The National Highway Traffic Safety Administration (“NHTSA”) is the imperfect brain behind the standards that guide DUI investigation around the country, and a recent research note by them confirms just how little is known about the relationship between THC concentrations in the blood and impairment from that THC.
As the NHTSA study described:
Every State has enacted a law defining drivers who are at or above .08 grams per deciliter BAC as “legally impaired,” but there are no similar, commonly accepted impairment levels for
other drugs. Some State laws have established levels for some drugs at which it is illegal to operate a motor vehicle (Lacey, Brainard, & Snitow, 2010; Walsh, 2009). The alcohol laws are based on evidence concerning the decreased ability of drivers across the population to function safely at these BACs. Such evidence is not currently available for concentrations of other drugs.
What a gentle way of saying Colorado doesn’t have a scientific justification for its per se DUID law! Colorado presumes that a driver with greater than 5ng of active THC per milliliter of blood is impaired by that THC. This per se limit was clearly a measure Colorado felt it had to do with marijuana use on the rise, and so it set an arbitrary level fast rather than waiting to develop a scientifically valid level (or no per se level, which might just be what the science dictates).
This is not to encourage driving while stoned—please don’t. But fairness demands that the heavy sanctions of DUI or DUID prosecution only accrue to those who have actually put the public in danger. Just as most drivers can have a couple drinks and be safe to drive, so are there many marijuana users who can have some active THC (even greater than 5ng) and not be too impaired to drive. When in doubt, don’t drive. But if you get charged, speak with a criminal defense lawyer who can challenge the bad science involved and hold the prosecution to its burden.