The biggest problem with drawing conclusions from the 40% number is that THC use correlates with other well established crash death risks like being a young driver or the use of other impairing substances.

The fact that legalization did not impact the crash rates is also a strong signal that THC itself is not causing the crashes.

The presence of THC in the blood is not a reliable signal for intoxication, so further research is needed to draw any type of conclusion.

Finally, it's also been noted that there are some sample bias concerns because the data comes from fatal crashes where it was determined that a drug test should be administered after the crash.