>> Correlation is not causation.
It’s time to retire this cute, meaningless expression.
Anything that impairs perception, responsiveness, and control is likely a contributing factor to car crashes.
Nervous, weary drivers are just as dangerous as reckless drivers.
The expression is meaningful, because correlation is so horribly abused as being causative. The book How To Deceive With Statistics is to a great extent about that.
Do rising hem heights in women’s dresses make the stock market go up? Close to a 100% correlation.
In this case, significant impairment with marijuana lasts just a few hours, but significant amounts of marijuana last in the body for two weeks. So unless someone involved in an accident admits that they smoked marijuana in the few hours before an accident, causation is less than marginal, it is incidental. You might as well blame it on coffee or soda.
If you examine these accidents on an individual basis, you would likely find that at some point an officer smelled marijuana, so *assumed* marijuana impairment. This is because there is no test for marijuana impairment, just its presence in the blood.
However, vehicle contamination with the marijuana smell can last for months. Some US government facilities that regularly dog-test for marijuana actually issued waivers for newly purchased used vehicles that are so contaminated they will always smell of marijuana.