This is really not a complicated issue, when civility and intelligence (not the brutish and deluded sense of control) form the priority.
There is no clear line to determine when someone using cannabis is incapable of driving with one obvious exception… the demonstration of impaired driving.
Impaired driving is obviously not limited to (inclusively pharmaceutical) drug intake. A highly agitated emotional state can be terribly distracting, noting that the right form of cannabis intake can ironically alleviate that problem (so improves driving focus).
Police should only pull people over, when they demonstrate an inability to safely drive.
That inability should obviously be illegal, and proper legal remedy should then be applied — e.g. detaining someone until they are able to drive safely, apply a fitting fine, and a fitting negative mark on their record that logically negatively affects that person’s auto insurance rate.
Otherwise, the law itself becomes the metaphorical net scraping the bottom of the ocean to wrongfully catch dolphins and so heinously on.