An increasing number of drivers involved in fatal crashes are testing positive for drugs, especially opioids and marijuana, according to a new study by the Governors Highway Safety Association — though it is unclear whether drug use is actually the culprit in those crashes.

The report raises serious concerns at a time when the U.S. is facing an epidemic of opioid usage and as more and more states legalize marijuana for medical and recreational usage. But the GHSA also cautions that it is difficult to fully understand the extent to which “drugged driving” is becoming a problem.

“Drugs can impair, and drug-impaired drivers can crash,” said the report's author, Dr. Jim Hedlund, a former senior official with the National Highway Traffic Safety Administration. “But it’s impossible to understand the full scope of the drugged driving problem because many drivers who are arrested or involved in crashes, even those who are killed, are not tested for drugs. Drivers who are drug-positive may not necessarily be impaired.”

Authorities in the U.S. and abroad have aggressively cracked down on drunken driving over the last several decades. It is relatively easy to test motorists to determine whether they are under the influence of alcohol and determine whether it was a factor in a crash.

The research found that the simultaneous use of multiple drugs is becoming more common.

But what federal data shows is that, where tests were performed, 44 percent of drivers fatally injured in a crash tested positive for drugs in 2016, up from 28 percent a decade earlier. Of those who tested positive for drugs in the latest study, 38 percent had used marijuana, 16 percent had used some form of opioid, and 4 percent tested positive for a combination of both.

The research found that the simultaneous use of multiple drugs is becoming more common. Of the drivers killed in crashes in 2016 who were found to be using alcohol, 49 percent also tested positive for drugs.

“Alcohol-impaired driving and drug-impaired driving can no longer be treated as separate issues,” said Ralph Blackman, president and CEO of Responsibility.org, a Virginia-based nonprofit focused on drunken driving. While the use of alcohol by motorists was responsible for 28 percent of all U.S. traffic fatalities in 2016, Blackman said, “We have to think about the combination of substances drivers are often putting into their systems at the same time.”

“Research has demonstrated the potential of marijuana to impair driving-related skills,” NHTSA advised Congress in a report last July, while noting that it can be difficult to determine when a driver using marijuana is unfit to be behind the wheel. It also questioned the accuracy of tests used to determine how much THC — the active ingredient in cannabis — is in the blood.

Nonetheless, the GHSA says the issue of drug driving cannot be ignored. It calls for new testing procedures, an increase in public awareness campaigns, and increased law enforcement.

“Too many people operate under the false belief that marijuana or opioids don’t impair their ability to drive, or even that these drugs make them safer drivers,” said GHSA Executive Director Jonathan Adkins. “Busting this myth requires states to expand their impaired driving campaigns to include marijuana and opioids along with alcohol to show drivers that impairment is impairment, regardless of substance.”