Standard Field Sobriety Tests Cannot Measure Marijuana Impairment

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Commonwealth v. Gerhardt, 2017 WL 4127666 (Mass. 2017)

A Massachusetts state trooper stopped Thomas Gerhardt just after midnight for driving without lights. Approaching the car, the trooper saw smoke inside the car and at the side window, he smelled burnt marijuana. Gerhardt admitted to smoking approximately one gram of marijuana three hours before driving. Recreational use of marijuana is lawful in Massachusetts.

The trooper administered three standard field sobriety tests (SFSTs): the horizontal gaze nystagmus test, the nine-step walk-and-turn test, and the one-leg-stand test. Gerhardt was then charged with driving under the influence of marijuana, but later challenged the scientific validity of these tests as an indicator of being under the influence of marijuana.

The Massachusetts Supreme Judicial Court held that an officer cannot opine on the question of a driver’s impairment from marijuana based on SFSTs. The tests are admissible “to establish a driver’s balance, coordination, and mental acuity.” The jury then can reach its own conclusion, relying on its common sense (and experience with getting high?) about whether the driver was impaired.

The court cited differing expert opinions on the usefulness of SFSTs to assess impairment from marijuana: “The scientific community has not reached a consensus as to whether a defendant’s performance on any combination of FSTs, or any individual FST, is correlated with marijuana use or impairment … Because the effects of marijuana may vary greatly from one individual to another, and those effects are as yet not commonly known, neither a police officer nor a lay witness who has not been qualified as an expert may offer an opinion as to whether a driver was under the influence of marijuana.”

Though the decision sharply limits the officer’s ability to testify as to his or her conclusion about whether the defendant was under the influence, the officer can carefully record and testify about the actual performance on the SFSTs. New tests are being developed to measure impairment from marijuana consumption, including a buccal swab test. This case illustrates the challenge of legalizing the use of psychoactive drugs while hoping to limit the public risk of drivers who become lawfully stoned.


CHIEF KEN WALLENTINE is a Special Agent who directs the Utah Attorney General Training Center, overseeing use of force training and investigation and cold case homicide investigations. He is also a consultant and Senior Legal Advisor for Lexipol. Ken formerly served as Chief of Law Enforcement for the Utah Attorney General, serving over three decades in public safety before a brief retirement. He also serves as the Chairman of the Peace Officer Merit Commission of Greater Salt Lake County.

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