Marijuana and alcohol driving impairments are so extremely different.  In general marijuana impairs cognitive sensory functions and alcohol impairs motor, with overlap on both.  Strategies to detect impairment are different for each.  And currently there is no effective means to determine impairment due to marijuana alone.  But what is clear there are negative consequences with increasing events related to marijuana impairment in those states with legal recreational use.  The state of Washington data indicates that a driver involved in a fatal crash testing positive for marijuana alone, no alcohol or other drug, is a the very least five times more likely to have killed someone other than themselves compared to a driver positive for alcohol alone.  IMMAD is a test for law enforcement that is in tablet form and measures sensory cognitive deficits with marijuana use.  It capitalizes on retinal cell dysfunction.  It is simple, short and objective.  More research is needed.