alcohol on your breath is a dead giveaway and its easy to check for with a quick blood sample....weed not so much....weed users can act laid back and the cop would never know their high except they just crashed their car into a light pole.
and if and when they develop a quick and easy test for weed my intuition and background tell me that a good number of “alcohol” related accidents would also include weed use...its just not checked for....