The key is whether your ability to drive is impaired, and to what degree. The law contains a presumption with respect to the alcohol content of your blood and its effect on your driving, so it always becomes a fight about BAC, even though the law speaks in terms of impairment. No such presumption exists with respect to mj, so a DWAID/DUID case would have to revolve around whether your ability to drive is impaired to the slightest degree (DWAID), or substantially (DUID). I think this is the correct terminology, it's been a long time. Watch for the law enforcement lobby (why should such a thing be allowed to exist?) to attempt to convince our idiot legislators, many of whom are ex-prosecutors, to codify some form of presumption for mj, either time from consumption to driving or something else. One of the reasons the law enforcement community has been against legalization (other than its effect on their livelihood0 is the lack of a quick and easy test to determine whether one is currently under the influence. I have heard that such a test is now available, although very expensive. If it comes down in price, and especially if legalization comes to pass, roadside tests for mj will become commonplace.