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The percentage of drivers involved in fatal crashes who had traces of marijuana in their blood has doubled since marijuana was legalized in Washington state, a new study suggests.
A separate study suggests that some of the legal limits that are in use for the level of THC — the main psychoactive ingredient in marijuana — that is concentrated in the blood are higher than the THC levels seen in many drivers who are actually impaired by the drug.
"Marijuana use in driving is a growing, contributing factor to fatal crashes," said Jake Nelson, the director of traffic safety advocacy and research at the American Automobile Association (AAA) said. "It's a highway safety problem that we should all be concerned about." [Where Americans Smoke and Grow Marijuana]
The findings, which were released by the (AAA), suggest that states that have legalized marijuana use need better rules to protect drivers on the road, Nelson said.
High tide
In recent years, a green tide has swept the U.S., with Colorado, Washington, Alaska and Oregon passing laws that allow people to use marijuana recreationally. In addition, using marijuana for medical reasons is now legal in nearly half the states in the country.
Meanwhile, road safety laws are struggling to keep pace with the new laws. In Colorado and Washington, anyone driving with more than 5 nanograms per milliliter of THC in their blood can be prosecuted for driving under the influence. But Oregon and Alaska have no clearly defined legal limit, and instead rely on police officer observations of impaired driving.
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Impaired while driving
To better understand how legalization has affected driving, the AAA looked at crash data from 30-day periods between 2010 and 2014 that was collected from the Washington Traffic Safety Commission. Recreational marijuana was legalized in Washington in 2012.
The team found that prior to legalization, about 8.3 percent of drivers involved in fatal crashes had THC in their blood, but after legalization, 17 percent of drivers had THC in their blood. Of that 17 percent, about two-thirds also had some other drugs or alcohol in their system. The total number of fatal crashes also went up slightly, the study found. [Marijuana vs. Alcohol: Which Is Really Worse for Your Health?]
"It was a small bump, but it was a small bump at a time that fatal crashes were going down in the rest of the country," Nelson told Live Science.
While the study can't prove that marijuana was a key cause of those crashes, it is likely that marijuana is at least one contributor to those fatal crashes, Nelson said.
Legal limit
If that's the case, then states need new rules to mitigate the risk of people driving under the influence of marijuana, he said.
However, the science isn't clear on what exactly constitutes driving while high, and how to measure this, Nelson said. For instance, while a person's blood levels of alcohol reliably predict his or her level of impairment, the actual impairment caused by marijuana occurs when THC enters the fatty tissue in the brain, and THC in the blood may spike before the peak effects of the drug take hold, Nelson said.
To understand how impairment may be tied to blood levels of THC, a separate group of researchers analyzed data from traffic stops of people who were impaired while driving. Police gave those people field sobriety tests, such as asking them to touch their nose, stand on one foot or walk along a line. Next, drug-recognition experts, or police officers who used an hour-long assessment to identify which drugs might have been contributing to the suspect's impairment, analyzed those who failed the field tests. Many of these impaired drivers then submitted to blood tests to confirm that a substance such as marijuana, alcohol or other drugs contributed to their impairment.
The researchers found that 70 percent of drivers who failed these sobriety tests, and whose impairment was attributed to marijuana by drug-recognition experts, still had blood levels of THC lower than 5 nanograms per milliliter.
"There is no number that we can use to reliably predict impairment," Nelson said. "Alcohol and cannabis are very different drugs. They behave in the body in very different ways, and trying to use the system from alcohol is not the way to go."
Instead, states should use a combination of field sobriety tests, blood tests and evidence from drug-recognition experts to identify drivers who are impaired by marijuana, Nelson said. One strategy would be to pass a law that says that if any marijuana is found in the body, the driver or the driver's lawyer would need to prove that marijuana was not the cause of the person's impaired driving, he said.
Overall safety issue
While it's possible marijuana legalization increased the percentage of drivers who are smoking pot before getting behind the wheel, the data doesn't prove that legalizing pot worsens road safety, said Benjamen Hansen, an economist at the University of Oregon in Eugene and at the National Bureau of Economic Research. Hansen has conducted other research on marijuana legalization in relation to driving accidents.
For instance, it's possible that police are simply testing for THC more often now that the drug has been legalized, and are therefore catching people who might have been missed in previous years, Hansen said. It's also possible that people who are found to have detectable levels of THC in their blood were not impaired at the time of the crash, he added.
Even if marijuana does cause more bad driving, it's still not clear that marijuana legalization reduces overall traffic safety, Hansen said. Legalizing the drug has widespread societal impacts, some of which may reduce the general risk of dangerous driving, he said.
For instance, if more people stay home, "getting high in their basements," rather than getting drunk in a bar, that may lower the number of drunk drivers on the roads, Hansen said. And a few studies suggest that driving while high may be safer than driving while drunk: In driving simulators, for instance, people who are high are less likely to weave out of their lane and speed than drunk drivers, a 2015 study in the journal Drug and Alcohol Dependence found.
On the other hand, legalization may make recreational marijuana use more socially acceptable, and so more people may venture out of their homes to smoke pot, thereby increasing the risk of people driving while high, Hansen said.
Either way, researchers need more information on the impacts of marijuana, Hansen said.
"At the federal level, we need to do a much better job of collecting information on THC and CBD and the active ingredients in marijuana," Hansen told Live Science.
Follow Tia Ghose on Twitter and Google+. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.
Tia is the editor-in-chief (premium) and was formerly managing editor and senior writer for Live Science. Her work has appeared in Scientific American, Wired.com, Science News and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.