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As Massachusetts opens more pot stores, Governor Charlie Baker wants to combat stoned driving by increasing the power and number of police trained to detect whether someone is high and a danger behind the wheel.

But defense attorneys and civil liberties advocates say the evaluation system that Baker is promoting, which relies on a series of observations by specially trained officers, is unscientific, deeply flawed, and could punish innocent people.

The “drug recognition expert” program, or DRE, has been rejected in numerous cases by Massachusetts judges who ruled it lacks scientific validation. Studies have found that the program’s claims of accuracy are overstated, and that it is biased and prone to false alarms.

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“The question in a criminal trial is supposed to be, ‘Is this particular person guilty of this crime?’ and these tests are useless for that,” said Dr. Greg Kane, a Colorado physician who wrote a 2013 study debunking DRE.

The concerns highlight a key challenge for authorities in the age of legalized marijuana: how best to prosecute high drivers who could endanger other motorists and themselves, without ensnaring law-abiding cannabis consumers.

Unlike with alcohol, there isn’t a device that can prove marijuana impairment. Officials believe a breathalyzer-like device for pot is years away. Blood levels of cannabis don’t correlate with impairment. Blood, saliva, and urine tests reveal only past use, which can be days old.

The DRE tests occur at the police station after a suspected high driver has been arrested, usually based on driving violations, and on observations made by an officer, such as the odor of marijuana, behavior, and field sobriety tests such as standing on one leg. A second officer trained in the DRE protocol conducts the 12-step test that includes measuring the driver’s pupils and blood pressure, interviewing the arresting officer and the driver, and seeing whether the driver’s eyes cross or jerk under certain conditions.

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The DRE officer then makes an assessment of which category of drug the driver is suspected of taking, and follows up with a blood or urine test.

Baker filed a bill this year that would increase the number of DRE-trained officers from 150 to 350 statewide and have them qualify as expert witnesses for court testimony. The bill would automatically suspend the licenses of suspected drugged drivers who refuse parts of the DRE tests — similar to a suspected drunk driver refusing a breathalyzer. Judges would receive education from federal traffic authorities and police groups that support the program.

Baker spokesman Brendan Moss said the bill would “make Massachusetts safer,” and was based on mostly unanimous recommendations of a special commission created under the Legislature’s update to the marijuana law, and included a defense lawyer, police and prosecutors, doctors, and a civil liberties group. The commission found that the DRE program, while imperfect, is the best tool available to detect impaired drivers.

Marchese held a device used to judge pupil size.
Marchese held a device used to judge pupil size.Matthew J. Lee/Globe Staff/Globe Staff

“The value of the DRE is that, through their examination, they’re able to interview people in a way where they can get at the truth,” said commission member John Scheft, a former Middlesex County prosecutor.

He acknowledged that DRE needs more rigorous scientific validation, but said, “the evidence is sufficient to allow these certified experts to get up and say what they found.”

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Matt Allen, field director for the American Civil Liberties Union of Massachusetts, was the sole commission member who opposed boosting the DRE program’s status in court.

“If we’re going to attach a scientific test to a legal penalty, we need to make sure the test measures impairment,” Allen said. “Expedience in court should not trump science, evidence, and civil liberties.”

Since Massachusetts voters legalized marijuana in 2016, law enforcement officials have warned of a potential massive increase in stoned drivers. It’s not clear, however, whether the state has seen an increase in stoned-driving crashes. Police say they’re seeing more drugged drivers in general, including people on opioids, stimulants, prescription pills, or multiple substances — though cases of drunk drivers still far outnumber drugged ones.

Studies show that marijuana can affect driver coordination, reaction time, and decision-making. Compared to sober drivers, stoned drivers are anywhere from no more likely to three times more likely to crash, researchers estimate, while drunk drivers are five to 20 times more likely, according to a congressional report.

One-third of the state’s pot consumers said they drove high in the previous month, about 7 percent of all adults in Massachusetts, a survey found last year.

Police say there are clear dangers. They point to the 2016 death of state Trooper Thomas Clardy, who prosecutors say was killed by a stoned driver who had just left a medical marijuana dispensary.

The DRE program was created by Los Angeles police officers in the 1970s. The National Highway Traffic Safety Administration found the test to be accurate, prompting Congress to fund a 112-hour training program for police nationwide.

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In Massachusetts, a large portion of the state’s police forces do not have their own DRE officer, so they share.

Chelsea police Officer Paul Marchese, who received his DRE training a year ago, said he handles a few drugged-driving calls per month in Chelsea, and for Revere, Everett, Malden, and Salem. He believes the test is nuanced and thorough, and people have ample opportunity to prove they’re not impaired.

“It’s not a black-and-white line — you can have a beer and drive, or maybe a joint and drive,” Marchese said one recent night, as he steered his cruiser past brick triple-deckers. “But you’re driving a weapon, an over-thousand-pound vehicle that could kill somebody if your reaction time is slow.”

Last summer, Marchese said, he pulled over a young man whose car was stopped at a green light. He had bloodshot eyes and slurred speech, but claimed to be sober and just texting. Marchese put him through tests because, “God forbid, I let him leave and he hits somebody, that’s on me.”

The man passed. Marchese let him go.

But critics say not all officers may do that. Last year, three drivers in Georgia alleged that a DRE officer arrested them for stoned driving while they were sober. Video backed their claims. They weren’t prosecuted.

“This is one of the few areas of the law where someone can be charged with something based on nothing but an opinion of a police officer,” said Joseph Zlatnik, a Pittsfield defense lawyer who in one case successfully fought to bar a DRE officer from concluding at trial that his client was impaired while driving.

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In his analysis of DRE in 2013, Kane found that the three studies that proponents often cite as proving the reliability of the tests all used flawed methodologies, such as concluding the DRE is accurate because blood and urine tests showed the presence of marijuana, although those tests do not measure impairment. He concluded the assessments of impairment made by most officers actually relied on admissions from suspects, or on the discovery of drugs.

Other studies have found DRE to have accuracy rates as low as 42 percent, and that it could falsely identify up to 57 percent of sober subjects as high.

For marijuana, the DRE protocol looks for red eyes, eyelid tremors, dilated pupils, high pulse, and the inability to cross one’s eyes. Scientific backing for these tests is mixed.

Judges across Massachusetts, who must consider an expert’s qualifications and scientific validity, have in some cases limited DRE officers from concluding whether a defendant was impaired. That frustrates police, who feel that many drugged drivers get off easy and return to driving stoned.

“You go to court and there’s basically nothing allowed, other than their simple observations,” said Walpole Police Chief John Carmichael. “You know you’re probably going to lose.”

Plymouth County District Attorney Tim Cruz added: “All I want is to have a fair shot to make sure we can advocate on behalf of the people who want to be safe.”

Meanwhile, defense lawyers criticized a provision in Baker’s bill that would require judges to accept DRE-trained officers as experts. If the state wants to solve the issue, they said, it should invest in solidifying the science behind drug impairment.

“We need to let statistics and science guide our public policy,” said Stuart Hurowitz, with the state’s Committee for Public Counsel Services.


Naomi Martin can be reached at naomi.martin@globe.com. Follow her on Twitter @NaomiMartin.