Editor's Note: (This story has been updated to reflect more precise language on the presence of drugs found in the bodies of those involved in 2015 auto fatalities. )
(CNN) Driving while on drugs was associated with more deaths in 2015 than driving with alcohol in one's system, a new report found. Still, some safety experts caution that drunken driving remains a bigger problem and say that "drugged driving," as the report refers to it, needs more research.
Positive drug tests were more common than the presence of alcohol among the fatally injured drivers who were tested in 2015, according to the report (PDF) "Drug-impaired Driving," released Wednesday by the Governors Highway Safety Association and the Foundation for Advancing Alcohol Responsibility, a nonprofit funded by alcohol distillers.
Of those tested, 43% of motorists who died had drugs in their system, the report said. This number surpassed the 37% of motorists who died who tested positive for alcohol in the same year.
"Data in the report showed that for the first time, there are more dead drivers for which we have test results that are positive for drugs than there are who were positive for alcohol," said James Hedlund, an independent safety expert with Highway Safety North in Ithaca, New York. The new report adds to earlier research conducted by Hedlund that addressed behavioral highway safety issues, including drug-impaired driving.
"As states across the country continue to struggle with drug-impaired driving, it's critical that we help them understand the current landscape and provide examples of best practices so they can craft the most effective countermeasures" to combat the issue of drug-impaired driving, governors association Executive Director Jonathan Adkins said.
Driving while impaired is illegal in all 50 states. However, laws and interpretations vary about the definition of drug impairment. Testing practices can also vary amongst states, and there are no uniform laws to determine how often testing is used and what drugs are screened for.
Of the more than 400 drugs that the federal National Highway Traffic Safety Administration tracks, marijuana accounted for 35% of positive tests reported, the new research said. Although usage laws vary -- marijuana for medical purposes is legal in 29 states and the District of Columbia, and laws permit recreational use in eight states and DC -- driving while impaired at any level is illegal anywhere in the United States.
Amphetamines accounted for 9% of substances detected, and more than half of the positive tests in the report were caused by "other drugs." These figures reveal the wide range of known and unknown substances that can contribute to drug impairment.
Currently, there are no drug field tests comparable to a preliminary alcohol screening using a breathalyzer. Law enforcement officials are trained to recognize signs of drug impairment and can make the decision to take a driver into custody for further testing.
"Drug impairment is a complicated topic," Hedlund said. "Drugs can affect people in different ways. Some things make you super excited, and some things slow you down."
The report acknowledged that "many officers are not trained to identify the signs and symptoms of drivers impaired by drugs other than alcohol." The International Association of Chiefs of Police and the National Highway Traffic Safety Administration offer specialized training courses to teach law enforcement officials how to recognize the behavioral signs of drug impairment, but the courses are not required. Often, a full evaluation cannot be done during a roadside stop.
Officials hope that this report will bring more attention to the need for more training and resources to combat this problem. For drivers, Hedlund said, "it's illegal to drive while impaired by drugs in the same way that it's illegal to drive while impaired by alcohol. And you just plain shouldn't do it."
Russ Rader, spokesman for the Insurance Institute for Highway Safety, is skeptical of the report's findings and said alcohol remains the bigger concern.
"There's no question that alcohol remains our biggest highway safety problem," Rader said.
Although the impact of alcohol has been studied for decades, drug impairment and driving has only recently been studied, he said, and the current evidence is weak.
The report cautions that these data do not paint the whole picture. The authors note that only 57% of drivers who were killed in car accidents were tested for drugs. That figure, critics say, is reason enough to be wary of taking this conclusion too seriously.
"There are a couple of problems with drawing the conclusion that drugged driving is now somehow a bigger problem with alcohol," Rader said. "For one, there isn't very consistent testing for drivers who are killed in crashes with regard to drugs."
He's concerned that the new report could detract from efforts to curb alcohol-impaired driving and shift funding instead toward driving under the influence of drugs. Nobody knows how to address the problem of drug-impaired drivers, he said.
"We don't have a good handle on what to do about it, but we do know how to address alcohol impairment," which remains a major problem, he said. "Another problem, particularly with marijuana, is that people often combine the two, so how do you separate them?"
Although critical of the report's findings, Rader said there is no denying that drug-impaired driving is an issue, but "we need research."
"If somebody's impaired," he said, "they are impaired."