As recreational marijuana sales come online, more people are expected to drive while impaired. But until Massachusetts law — and science — catch up with cannabis culture, keeping roads safe will remain a challenge for police and prosecutors.

"In Massachusetts, we put the cart before the horse," said John Carmichael, Walpole's police chief and spokesman for the Massachusetts Police Chiefs Association. "We legalized three years ago, and now we're trying to figure out how to deal with the OUI problem."

A special commission is on the case, but its report isn't due until January.

In the meantime, police departments increasingly are relying on one existing tool — drug-recognition expert training for officers — to identify impaired drivers.

But not everyone is on board with the idea. Civil liberties advocates and some judges say drug-recognition evaluations are too subjective.

To be sure, police officers can take reckless, marijuana-impaired drivers off the road. Operating a motor vehicle to endanger is a criminal offense punishable by up to two years in jail and loss of driving privileges.

Still, it's difficult to know what to expect on Massachusetts roads once marijuana becomes something adults can pick up whenever the mood strikes. Law enforcement officials are anticipating more stoned drivers on the road.

In Colorado, where recreational marijuana sales began in 2014, the number of fatal motor vehicle accidents increased by 40 percent from 2013 through 2016, according to a Colorado state analysis of National Highway Traffic Safety Administration data.

It isn't clear that cannabis was to blame. A 2017 study published in the American Journal of Public Health found that when compared with eight states that did not have legal recreational marijuana, the number of fatal car crashes in Colorado from 2009 to 2015 increased at the same rate as states where recreational cannabis is illegal.

Colorado law says that drivers whose blood contains 5 nanograms of active THC, the high-inducing chemical in cannabis, can be prosecuted for driving under the influence.

The state also has an open container law that requires a vehicle's passenger area to be free of open packages of marijuana, meaning those with a broken seal. It also is illegal to consume marijuana on a public road. People who refuse to undergo a blood test to detect THC get an ignition lock put on their vehicle for two years.

"What we're seeing — and this is national — [is] less people are driving drunk under the influence of alcohol and more people are now under the influence of drugs, marijuana, or it's a combination case where someone has a beer to wash down a Percocet," said lawyer John Schefft, a member of the state's Special Commission on Operating Under the Influence and Impaired Driving.

"Now, the dilemma is, everyone wants to find the silver bullet, the equivalent of the alcohol Breathalyzer to drugs," Schefft said.

How high?

In Boston, that 13-member commission is analyzing Massachusetts' limit and enforcement options by looking at available science, best practices and how other states are tackling the problem. The group began meeting in June and is charged with making recommendations to the Legislature by January.

Sitting in a half-circle behind desks in a community room at the Arlington Police Station, members noted that their comments would be heard across Massachusetts: The audience was made up of about two dozen journalists.

One issue is basic: How high is too high to drive? In Massachusetts, the answer is debatable.

Without a standard marijuana sobriety test or a THC blood-limit standard, removing cannabis-impaired drivers from roads remains a challenge for police officers.

Adding to the difficulty, recent Massachusetts court cases changed how an officer can describe a suspect's impairment in court. Tina Cafaro, author of the article "Slipping Through the Cracks: Why Can't We Stop Drugged Driving?" and a Western New England School of Law professor, pointed to the 2017 Commonwealth V. Gerhardt case, which limited how officers can describe, in court, a suspect's impairment.

"The Gerhardt case makes it more difficult to establish that someone is under the influence of marijuana than it is to establish someone is under the influence of alcohol," Cafaro said. "Officers have to get in the habit of referring to `field sobriety tests' as `roadside assessments' and need to remember that when they are testifying, they may not offer an opinion regarding whether or not the operator was impaired [something they are allowed to do in OUI alcohol cases]."

"Without that testimony," she said, "it makes it more difficult for the prosecutor to convince a jury, beyond a reasonable doubt, that the defendant was impaired."

Lee Police Chief Jeffrey Roosa said that when marijuana was decriminalized by voters in 2008, everything Massachusetts police thought they knew about how to stop marijuana-impaired driving started to change. Because there is no reliable, scientific way to measure marijuana impairment and intoxication, before 2008 police officers who suspected someone of driving high would often seek to charge the person with more easily provable crimes, such as possession of marijuana, Roosa said.

"We need to change the laws. A guy can drive by, you can smell it and you can see him use it — that's under the influence, it's a good reason to stop [the driver], and then it gets complicated," he said.

"It takes four hours to process a marijuana OUI — is that worth the time when I only have two guys out on patrol?" Roosa asked.

Adult marijuana possession is now legal, within limits, and since possession is no longer a criminal offense, in most cases, the best an officer can do is cite a public pot smoker for breaking a town's anti-public intoxication bylaw — typically a $500 fine.

Unless written into the bylaw, there are no penalties for failing to pay the citation. The town would have to sue a scofflaw in small claims court to recoup the fine.

"That $500 fine isn't worth the paper it's written on," Roosa said.

On the case

When Massachusetts voters in 2016 approved recreational marijuana, the measure required investigation into the impact of weed-impaired driving. But neither the ballot question nor the final version of the bill sets limits on marijuana consumption, intoxication and a person's ability to drive a car.

The law did set up the special commission, which got to work only a month before recreational sales were scheduled to begin.

