Perhaps one issue that I would like to bring forward is that if you look at DUI statistics, the number of drivers who test positive for cannabis alone is really not very impressive. In Europe, it's really somewhere down to 3% or 4% at maximum. In the U.S. it's maybe twice as high. In Canada I'm not quite sure, but probably it's comparable to the U.S. But if you look at the percentage of drivers who are positive for THC as well as any other drug or alcohol, then we're actually looking at a more significant number of drivers that I think may even be a more important target for any DUI legislation than just the driver who is under the influence of marijuana per se.
Of course that's not unimportant, but from statistics you can actually say that the combination of cannabis and alcohol is actually much more common. What you've seen, at least in the scientific literature, is that any combination of cannabis and alcohol already increases drug impairment and increases crash risk.
My advice would actually be to copy some of the laws that have been installed in Europe that basically take a zero-tolerance position for any combination of cannabis and alcohol, independent of the actual concentration. Even with BACs below .05 and THC levels beyond five nanograms, the combination will always lead to a very significant increased crash risk. I see that, in your bill, there is an effort to also make a law for combined use of cannabis and alcohol, but it actually has lower limits. Anything above blood alcohol concentration of .05 is considered relevant.
I would argue that any combination, independent of the actual level of concentration, should be an offence. It would actually make more impact, because this is really the most frequent occurrence on the road.