Or they can't tell yet, at least not until some government research lab perfects a saliva test that will detect recent marijuana use. For now, police departments in states that permit medical marijuana use are struggling to determine the levels at which the drug impairs one's ability to competently operate an automobile, or, in other words, the level of impairment when actual driving seems more like a video game in which the object is to always stay five cautious miles-per-hour under the posted speed limit.
Marijuana advocates in states like Colorado and Washington are scrambling to establish impairment limits in large part because more drivers are admitting to using the drug while behind the wheel, incorrectly believing that, because weed is semi-legal where they live, police are unable to arrest them. Not so, says Sean McAllister, an attorney who defends people charged with driving under the influence of marijuana. "The explosion of medical marijuana patients," explains McAllister, "has led to a lot of drivers sticking the (marijuana) card in law enforcement's face, saying, ‘You can't do anything to me - I'm legal.'"
Though driving under the influence of any drug is illegal, police are hard-pressed to determine a driver's recent marijuana use because the drug remains in a person's bloodstream long after its effects have faded. Most convictions for stoned-driving, however, are based on blood tests (the only definitive indicator) and police observation, the latter of which, if you're familiar with the hopelessly subjective nature of humanity, can sometimes prove an unreliable metric. Authorities believe that eventually a threshold test for driving under the influence of marijuana could resemble the blood-alcohol standard used to determine drunk driving, but introducing such a wide-reaching new standard might entail a new federal law legalizing the drug (spoiler alert for America?).
But what's the big danger in smoking before/while you're behind the wheel of a car? For instance, Angeline Chilton, a Denver woman who suffers from multiple sclerosis, says that she never drives anywhere without getting super high first, and, if Anthony Hopkins's words to Alec Baldwin in The Edge hold any currency in this debate, "What one [woman] can do, another can do." Researchers (squares) caution, however, that since marijuana can cause dizziness and slowed reaction time, drivers are more likely to make whoever is tailgating them in rush-hour traffic really frustrated. They're also more likely to swerve, which presents a serious problem that's easily solved not with drug-testing or any such nonsense, but with wider, more luxurious lanes, like in Seinfeld when Kramer adopts a highway.