GOV. CHARLIE BAKER has been pushing for legislation he says would give “law enforcement more tools to keep our roads safe from impaired drivers.”  

John Amabile, president of the Massachusetts Association of Criminal Defense Lawyers, used other words to describe the bill: “a classic use of nonscientific junk science to potentially cause a person to be convicted of a serious crime.

When recreational marijuana was legalized by a 2016 ballot question, the tagline of the legalization campaign was “regulate marijuana like alcohol.” A special commission was formed to explore ways to update the state’s drunk driving laws to address marijuana use, and Baker has been advocating for passage of their recommendations. A legislative committee recently sent Baker’s bill to study, effectively killing it for the second time.

Impaired driving due to any substance is already illegal. Baker’s bill would impose an automatic license suspension on someone who refuses to take a roadside drug test, as is already the law if someone refuses a breathalyzer test. It would direct judges to accept as a fact that marijuana impairs driving, among other provisions.

Speaking on this week’s Codcast, Amabile and Dr. Staci Gruber, director of the Marijuana Investigations for Neuroscientific Discovery program at Harvard’s McLean Hospital, explained why treating marijuana like alcohol is not so simple.

For alcohol, someone who has a blood alcohol level of 0.08 percent on a breathalyzer test is considered impaired. There is no similar test for marijuana. “Cannabis is not alcohol. Cannabis is complicated,” Gruber said. “And in terms of the ways in which people use and the very, very long half-life of cannabis means that we are going to detect the presence of its metabolites.”

For example, Delta 9 THC, an intoxicating part of the marijuana plant, can be detected in someone’s body for a month after they used it – long after the point the person is no longer impaired. Some people might use non-intoxicating products, which can still be detected in their blood. “That’s a huge distinguishing fact – the difference between using something and being intoxicated or impaired,” Gruber said. “There is no empirically sound metric at this point that ties the level of impairment to cannabis use.”

Today, the police rely heavily on field tests performed by specially-trained drug recognition experts, who ask drivers to complete certain tasks. But a recent study by Massachusetts General Hospital researchers, aimed at testing new portable brain-scanning technology that could be used to detect marijuana impairment, found that drug recognition experts actually produced false positives in 34 percent of subjects.

“That means more than a third of your folks are going to be incorrectly classified or characterized as those who are impaired,” Gruber said. “That’s a problem.”

Is there a role for individuals to be able to assess real world folks in real world situations? Absolutely,” Gruber said. “Are we there yet with regard to their ability to do this in a very valid way that is reliable over time? I don’t think so.”

For now, Gruber said, she believes education is key to stopping people from driving while impaired – reminding people of the dangers of driving while high and ensuring, for example, that everyone who buys a marijuana edible knows how long it takes for the intoxicating effects to kick in.

“What doesn’t need to be done is to convict innocent people based on shoddy and unreliable so-called scientific evidence,” Amabile added.

Baker has portrayed the bill as urgently needed. He named the latest version of it after State Trooper Thomas Clardy, who was fatally struck by a driver who had THC in his blood. Baker cites data from the National Highway Traffic Safety Administration showing an increase in driving under the influence of alcohol and drugs during the COVID pandemic nationally. One NHTSA study of five trauma centers, including one in Worcester, in 2021 found 56 percent of seriously and fatally injured drivers tested positive for at least one impairing substance, up from 50.8 percent before the public health emergency.

But Amabile said there is no evidence that rates of drugged driving have increased with marijuana legalization. “This whole sort of hysteria around drug-impaired driving is really a solution in search of a problem,” Amabile said. “There is no actual evidence that there’s some epidemic of drug-impaired driving out there. That is an absolute falsehood. And the notion that that would be linked to the legalization of cannabis is totally unproven.”