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State high court: Field sobriety tests inadmissible for pot DUI

The Supreme Judicial Court of Massachusetts has ruled that field sobriety tests are not an appropriate measure for whether a driver is under the influence of marijuana. Although the ruling doesn't apply directly in Tennessee, it could be influential because it involved a review of the current science on accurately detecting marijuana intoxication.

Specifically, the Massachusetts high court addressed how police officers can legally testify about field sobriety tests they administer to drivers they suspect of being under the influence of marijuana. It is not illegal to administer the tests, but the court determined they have no scientific value. Therefore, officers' testimony must be limited.

The issue arises because marijuana is now legal in Massachusetts, so it's no longer illegal to drive with any amount of marijuana in your system. Instead, police must prove drivers are impaired.

There is currently no chemical test that can determine whether someone is "high" on cannabis. There are tests that can determine that a person has used marijuana, but they can only say whether the subject used within the last month or so, along with how much of the drug's psychoactive compound is present. However, the effects of marijuana differ from person to person, so no particular level in the blood has been conclusively tied to being high.

That lack of research is the basic reason for the Massachusetts' court's ruling. In the general rules of evidence, only expert witnesses can give opinions about the subject of a case. Police are routinely deemed expert witnesses on Breathalyzer test administration because the science connecting blood-alcohol level to intoxication is generally accepted by most scientists in the field.

When there is no general consensus around a particular scientific theory, however, the courts are not to certify expert witnesses to give opinions on that theory. Since there is no scientific consensus that field sobriety tests accurately predict who is impaired by marijuana, police cannot be certified as experts in this area, according to the court.

"The research on the efficacy of FSTs [field sobriety tests] to measure marijuana impairment has produced highly disparate results," the court held. "Some studies have shown no correlation between inadequate performance on FSTs and the consumption of marijuana; other studies have shown some correlation with certain FSTs, but not with others; and yet other studies have shown a correlation with all of the most frequently used FSTs."

Therefore, police can only testify about their observations about what occurred when a field sobriety test was administered. They cannot give their opinion about whether the subject passed or failed the test, or whether the officer concluded they were impaired. They also cannot imply that failure of these tests is conclusive evidence of marijuana impairment.

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