I don't know what "THC in their system" is supposed to mean. I could've just driven with THC in my system but it's been a good 18 hours since I did anything, so if I crash does that still count?

The raw numbers are (relatively) flat despite the change in THC-positive drivers so it is impossible to conclude that re-legalization has caused an increase in traffic fatalities in WA. It is more likely that a) more drivers are being tested, to see if they are above the 5ng per se limit (which is relatively new), and b) the positive tests reflect relatively recent cannabis use but not impairment. There is nothing to suggest that any of these drivers were impaired by cannabis when the accident occured. This doesn't mean that they weren't; it just means that these particular data cannot possibly answer that particular question, which is by the way the only one that matters.

Obviously, given the growth in population, we need to crack down on people driving. That's the root cause.

Don't forget we've also had a lot of California transplants. Many are young men with fast cars and prior exposure to cannabis, and they're unfamiliar with Washington roads.

Yes, obviously cannabis is causing drivers to kill kids, just like the opposition campaigns predicted for recreational pot legalization and taking the state out of running liquor stores. We're well on our way to bedlam and if you can't see that then you're just not properly informed on the matter.

Additionally, texting and other forms of distracted driving should be considered as factors.

A majority of the data points for this study come from BEFORE legal marijuana was a factor or likely a factor. To quite the frickin' Seattle Times:

"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."

One key number is missing here. That's total miles driven in Washington State. If miles driven doubled but crashes remained the same, that would obviously mean that the roads were getting safer. Fortunately, WSDOT provides that number. In 2013, we get 0.00001378 fatalities per mile driven. In 2014, we get 0.00001436.That's a 4.2% bump. It's not statistically significant (73% confidence). Basically, total fatalities are flat. We look at those that are marijuana positive, and we see a 52.1% bump, which is not as dramatic as doubling, but is damn well statistically significant (99% confidence). Alcohol involved goes down 14%. Not statistically significant change (88% confidence). So yeah, there was a real, measurable, significant difference in pot deaths in 2014. Causation is another question, but there's definitely a real increase.