New research from the AAA Foundation of Traffic Safety shows that it's near-impossible to measure when marijuana use by drivers becomes dangerous. They recommend that trained police officers be used to pull over and test drivers suspected of marijuana impairment. (AAA Foundation for Traffic Safety)

New research from the AAA Foundation of Traffic Safety shows that it's near-impossible to measure when marijuana use by drivers becomes dangerous. They recommend that trained police officers be used to pull over and test drivers suspected of marijuana impairment. (AAA Foundation for Traffic Safety)

There is a legal limit for drunk driving, but when it comes to marijuana, new research shows it may be impossible to say just how high is too high to drive.

There’s no breathalyzer for pot, and researchers say blood tests are useless when it comes to telling whether someone who has been smoking is fit to drive.

The question matters now that four states have made it legal for any adult to smoke marijuana, and more than 20 others have approved its use for medical reasons. In Washington, one of the first states to approve recreational marijuana use, a study released this week found that 17 percent of drivers involved in fatal crashes two years after marijuana was legalized had THC, the component that creates the high, in their system.

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At least 20 states have approved laws on marijuana use by drivers. A dozen of them have made any use of the drug by those behind the wheel illegal; six others have set a legal limit similar to the .08 alcohol level, with any driver testing above it subject to a DUI conviction.

Marijuana plants (Raul Arboleda/AFP/Getty Images)

A report by researchers at the AAA Foundation for Traffic Safety said there is no threshold that indicates when a marijuana smoker may be too impaired to drive.

“There is no reliable number that has any meaningful value in terms of predicting impairment,” said Jake Nelson, AAA’s director of traffic safety and advocacy.

The AAA Foundation recommends doing away with setting legal limits for THC. Instead, it says each police department should put a cadre of officers through 72 hours of training, followed by field testing, to be certified as drug recognition experts (DRE).

If an officer suspected marijuana use, a DRE could be called in to conduct an hour-long series of tests, and if they provide confirmation, a blood test would follow.

“It shifts the burden of evidence from the prosecution to the defense,” Nelson said. “If you can prove that you weren’t [using pot], you can avoid conviction, but if you can’t beat this case, then you’re going to jail.”

Nelson says that since there is no threshold, some people who are not fit to drive will get off the hook if they are stopped or involved in a crash.

“People who are driving impaired are still going to crash,” he said. But “if you have a blood concentration of THC below whatever stated threshold it is in your state, essentially the courts will presume sobriety. The odds of getting convicted of impaired driving, even if you’re all over the road, are slim to none because of that number. And because we know from the research that the number is meaningless, that there’s no research that supports any stated threshold, so to have those laws on the books is bad news.”

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The second report released by the AAA Foundation this week examines the effect of marijuana use in Washington state, where recreational use has been legal for more than three years.

The numbers in Washington are made soft by a number of variables. For example, drivers who died within two hours of a fatal crash were likely to be tested for drugs and alcohol, those who died later were less likely to be tested, and those who survived were unlikely to be tested. What’s more, many drivers who tested positive for THC also had alcohol in their system, considered a more potent mix than simply using one of the two intoxicants.

Still, the report found that in 2013, 8 percent of drivers in fatal crashes tested positive for marijuana use. In 2014, the number more than doubled to 17 percent.

“Of all the fatal crashes in the state, the proportion that involved a driver that had recently consumed marijuana more than doubled in one year,” Nelson said. “That doesn’t say that people who had smoked marijuana and got behind the wheel were responsible for an increase in fatal crashes. It means that recent marijuana use is a growing contributing factor in traffic crashes that kill people.”

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