Are the signs of marijuana intoxication so familiar to the public that prosecutors can rely on the observations of police in stoned driving cases, or are experts required?

That’s a question the state’s highest court is considering, hearing arguments Friday in the case of Thomas Gerhardt, who was charged with driving under the influence of drugs after a February 2013 traffic stop off Route 146.

“Like alcohol, marijuana usage at this point is so widespread - it is in the common experience of a juror - that an expert is not required,” Worcester prosecutor Michelle King argued before the Supreme Judicial Court.

Public defender Rebecca Jacobstein argued experts are necessary witnesses at trial to explain a police officer’s observations of an allegedly stoned driver, and said roadside impairment tests used to identify drunk drivers are not suitable to detect marijuana intoxication.

“There are no physical characteristics that permit an inference of impaired driving due to marijuana use, and a layperson does not have sufficient understanding of marijuana’s physical manifestations,” Jacobstein told the justices.

“What if he stumbles and he says, ‘Oh my goodness, I just had five joints,’” said Justice David Lowy. “Do you need an expert?”

“You still do,” Jacobstein replied.

“You still do?” Lowy asked.

“That’s the law in Massachusetts,” she said. “The Appeals Court has found on multiple occasions that evidence of drug use plus erratic driving is insufficient without expert testimony on how the drug use and the amount of the drug use impacts the driving.”

Concerns about marijuana-impaired driving and the difficulties in proving marijuana intoxication are likely 2017 agenda items for lawmakers seeking public-safety changes to the marijuana legalization law approved by voters in November.

Science has not yet produced a measure of marijuana intoxication on par with the blood-alcohol-content tests, which provide reliable readings of someone’s sobriety or drunkenness.

Marijuana hampers drivers’ ability to judge distance, reduces peripheral vision and coordination, and makes it more challenging to stay in a lane, according to American Automobile Association Massachusetts public and legislative affairs director Mary Maguire, who campaigned against the marijuana ballot question and said pot also impairs judgement and reaction times.

According to the prosecution’s brief, Gerhardt’s rear lights were not on as he traveled down Route 146, taking exit 8, after midnight. He was pulled over, and the state trooper who made the stop noticed smoke in the vehicle. Gerhardt told the officer he had smoked marijuana three hours prior, though a passenger said they smoked 20 minutes earlier, according to the brief.

Gerhardt failed to understand and follow directions on one field sobriety test, and when instructed to stand on one foot he swayed and put his other foot down multiple times, which indicated he was impaired, according to the prosecution.

Jacobstein said people can perform poorly on field sobriety tests for a number of health reasons, and she said chronic marijuana users tend to have difficulty standing on one foot even when they are not high.

King, the Worcester County prosecutor, said the field tests are merely one factor used to make a case, which might include other evidence that the driver was weaving, the discovery of marijuana in the car, and the aroma of marijuana smoke inside the car.

Justice Geraldine Hines said the science for measuring marijuana intoxication is in its early stages.
“Science has verified the tests for alcohol impairment,” Hines said. “We’re starting from the beginning here, and I’m just puzzled at how police officers are going to be able to testify to something that the science hasn’t verified. I mean, I don’t get that.”

King said studies have shown a correlation between marijuana intoxication and poor performance on field sobriety tests.

In a written brief, Jacobstein said while the average juror might lack an understanding of marijuana’s effect and its impact on driving, he or she is likely familiar with stereotypes of marijuana intoxication.

Jacobstein pointed out the prosecution’s brief quoted a National Geographic article that pot can “lead to temporary laughing sickness, intense shoe­gazing, amnesia about what happened two seconds ago, and a ravenous yearning for Cheez Doodles.”