By Pat Anson, PNN Editor, and Shefali Luthra, Kaiser Health News

With medical marijuana now legal in nearly three dozen states – and ten allowing its recreational use – state governments face two difficult questions: Are more people driving under the influence of marijuana? And at what point are they too impaired to drive?

The answer to the first question is troubling and the second one is elusive. 

According to a new survey, over half the people who take medical cannabis for chronic pain say they've driven under the influence of cannabis within two hours of using it.  And one in five say they've driven while “very high.”

Researchers at the University of Michigan Addiction Center surveyed 790 Michigan adults who sought medical cannabis certification for chronic pain in 2014 and 2015. The researchers asked about their driving habits over the past six months.

Fifty-six percent reported driving within two hours of using cannabis, 51% said they drove while a "little high" and 21% reported driving while "very high."

"We want people to know that they should ideally wait several hours to operate a vehicle after using cannabis, regardless of whether it is for medical use or not," said Erin Bonar, PhD, a psychiatry professor, clinical psychologist and lead author of the study published in the journal Drug & Alcohol Dependence. "The safest strategy is to not drive at all on the day you used marijuana."

The survey found that patients with higher pain levels were less likely to to drive while impaired. But Bonar says the overall risk of impairment could be higher for chronic pain patients who use medical marijuana daily and have trace amounts of tetrahydrocannabinol (THC) lingering in their system. THC is marijuana’s main psychoactive ingredient.

"When it comes to driving, we haven't yet figured out the best way to know how impaired marijuana users are at any given time," she says. “It's hard to quantify because there is a lot of variation in marijuana dosing, THC potency, and route of administration. We also don't have specific guidelines yet about when exactly it would be safe to operate a vehicle.

"We also need clearer guidelines about marijuana dosing and side effects with an understanding of how individual differences in things like sex and body weight interact as well."

The study was funded by the National Institute on Drug Abuse.

Measuring Impairment

Brain scientists, pharmacologists and law enforcement are still learning how measure if and to what extent marijuana causes impairment.

Blood and urine tests can detect marijuana use, but because traces of the drug stay in the human body for a long time, those tests can’t specify whether the use occurred earlier that day or that month. They also don’t indicate the level at which a driver would be considered under the influence.

“It’s a really hard problem,” said Keith Humphreys, PhD, a psychiatry professor and drug policy expert at Stanford University in California, the first state to legalize medical marijuana and where recreational cannabis use among adults became legal in 2016. “We don’t really have good evidence — even if we know someone has been using — what their level of impairment is.”

For alcohol, there is a clear, national standard for impairment. If your blood alcohol content is 0.08 percent or higher, you’re considered cognitively impaired at a level that is unsafe to drive.

Extensive research supports this determination, and the clarity makes enforcement of drunk driving laws easier.

Setting a marijuana-related impairment level is a murkier proposition. And states that have legalized cannabis have to figure it out, experts said.

“You can’t legalize a substance and not have a coherent policy for controlling driving under the influence of that substance,” said Steven Davenport, an assistant policy researcher at the nonprofit Rand Corp., who specializes in marijuana research.

With marijuana going mainstream around the country, regulators are “playing catch-up,” according to Thomas Marcotte, a psychiatry professor at the University of California-San Diego and one of a number of academics who is researching driving while high.

States have put forth a bevy of approaches. At least five outlaw driving if someone’s blood level of THC exceeds a certain amount. Colorado, where voters approved recreational marijuana in 2012, has this type of driving law on the books. But it took three years to pass amid fiery debate and deems “intoxicated” any driver who tests higher than 5 nanograms of THC per milliliter of blood.

Rhode Island, Pennsylvania and Indiana are among states that forbid driving at any THC level.

Still others say drivers should be penalized only if they are impaired by THC -- a standard that sounds reasonable but quickly gets difficult to measure or even define.

None of these approaches offers an ideal solution, experts say. “We’re still definitely evaluating which policies are the most effective,” said Ann Kitch, who tracks the marijuana and driving issue for the National Conference of State Legislatures.

States that set a THC-level standard confront weak technology and limited science. THC testing is imprecise at best, since the chemical can stay in someone’s bloodstream for weeks after it was ingested. Someone could legally smoke a joint and still have THC appear in blood or urine samples long after the high passes.

There’s general agreement that driving while high is bad, but there’s no linear relationship between THC levels and the degree of impairment. States that have picked a number to reflect when THC in the bloodstream becomes a hazard have “made it up,” argued Humphreys.

“The ones who wrote [a number] into legislation felt they had to say something,” he said. But “we don’t know what would be the analogy. Is the legal amount [of THC] equal to a beer? Is that how impaired you are? Is it a six-pack?”

Roadside testing for THC is also logistically difficult. Blood and urine samples need to be collected by a medical professional and analyzed in a lab.

In Canada, where recreational cannabis was legalized last year, law enforcement will test drivers with a saliva test called the Dräger DrugTest 5000. But that isn’t perfect, either.

Some private companies are trying to develop a sort of breathalyzer for marijuana. But Jonathan Caulkins, a drug policy researcher at Carnegie Mellon University, said, “There are fundamental issues with the chemistry and pharmacokinetics. It’s really hard to have an objective, easy-to-administer roadside test.”

Until a reliable system is found, states are relying on law enforcement to make a subjective assessment of whether someone’s driving appears impaired by marijuana.

In California, every CHP officer learns to administer field sobriety tests — undergoing an extra 16 hours of training to recognize the influence of different drugs, including marijuana. Because medical marijuana has been legal there since 1996, officers are “very used” to recognizing its influence, says Glenn Glazer, the state’s coordinator for its drug recognition expert training program.

That kind of training is taking off in other states, too. Lobbying groups such as Mothers Against Drunk Driving are pushing for more law enforcement training to help officers assess whether a driver is impaired.

In the meantime, the public health threat is real. States with legalized cannabis appear to have more car crashes, though the relationship is muddled.

“This is going to be a headache of an issue for a decade,” Caulkins said.

Kaiser Health News is a nonprofit news service covering health issues. It is an editorially independent program of the Kaiser Family Foundation, which is not affiliated with Kaiser Permanente.