In Michigan, you can smoke marijuana and still drive a car. That's what the Michigan Supreme Court ruled this Tuesday, albeit on a technicality. Though Michigan has a zero-tolerance policy for driving "under the influence" of marijuana, it also has a law on the books that exempts medical marijuana users from any sort of persecution for its use. The court had to decide which of the two laws it wanted to uphold.
Since Michigan doesn't actually specify an amount of marijuana in a user's system that impairs driving judgement enough to be considered "under the influence," simply outlawing drugged driving altogether went too far, argued the court. If the state could prove that a medical marijuana user was under the influence, the court decided, then they could be legally convicted for driving after using the drug.
The easy way to do that, though, might simply be to revise the law. Washington, Nevada, Ohio, and Colorado have indeed set specific legal driving limits of a certain number of nanograms of THC in a driver's blood, and the court recommended that Michigan do the same before long.