Is Marijuana Causing More Car Crashes in Washington?

The presence of THC does not necessarily indicate impairment.


Data released by the Washington Traffic Safety Commission (WTSC) this week indicate that the number of drivers involved in fatal crashes with active THC in their blood jumped from 38 in 2013 to 75 last year. As The Seattle Times notes, the reasons for that increase are not entirely clear:

One obvious reason is that state-regulated pot stores opened in 2014, providing access to legal weed. But the first few stores didn't open until July, and their supply was scarce. Seattle, allotted 21 stores by state officials, saw only one shop selling pot until late September.

What's more, there were more marijuana-involved fatal crashes in the first half of 2014, before stores opened, than in the second half of the year.

Contrary to comments by Staci Hoff, the WTSC's director of data and research, the presence of active THC does not necessarily indicate that a driver was impaired by marijuana at the time of the crash, let alone that marijuana caused the accident. Noting that 85 percent of "cannabis-positive" drivers involved in fatal accidents had active THC (as opposed to an inactive THC metabolite) in their blood last year, Hoff concludes that "most of them were high." That is not a safe conclusion to draw, because (as the National Highway Traffic Safety Administration points out) there is no reliable way to relate THC blood levels to impairment.

About half of the 75 drivers who tested positive for active THC had blood levels higher than five nanograms per milliliter, which is Washington's definition of driving under the influence. But that standard is highly controversial because regular users, who develop tolerance and accumulate THC in their fatty tissue, may exceed five nanograms all the time, even when they are not impaired. So while some of the fatally injured drivers may indeed have been "high" at five nanograms, it is not reasonable to assume that all of them were, let alone that the drivers below that arbitrary threshold were.

The picture is further complicated by the presence of other drugs. The Times notes that "half the drivers with active THC in their blood also were under the influence of alcohol, and the majority of those were legally intoxicated." Alcohol has a much more dramatic impact on driving ability than marijuana does, and the two together have a greater effect than either alone. The Times adds that the WTSC's analysis "doesn't account for prescription drugs in the marijuana-positive drivers."

Although marijuana's contribution to traffic accidents is hard to pin down, it is possible than an increase in cannabis consumption following legalization would lead to more stoned drivers on the road, resulting in more crashes. Alternatively, if more pot smoking is accompanied by less drinking, the net result could be fewer crashes, since alcohol impairs drivers a lot more than marijuana does. It is not clear yet whether either of those scenarios is materializing in Washington.

WTSC data show the total number of traffic fatalities rose by 6 percent last year (from 436 to 462) after falling the previous six years (including 2013, the first full year in which recreational use was legal, although state-licensed pot stores were not open yet). The number of fatalities from accidents in which the driver tested positive for marijuana (which does not necessarily mean he was impaired by marijuana) rose by 55 percent (from 64 to 99). Meanwhile, the number of fatalities from accidents in which the driver was deemed to be impaired by alcohol fell by 13 percent (from 127 to 111). That number had declined or remained steady in the previous six years, except for a 14 percent increase in 2009.

The 6 percent increase in total fatalities is consistent with the idea that legalization raises the number of dangerously impaired drivers. But that increase occurred entirely in the first half of 2014, before the pot shops started to open, which is a bit of a puzzle. By comparison, Colorado, where state-licensed marijuana merchants were open for business throughout 2014, saw only a 1.5 percent increase in total traffic fatalities that year. To get a better idea of what is happening, we will need more years of data, plus comparisons to trends in other states that have not legalized marijuana.