Further, pot is fat-soluble and is detectable in the blood for months when using mass-spectroscopy. There is no clear blood-pot-level for impairment as there is with ethanol. So the odds of finding some level of pot despite the length of time since used is greater than for similar ethanol use.
I'd rather be on a drug-free highway but if I had my choice I would pick the slow-driving potheads than the reckless boozehounds.
Statistics for Dummies: note the word “involved” - in a multiple car accident it means all regardless of who caused the accident.