The commission, which includes lawyers, law enforcement, professors and government officials, is charged with researching how Massachusetts can address drug-impaired driving and making recommendations to the Legislature no later than January.

The commission held its first meeting in June, when members spent time discussing the value of drug-recognition officers.

The panel is mulling expansion of the drug-recognition program throughout the state as one approach to policing impairment, though civil rights advocates are against it.

Critics say the drug-recognition analysis are overly subjective and lack supporting scientific research.

"There's also a concern with specialized policing units and how much racial bias plays into their actions," said Rahsaan Hall, director of racial justice for the American Civil Liberties Union of Massachusetts. Studies have shown that people of color are more likely than white people to be stopped by police, put in handcuffs, pepper-sprayed, pushed to the ground or shot by the police.

State Rep. William "Smitty" Pignatelli, D-Lenox, said he understands people want an answer to the question on impaired driving enforcement but said the Legislature is holding off on making decisions regarding drugged driving until the commission reports its findings.

"People are generally concerned about this. There's a police training program to tell what drugs a person is under, but right now, I don't think there is a perfect science to it," said Pignatelli, who expressed interest in creating a blood-weed standard but isn't sure how one could be accurately administered.

"If you smoke marijuana two weeks ago, technically, it's still in your system," he said. "Does that mean you're too impaired to drive? What is the equivalence of a blood-alcohol test?"

Need for tools

Meanwhile, police departments across the state can't simply wait. They are dealing with marijuana-using drivers who might or might not be a threat to other people — and being asked to act.

From 2007 to 2014, National Highway Traffic Safety Administration research shows a 48 percent increase nationwide in people driving with marijuana in their systems.

Whether these people shouldn't have been driving because of impairment, though, can't be said for certain.

But police officers often have to make a determination.

Carmichael, the chiefs' association spokesman, said officers need the right tools to assess impairment. He believes that includes compelling drivers suspected of operating under the influence of cannabis to take an hourlong drug-recognition evaluation, administered by a certified drug-recognition officer.

It would be similar, Carmichael said, to how drivers suspected of being drunk are compelled to take a Breathalyzer alcohol-detection test.

In Massachusetts, if you refuse a Breathalyzer, you lose your driver's license for six months. Now, the drug-recognition screening is considered voluntary without repercussions.

To receive drug-recognition certification, a police officer must be able to identify drug use in a person with at least 75 percent accuracy, according to the international standards for drug-recognition officers. A 2017 study in Canada found that its drug-recognition officers are able to detect drug impairment with 95 percent accuracy.

"Ultimately, what we want ... is [for] drug-recognition experts to be seen for what they are — experts," Carmichael said. "Most grads can testify in court now, but some Massachusetts courts accept it and some don't."

At the June commission meeting, Harvard University Medical School professor Margarita Alegr a and American Civil Liberties Union lawyer Matt Allen, both of them panel members, cited a lack of scientific evidence supporting the accuracy of drug-recognition experts.

"My concern is to ensure any recommendation doesn't infringe on people's civil liberties. All the research we support are based on evidence," Allen said. "I'm not sure of the assumption that this training is the solution."

In Berkshire County

In the Berkshires, if a police department needs help identifying an impaired driver, it calls Lanesborough Police Officer Brennan Polidoro, the lone drug-recognition officer in the county.

But Polidoro is often not considered an expert in Massachusetts courts when he gives testimony.

Polidoro has spent 80 hours in a classroom studying drugs and their effects on people, as well as hours in the field evaluating suspect drivers. He spent a week in Arizona in a specialized drug-recognition boot camp.

"The most preventable death is an impaired-driving death," said Polidoro, a 27-year-old Adams native with about six years of experience in the Egremont, Sheffield and Lanesborough police departments.

Before they were able to meet, his grandfather, a Pittsfield firefighter, was killed by a drunken driver, Polidoro said. That might have influenced his decision to specialize in drugged and drunken driving prevention.

"I'm good with impaired driving and recognizing it," he said.

Since Polidoro began his policing career in Egremont, he has arrested 17 people for driving under the influence of drugs or alcohol — a significant number of DUI collars for an officer in rural Berkshire County.

There are now 116 certified drug-recognition officers statewide, according to 2016 data, the most recent numbers available; 60 law enforcement departments have recognition officers on staff.

When Polidoro is not allowed to testify as an expert in court, he will describe what the person looked like, how they acted, what evidence of drug use there was at the incident and what the suspect did during a sobriety assessment, but Polidoro cannot conclude that the person was high. Only an expert, such as a doctor, could do that, Massachusetts courts have said.

A drawback of the drug-recognition program is its cost. The program is expensive and time-consuming for municipal police departments, which must send officers away for training and, every two years, for recertification.

"For us to send an officer to DRE [drug-recognition expert] training, the expense would be enormous," said Roosa, the Lee chief. "There's an overtime burden there's a lot of economical challenges."

Drug-recognition training, though, isn't helpful if drivers are able to consider the evaluation optional.

So, in addition to supporting drug-recognition training, the Massachusetts Chiefs of Police Association supports compelling drivers suspected of being high to submit to an hourlong, in-station drug-recognition evaluation with a certified officer.

"There needs to be a balance in the court system so that we can take to trial what happened," Polidoro said. "They're giving the benefit of the doubt to people who put other people's lives in danger."

Kristin Palpini can be reached at, @kristinpalpini on Twitter, 413-629-4621.