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Share of fatal crashes involving THC more than double since 2012, study finds

The percentage of drivers involved in fatal traffic crashes who had the active ingredient in marijuana in their blood has more than doubled since recreational marijuana became legal in Washington, according to newly released estimates.

The AAA Foundation for Traffic Safety announced its latest study of marijuana and driving this week, which builds on the foundation’s 2016 research.

Between 2008 and 2012, the five years before Washington legalized marijuana, an estimated 8.8 percent of Washington drivers involved in fatal crashes were positive for tetrahydrocannabinol, the compound in marijuana that makes users high.

That percent increased to 18 percent for the five years after the drug became legal in December 2012. For 2017, the final year covered by the study, an estimated 21.4 percent of Washington drivers involved in fatal crashes were THC positive, the highest percentage during the 10 years examined.

“This study enabled us to review a full 10 years’ worth of data about the potential impact of marijuana on driving safety, and it raises significant concerns,” David Yang, executive director of the AAA Foundation for Traffic Safety, said in a statement. “Results from the analysis suggest that legalization of recreational use of marijuana may increase the rate of THC-positive drivers involved in fatal crashes.”

Testing and imputation

According to the study, 6,721 drivers were involved in fatal crashes over the 10-year period. Eighty-eight percent of drivers who died were tested for THC, but only 29 percent of surviving drivers were tested.

The study used an imputation method to estimate drivers who were not tested but were still THC positive. Those estimates accounted for 31 to 56 percent of drivers classified as THC positive.

However, even if those imputed estimates are ignored, the numbers still show a significant spike in drivers involved in fatal crashes who tested positive.

The Washington State Department of Transportation reports that fatal crashes have fluctuated in recent years.

In 2012, the year cannabis became legal, there were 409 fatal crashes. That number peaked at 534 in 2017, the final year covered by the AAA study, before dropping to 493 fatal crashes in each of the past two years.

The AAA study, which uses Washington Traffic Safety Commission data, did not attempt to determine if marijuana use caused or contributed to fatal crashes. It considered only the prevalence of drivers estimated to have THC in their blood, defined as at least 1 nanogram per milliliter.

Jennifer Cook, AAA’s senior manager for corporate communications in Bellevue, said that THC level could last for up to six hours after marijuana use for someone who is not a regular user or did not smoke or consume an unusually large amount of marijuana.

However, daily or heavy users may continue to have 1 nanogram or more of THC per milliliter in their blood for days or even weeks after their last use, she said.

Cook emphasized that the study only estimated whether drivers had THC in their blood, not whether they were impaired.

“Science hasn’t shown us yet how much marijuana makes someone impaired,” she said.

Too high to drive

Washington and Colorado were the first states that voted to legalize recreational marijuana in November 2012. Since then, nine others — Alaska, California, Illinois, Maine, Massachusetts, Michigan, Nevada, Oregon and Vermont — have legalized weed, along with Washington, D.C. Several other states, including New York, are mulling legalization.

As more states legalize marijuana, questions remain for determining when a user is too high to drive.

An AAA survey last year found that nearly 70 percent of Americans believe it’s unlikely someone will get caught for driving shortly after using marijuana. The same survey estimated that 14.8 million people had driven within one hour of using marijuana during the past 30 days.

In Washington, it’s illegal to drive with more than 5 nanograms of THC per milliliter of blood. There’s zero tolerance for drivers younger than 21; it’s illegal for them to drive with any THC in their blood.

The Washington State Liquor and Cannabis Control Board says multiple factors can affect how long users should wait before they can legally or safely drive.

“Published research says it can take 3 hours for some people to drop below 5 ng/ml after using marijuana, but it can take longer depending on multiple variables, such as gender and body size,” the board’s website says. “Some people may still be impaired with less than 5 ng/ml of THC in their blood.

“It is less risky to wait at least 5 hours before operating a vehicle. It is recommended that you wait even longer after consuming edible marijuana products as they can remain in your system much longer.”

Arbitrary limits?

Legal limits for determining when a marijuana user can drive have been criticized. In 2016, AAA concluded that such limits are arbitrary and cannot be scientifically supported.

AAA argues that states should use a two-part approach requiring:

• A positive test for recent marijuana use.

• Behavioral and physiological evidence of driver impairment.

AAA continues to oppose legal recreational marijuana because of traffic safety and difficulties in writing legislation that protects the public and treats drivers fairly.

“As an organization,” Cook said, “our No. 1 goal is to keep people who are using our roadways safe.

“Marijuana is legal, and people can use it legally,” she said. “Just don’t get behind the wheel.”


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