more car accidents or fatalities.

In fact, there is a lot of research to suggest that there is no link between a positive test and safety behind the wheel, either because the test does not actually gauge impairment, or marijuana does not have the same effect on driving ability that alcohol does. It’s probably a combination of both.

In any case, driving while impaired on marijuana is already illegal in Maine. Police now use officers specially trained in drug recognition to identify those drivers.

In court, that’s not as clear-cut as a blood-alcohol test. But until tests for marijuana improve, it’s the best we’ve got.

Maine should increase the number of drug recognition officers — as recommended by the study group — and forget about a law that would do nothing but fill court dockets.

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