BOSTON (CN) — Massachusetts’ highest court ruled this week that field sobriety tests are not good enough to determine whether a driver is too high on marijuana to be behind the wheel.

The Supreme Judicial Court ruled Wednesday that field sobriety tests designed to detect alcohol use were not appropriate to determine whether a driver has been impaired by marijuana use, because there has not been enough research done about the effects of marijuana, which differs from person to person.

The three typical sobriety tests for drunkenness are a horizontal gaze test, walking in a line and turning around and maintaining balance on one foot. But there is no scientifically valid method to for marijuana intoxication.

“The research on the efficacy of FSTs [field sobriety tests] to measure marijuana impairment has produced highly disparate results,” Associate Justice Frank Gaziano wrote for the full, seven-member court. “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.”

Gaziano said that while it is valid for a police officer to asses a driver’s coordination, alertness and balance, it is inappropriate for the officer to testify whether the driver is too stoned to drive.

“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,” Gaziano wrote.

Thomas Gerhardt was charged with driving under the influence of marijuana on Feb. 13, 2013, just months after Massachusetts legalized medical marijuana through a 2012 ballot measure.

The police officer reported that he saw smoke coming out of Gerhardt’s car and that he smelled burned marijuana. Gerhardt and his passengers all acknowledged having recently smoked.

Gerhardt passed his field sobriety tests except for walking and turning. He wobbled while standing on one foot, but remained upright.

The officer decided to charge Gerhardt, who appealed, resulting in four questions going to the state’s Supreme Judicial Court.

The court determined that a police officer may testify only that he observed signs of a person being high, such as drowsiness or bloodshot eyes, but not whether he believed the person to be high on marijuana. A lay person may not testify whether he or she thinks someone is high, but a “Jurors are permitted to utilize their common sense in assessing trial evidence.”

In a brief appendix to the ruling, the court provided model jury instructions for such cases.