Roughly 10 percent of Washington state drivers involved in fatal car crashes between 2010 and 2014 tested positive for recent marijuana use, with the percentage of drivers who had used pot within hours of a crash doubling between 2013 and 2014, according to a new study by the AAA Foundation for Traffic Safety.

Although the uptick in fatal crashes comes after Washington citizens voted in 2012 to legalize marijuana — and as other states are expected to consider similar measures — a second AAA study discourages lawmakers from adopting “arbitrary legal limits” on marijuana use because of a lack of adequate methods to determine impairment by the drug.

AAA officials said the studies about marijuana and driving, released Tuesday, are meant to encourage more comprehensive enforcement measures to improve road safety.

Authorities in Washington recorded 436 fatal crashes in 2013, and determined that drivers involved in 40 crashes tested positive for THC, the active chemical in marijuana, according to the study. In 2014 they found that of 462 fatal crashes, 85 drivers tested positive for THC.

The fatal crash study does not determine whether drivers were impaired, and it notes that there was no sign of an increase in fatal crashes among those with marijuana in their systems until a full 39 weeks after marijuana possession was legalized in the state.

But the uptick in crashes by people testing positive for recent marijuana use also raises concern over the lack of adequate methods to determine whether drivers are actually impaired by marijuana when they get behind the wheel.

“The significant increase in fatal crashes involving marijuana is alarming,” said Peter Kissinger, president and CEO of the AAA Foundation for Traffic Safety. “Washington serves as an eye-opening case study for what other states may experience with road safety after legalizing the drug.”

Michael Green, a spokesman for AAA, said tests used were meant to detect active THC, and are generally thought to detect marijuana use within the last four to six hours.

Unlike tests used to detect drunken driving, there are no proven blood or urine tests that can determine how high a person is from marijuana, only how much marijuana is in their system. As a result, AAA officials said adopting “per se” limits that specify the maximum amount of active THC that drivers can have in their system are not useful.

“There is understandably a strong desire by both lawmakers and the public to create legal limits for marijuana impairment, in the same manner as we do with alcohol,” said AAA President Marshall Doney. “In the case of marijuana, this approach is flawed and not supported by scientific research.”

The second AAA study instead encourages states looking to limit driving under the influence of marijuana to place more emphasis on teaching police officers to conduct roadside tests that would detect signs and symptoms of marijuana.

It’s a suggestion that Paul Armentano, deputy director of the pro-marijuana group NORML, supports so long as field sobriety tests are developed to measure the known effects of marijuana rather than trying to adapt methods developed to test alcohol impairment to fit the drug.

Because of the lack of adequate tests to determine impairment, Mr. Armentano questions whether AAA’s findings on fatal crashes can claim any causality link to marijuana usage.

“We should not conflate the detection of certain substance with the notion that the driver was necessarily impaired with certain substances,” he said. “This data may reflect that a greater portion of the public is using cannabis, that police are more routinely screening for cannabis or that there have been procedural changes in the way drug screening in accidents is performed.”

Mr. Armentano instead points to a 2015 study from the National Highway Traffic Safety Administration that found that motorists who used marijuana prior to driving were no more likely to be involved in a car crash than individuals who had not used any drugs or alcohol prior to getting behind the wheel. Meanwhile, the study found drivers with a blood alcohol concentration of .05 or above were seven times more likely to be involved in a crash.

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