Governor Charlie Baker’s office this week launched the first campaign to crack down on impaired driving since retail marijuana shops opened. The campaign includes television and online ads and a state-funded effort encouraging overtime for police officers to keep impaired drivers off the roads.

According to Massachusetts Highway Safety Director Jeff Larason, an increased number of drivers under the influence of marijuana is a major concern on Massachusetts roads.

“It’s a big problem,” Larason said. “And with legalization, as we’ve seen in Colorado and Washington State, those numbers have grown in those states, and we expect that’s going to be the same case in Massachusetts.”

The new campaign illustrates a gray area in a state adapting to recreational marijuana laws: the ads don’t specify an acceptable amount of the substance when it comes to driving. Rather, the emphasis is on impairment.

“Some people can be affected by a fairly low amount of marijuana. Some people will not be affected by a low amount of marijuana,” Larason said. “We’re not concerned about how much you have used, we’re only concerned about whether you’re impaired.”

But with no chemical test to measure impairment, who determines what, exactly, “impaired” means? And who determines if someone is too high to drive?

That’s where Drug Recognition Experts (DREs) come in.

DREs are specially trained police officers, usually called in by other officers who have pulled drivers over under suspicion of impairment.

“If they blow a zero on the breathalyzer,” Larason said, “then they can pull in a drug recognition expert and go through a battery of an evaluation to make a determination about the level of impairment and what that person is impaired by.”

The officers attend a two-week training through the International Association of Chiefs of Police, funded by the National Highway Transportation Safety Administration. In this training, officers learn how to identify the signs of impairment and how to determine which category of drugs the person may be impaired by — such as hallucinogens, stimulants, or inhalants.

When it comes to cannabis, officers are looking for a “totality of the circumstances,” according to Officer Mark Cote, the DRE for the West Springfield Police Department. This can include red eyes, marijuana odor, an involuntary jerking of the eyes, body and eye tremors, dilated pupils, high blood pressure and a high pulse.

Officers are trained to administer a marijuana-specific field sobriety test that includes a modified Romberg test, which entails “having somebody just stand with their head tipped backwards,” Cote said, “and we just have them estimate the passage of 30 seconds, and we watch whether they're tilting back and forth side to side.”

According to Cote, just one of these factors is not really enough to charge someone with impairment, as someone could naturally have high blood pressure, or even just a pre-existing marijuana smell in their car.

“I've had people stopped with the odor inside the car and I haven't got them out to do any field sobriety tests, because I felt that it's an old odor that's been in there and I could just tell by speaking to them that they're [sober],” Cote said. “They're talking normally, they're not lethargic, they're not talking with slurred speech or displaying the reddened eyes. ... I would never get them out to do that, unless I truly believe that they were under the influence.”

According to Massachusetts Defense Attorney Joe Higgins, there’s a huge legal gray area, because so much depends on an officer's in-person assessment.

“There is a considerable defense, particularly in OUI marijuana cases where a drug recognition expert is employed,” Higgins said, “because ... there is no test for that.”

In these cases, Higgins says he would usually file a motion challenging the protocol itself — including the training — as unscientific and therefore unable to hold up in court.

“Essentially what we would be arguing is that the protocol hasn't been recognized in the relevant scientific community as being reliable and accurate,” Higgins said. “The protocol was established by police officers at the International Association of Chiefs of Police, but there were no clinical studies establishing the reliability of the protocol for their analysis.”

This puts the burden on the government to establish the reliability of the system before it can be introduced as evidence in a trial.

“Most police officers, drug recognition experts included, aren't toxicologists. [They] aren't pharmacologist or physiologists,” Higgins said. “We challenge whether or not they have the requisite training and education qualification to be able to testify competently and reliably in court.”

The training program teaches officers to differentiate medical issues from drug-induced behavior, but not necessarily to identify neuroatypical conditions, according to Cote.

“It doesn't so much go through the neurological issues, they just explain certain different ways of looking,” Cote said.

And then there is the question of bias. According to a 2013 report from the ACLU, though marijuana use is roughly equal among black and white people, black people are 3.73 times as likely to be arrested for marijuana possession. A 2016 report from the Colorado Department of Public Safety noted that the arrest rate for black people for weed-related offenses is still nearly three times that of whites.

Despite this disparity, Cote said the training does not contain a racial sensitivity or diversity training. When pressed about the issue, Cote said he “didn’t like the direction this interview was going” and cut it short abruptly.

“I don't understand what that has to do with whether someone's high or not,” Cote said. “It has nothing to do with racial sensitivity. If you're high you're high, if you're not you're not.”

The Drug Recognition Expert program now has 147 certified DREs across the state, 19 of whom are DRE instructors.