What I understand is that among people who are acclimatized and use a lot of marijuana, the level of impairment is considerably less than that of people who are occasional users. People whose bodies have not adjusted to this use of marijuana would be more impaired with a lesser density of the drug in their system.
You're getting, then, a bit of a perverse kind of result, if you're solely relying on the amount of this drug in your bloodstream. Sure, maybe people should not be driving after they've been smoking at all, but the issue is whether they're driving impaired, and the blood test per se may not give you a fair testing of whether or not they're impaired. I think you need a better test to test whether or not they're impaired.