States like Maine that are considering legalizing marijuana and already are grappling with increases in prescription drug abuse should act to address the problem of driving while under the influence of drugs, according to a report published Wednesday.

The report by the Governors Highway Safety Association includes data indicating that driving while drugged has increased in states that have legalized medical or recreational marijuana use, including Colorado and Washington. However, the study also highlights the limits to existing data, the complexity of the drugged-driving problem and the challenge of determining if people are driving while impaired by drugs.

“It’s an increasing problem and one we haven’t been paying attention to,” said Dr. Jim Hedlund, the report’s author and a former senior official with the National Highway Traffic Safety Administration.

The call for action comes as a working group convened by the Maine Legislature grapples with how to deal with driving under the influence of marijuana. The group will report back to the Legislature, which is holding a bill that would establish an official blood-level limit for defining when someone is driving under the influence of marijuana.

How to deal with impaired drivers is likely to become a more pressing issue as Maine considers joining four other states and the District of Columbia in legalizing marijuana for recreational use. Two separate groups are collecting signatures to put legalization proposals for recreational use on the November 2016 ballot.

Drug-impaired driving is more complex than alcohol-impaired driving for many reasons, Hedlund said. There are hundreds of drugs that can affect drivers, including some that are illegal, others that are legal under certain conditions and some that are available over the counter.

Hedlund said data on the prevalence and impact of drug-impaired driving is incomplete because of the various ways it is tracked state to state and even by jurisdictions within a state. The report, citing the most recent data available, says that drugged driving is increasing while drunken driving is declining. The percentage of fatally injured drivers who tested positive for drugs – 40 percent – is almost the same as those testing positive for alcohol, according to the report.

DEVELOPING STANDARDS, PRACTICES

In Washington, where recreational marijuana was legalized in November 2012, the proportion of suspected impaired-driving cases where the driver tested positive for the chemical that produces most of the drug’s psychological effects, THC, is on the rise, according to the report. From 2009 to 2012, the proportion averaged 19.1 percent. That rose to 24.9 percent in 2013 and 28 percent in 2014. Preliminary data from the first four months of 2015 indicates it is now at 33 percent, according to the report.

In Denver, arrests for driving under the influence of drugs doubled from 2013 to 2014, from 33 to 66, according to a recent report from the Rocky Mountain High Intensity Drug Trafficking Area.

Although driving under the influence of drugs is illegal in all states, the laws can be complex and hard to prosecute, said Hedlund, the report author. Unlike with alcohol, where a roadside breath test can determine impairment, there is no simple test for drug impairment. Instead, officers are trained to recognize signs that people are under the influence of drugs.

“Every state must take steps to reduce drug-impaired driving, regardless of the legal status of marijuana,” said Jonathan Adkins, executive director of the Governors Highway Safety Association. “This is the first report to provide states and other stakeholders with the information they need. And we encourage the National Highway Traffic Safety Administration to issue guidance on best practices to prevent marijuana-impaired driving. We look to the federal government to take a leadership role in this issue similar to that of drunk driving and seat belt use.”

The report recommends that states take steps to assess and understand the issue; update drug-impaired driving laws; better train law enforcement, prosecutors and judges; test all fatally injured drivers for the presence of drugs; screen offenders for underlying drug, alcohol or metal health issues and refer them to treatment; and track all alcohol- and drug-impaired driving data separately to best assess the problem.

State Rep. Mark Dion, a Portland Democrat and former Cumberland County sheriff who has advocated for marijuana legalization, believes Maine’s impaired-driving laws are adequate, but more could be done to educate the public and support law enforcement officers dealing with drugged driving.

“I think where we’re lacking is data and tests to support the police officer’s conclusion that he or she is dealing with an individual impaired by drugs,” Dion said. “We have a long history with alcohol and traffic enforcement focused on impairment. We need to take more determined steps to deal with drug impairment.”

The working group charged by the Legislature with making recommendations about how to deal with driving under the influence of marijuana includes law enforcement officials, prosecutors and marijuana advocates. The group includes the leaders of Legalize Maine and the Campaign to Regulate Marijuana Like Alcohol in Maine, the two groups pushing for legalization referendums.

LEGAL POT WITHOUT ERODING SAFETY

David Boyer, head of the latter group, said now is a good time for Maine to deal with the issue because of the possibility of a 2016 vote. He said the work Maine is doing to be proactive is valuable, but there also needs to be study and discussion of the issue at a federal level.

“We think prohibition should end. We want to make sure our roads continue to be safe, or are safer than they are now,” Boyer said. “It’s also important to remember that once legalization happens, driving under the influence is still illegal. Field sobriety tests do a good job of checking for that. These have been proven to be good indicators of whether someone has the skills to operate a car.”

Paul McCarrier of Legalize Maine said he understands why much of the conversation in Maine is centered on driving after using marijuana, but he argued that the conversations also need to focus on people under the influence of prescription drugs.

“We really have to be aware that there are so many different substances out there that people are frequently using,” he said.