It’s pretty difficult to measure impairment from marijuana. That’s why it took so long for Colorado to approve a legal limit for marijuana impairment in drivers: five nanograms of THC per milliliter of blood. But that number has plenty of critics, and rightly so – it says almost nothing about impairment, only that you have marijuana in your system. Marijuana sticks around for days, sometimes longer, and five nanograms is an incredibly small amount.
Researchers at the University of Iowa have done an investigation on the five nanogram limit and found that it was far too low, and not at all similar to having a .08 BAC. That much was obvious, of course, but the researchers also proposed a new limit to more accurately mirror the legal standard for drunk driving: 13.1 nanograms per milliliter.
Picture sitting in a sedan encased in a 24-foot diameter dome surrounded on all sides by a simulated view of a city street, and you have an accurate image of what the test subjects were involved with; a sort of virtual reality drugged driving chamber. The subjects were each given a combination of alcohol, vaporized marijuana and placebos, and then were put behind the wheel of the simulation. Researchers measured each driver’s in-lane weaving, how often the car left the lane and how quickly they were weaving. From this data, the 13.1 nanogram number emerged.
This study is only one step along the road to sensible drugged driving laws. This is the first time we’ve heard a proposed THC limit backed up by experimental evidence, so it will be interesting to see how lawmakers react, and where further experiments lead. To learn more about Colorado drugged driving laws, visit our marijuana FAQ.
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