the pot is the reason the person is impaired.
now, how difficult is it to understand?
No. The reason he is impaired is because he used the pot. How difficult is that to understand? The pot is not the reason. The douche would likely have just used a different drug if pot were not available.
pot IS the reason of cognitive impairment. It CAUSES cognitive impairment. And he used it because it causes it - to feel high.
what are you actually trying to prove? that pot is not going to be charged, but the user? well, duh.
the problem is how to reliably measure the extent of the impairment to be able to file charges.
Is there any proof that he would not have had the accident if he had not smoked pot?