The Washington Post's editorial board on Monday came out against marijuana legalization in the District of Columbia, citing concerns about legalization's effects in Colorado. Among its points against legalization, the Post said Colorado has seen a spike in the number of marijuana-impaired drivers.
The available data seems to support the Post's claim. The latest numbers from the Colorado Department of Transportation found more drivers involved in fatal crashes tested positive for marijuana after 2008 and 2009, when medical marijuana dispensaries began opening widely across the state, but no further increase in 2013, the first full year of completely legalized possession. (Data for 2014, the first year of recreational marijuana sales, isn't available.)
But legalization advocates say there's a few reasons to remain cautious of the Post's argument: the data for marijuana-impaired drivers has never been very good, it's unclear if the drivers were actually high while driving, and it's still uncertain how much pot affects someone's ability to drive.
1) Data for Colorado's drugged drivers isn't very good
The Colorado Department of Transportation has reported numbers for drugged drivers for years, but it's never been clear how much can be reliably drawn from the data.
For one, less than half of drivers involved in fatal traffic accidents are actually tested for any drugs, much less marijuana. It's possible, although not very likely, that the drivers not being tested for drugs are relatively clean compared to the sample of tested drivers.
Even when drivers are tested, very few drivers are found to have only marijuana in their system. Among the 630 Colorado drivers involved in fatal car crashes in 2012, 35 came back positive for marijuana only. When the sample size is so small, year-to-year fluctuations could signify statistical noise more than genuine increases in the number of marijuana-impaired drivers on the road.
Mason Tvert of the pro-legalization Marijuana Policy Project pointed out that the data shows all drivers involved in fatal traffic accidents, not the person at fault for the crash. "If someone used marijuana six hours ago and then got hit by someone else, that's not a sign of a marijuana law that's not working," Tvert said.
Colorado, for its part, seems to be well aware of the data's historically poor quality. Starting in January, the Colorado State Patrol began tracking drug impairment by drug type when someone was pulled over for driving under the influence. Between January and May, the Colorado State Patrol found about 12.5 percent of drivers cited for a DUI tested positive for pot.
2) It's unclear if tested drivers were actually high while driving
Since marijuana can remain in someone's system for weeks at a time, it's unclear if the drivers testing positive for marijuana were actually high while they were behind the wheel. The increase could merely show people are driving within weeks of trying or using pot, not necessarily that more people are driving while high.
Kevin Sabet of the anti-legalization Project SAM (Smart Approaches to Marijuana) acknowledged testing positive for pot doesn't necessarily mean a driver was high. "Marijuana does stay in your system longer than alcohol because it absorbs in your fat," Sabet wrote in an email, "but on balance I think most Americans would agree that we want roads to be as safe as possible — and that means erring on the side of caution."
3) It's still uncertain how much pot affects someone's ability to drive
Unlike alcohol-impaired drivers, stoned drivers tend to be more aware that they're impaired. As a result, they might pay even more attention to their driving to avoid being pulled over or causing an accident. "The joke with that is Cheech and Chong being arrested for doing 20 on the freeway," Mark Kleiman, a drug policy expert at UCLA, told the New York Times.
One study from the Pacific Institute for Research and Evaluation (PIRE) found marijuana didn't increase a driver's chances of getting into a crash, and some research from Yale University found being high doesn't impair all drivers in the same way. But in another study, Columbia University researchers estimated that, while alcohol multiplies the chance of a fatal accident by more than 13 times, marijuana nearly doubles the risk.
Despite the conflicting evidence, most researchers seem certain marijuana affects a driver's capabilities in some way. The head of the PIRE study, which found marijuana didn't increase a driver's chances of getting into an accident, previously said that he's not convinced pot is safe on the road.
"Despite our results, I still think that marijuana contributes to crash risk," lead author Edward Romano told the New York Times, "only that its contribution is not as important as it was expected."
Update: The chart was updated with the addition of newly obtained 2013 data.