one joint and you are incapable of legally driving.
not so with alchol. We are talking about pot, not red herrings.
Undoubtedly, that is true in most states, but is it true in Colorado (or Washington)? I'm not sure what the law is regarding driving under the influence of marijuana in states where it's legal.
If you are talking about being impared, then that's a different question. There have been studies that show that driving ability is impaired even below legal limits. The question is one of degree. Do we really want to treat someone who's had one drink and is slightly impaired similarly to someone who's got a 0.15% BAC and is severely impaired? Probably not.
Similarly, I am sure that smoking one joint would cause impairement, but how much? Even if smoking one joint causes enough impairment to legitimately charge such a person with DUI, why should someone who uses marijuana be legally disallowed from driving? Do you really think that marijuana users are CONSTANTLY high? Why should someone not be allowed to drive to work in the morning, home in the afternoon, and then smoke a joint without doing any more driving (assuming that you're talking about a state like CO where that's legal)?