WASHINGTON — 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.

A demonstrator waves a flag with marijuana leaves on it during a protest calling for the legalization of marijuana outside the White House in Washington last month. The degree to which a driver is impaired by marijuana use depends a lot on the individual, a study concludes. The Associated Press

The question matters now that four states have made it legal for any adult to smoke marijuana, and more than 20 others, including Maine, 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.

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.

The problem is only growing as more states contemplate legalizing the drug. At least three, and possibly as many as 11 states will vote this fall on ballot measures to legalize marijuana for medicinal or recreational use, or both.

In Maine, which legalized medical marijuana use in 1999, a referendum to legalize marijuana for recreational use is on the November ballot. The Legislature considered a bill to set a 5 nanogram limit in drivers’ blood, but it was rejected by the House last month.

Currently, six states – Colorado, Montana, Nevada, Ohio, Pennsylvania and Washington – have set specific limits for THC, the chemical in marijuana that makes people high, in drivers’ blood, the Associated Press reported.

But the study by the AAA Foundation for Traffic Safety says the limits have no scientific basis and can result in innocent drivers being convicted, and in guilty drivers being released.

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

Another nine states, including some that have legalized marijuana for medical use, have zero-tolerance laws for driving and marijuana that make not only any presence of THC in a driver’s blood illegal, but also the presence of its metabolites, which can linger in a driver’s bloodstream for weeks after any impairment has dissipated.

DETERMINATION IS COMPLEX

That makes no sense, said Mark A. R. Kleiman, a New York University professor specializing in issues involving drugs and criminal policy.

“A law against driving with THC in your bloodstream is not a law you can know you are obeying except by never smoking marijuana or never driving,” he told the AP.

The problem is that determining whether someone is impaired by marijuana, as opposed to having merely used the drug, is far more complex than the simple and reliable tests that have been developed for alcohol impairment.

The degree to which a driver is impaired by marijuana use depends a lot on the individual, the foundation said. Drivers with relatively high levels of THC in their systems might not be impaired, especially if they are regular users, while others with relatively low levels may be unsafe behind the wheel.

Some drivers may be impaired when they are stopped by police, but by the time their blood is tested they have fallen below the legal threshold because active THC dissipates rapidly. The average time to collect blood from a suspected driver is often more than two hours because taking a blood sample typically requires a warrant and transport to a police station or hospital, the foundation said.

In addition, frequent marijuana users can exhibit persistent levels of the drug long after use, while THC levels can decline more rapidly among occasional users.

‘PICKED OUT OF THIN AIR’

Colorado’s 5-nanogram limit for THC in blood “was picked out of thin air by politicians,” said Robert Corry, a Denver criminal defense attorney. “Innocent people are convicted of DUI because of this.”

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.

If an officer suspected marijuana use, one of the experts 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.”

Melanie Brinegar, who uses marijuana every day to control back pain, was stopped by police two years ago for having an expired license plate, the AP reported. The officer smelled marijuana and Brinegar acknowledged she had used the drug earlier in the day. Her blood test showed a level of 19 nanograms, well over the state limit. She was arrested and charged with driving while impaired.

Brinegar, 30, who lives in Denver, said she spent the next 13 months working 80 to 90 hours a week to pay for a lawyer to help her fight the charge and eventually was acquitted. People like herself will always test positive for THC whether they are high or not because of their frequent use, she said.

“It took a good amount of my time and my life,” she said. “There is still that worry if I get pulled over (again).”

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.”

Studies show that using marijuana and driving roughly doubles the risk of a crash, Kleiman told the AP. By comparison, talking on a hands-free cellphone while driving – legal in all states – quadruples crash risk, he said. A blood alcohol content of .12, which is about the median amount in drunken-driving cases, increases crash risk by about 15 times, he said.

Driving with “a noisy child in the back of the car” is about as dangerous as using marijuana and driving, Kleiman said.

The exception is when a driver has both been using marijuana and drinking alcohol because the two substances together greatly heighten impairment, he said.

Related Stories