Again, looking at the 2012 study of chronic cannabis smokers and the physiological changes, what it concluded was that no significant differences were observed in critical tracking or divided attention task performance in the cohort of heavy chronic cannabis users. They found there was very minimal impairment in terms of psychomotor tasks in the chronic users. But of course, any amount of alcohol will impair—a glass of wine will impair—if you're going to drive.
With regard to multi-drug use, among the most common drugs used are antihistamines or cough medicines. We know those can impair one quite significantly. Should we therefore test people for antihistamines and cough syrup when we're checking for their impaired motor skills when they're driving?
The bottom line is that lots of drugs that people are taking for bona fide reasons, medications, etc., can impair, yet people get in their car and drive using antihistamines or cough medicines.