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Massachusetts' highest court ruled Tuesday that field sobriety tests typically used in drunken-driving cases cannot be treated as conclusive evidence that a motorist was operating under the influence of marijuana.
The Supreme Judicial Court said it was reasonable for police officers to testify -- as non-expert witnesses -- only to their observations about how individuals performed during sobriety tests. But officers are not allowed to tell juries if defendants passed or failed such tests, nor offer their own opinions on whether a driver was too high to be behind the wheel.
The ruling came in a case of a man who was charged with impaired driving in 2013.
The justices noted there currently is no reliable scientific test for marijuana impairment comparable to tests for blood alcohol content, though several potential tools are under development. In drunken-driving cases, results of field sobriety tests can be correlated with blood alcohol readings as evidence of impairment.
The lack of such a test for marijuana has taken on greater significance in states such as Massachusetts that have legalized the recreational use of marijuana for adults, but where driving under the influence of pot remains a serious crime.
"While not all researchers agree, a significant amount of research has shown that consumption of marijuana can impair the ability to drive," the court said in a unanimous decision. "There is ongoing disagreement among scientists, however, as to whether [field sobriety tests] are indicative of marijuana impairment."
Thomas Gerhardt challenged the admissibility of tests that were conducted by a state trooper in Millbury after he was pulled over on suspicion of impaired driving. The trooper reported smoke in the car, the odor of marijuana and found two marijuana cigarette butts.
Two passengers said they had smoked the joints about 20 minutes earlier, while Gerhardt maintained it had been about three hours since he used marijuana.
During his field sobriety test, Gerhardt was able to recite a portion of the alphabet and count backward, but was unable to properly follow instructions for a so-called walk-and-turn test, leading the officer to conclude he was under the influence of marijuana.
The state Legislature recently ordered creation of a special commission to study issues around driving while impaired by marijuana.
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