Usually when we imagine someone charged with a DWI, we think of someone getting behind the wheel after drinking one too many. The North Carolina DWI statute, however, is broader than just alcohol and includes driving a vehicle on a highway, street or public vehicular area in North Carolina “While under the influence of an impairing substance.” Marijuana is undeniably an impairing substance, but the ability to test for impairment is more difficult than with alcohol. How much marijuana must be in a person’s system for the person to be considered “under the influence of an impairing substance?”
Under G.S. 20-138.1, to be found guilty of DWI, a person must be driving a vehicle on a highway, street or public vehicular area in North Carolina:
(1) While under the influence of an impairing substance; or
(2) After having consumed sufficient alcohol that he has, at any relevant time after the driving, an alcohol concentration of 0.08 or more. The results of a chemical analysis shall be deemed sufficient evidence to prove a person's alcohol concentration; or
(3) With any amount of a Schedule I controlled substance, as listed in G.S. 90-89, or its metabolites in his blood or urine.
The test for alcohol in part (2) of the statute is very specific, and many people who have never dealt with DWI personally are familiar with the 0.08 alcohol limit. The North Carolina law also sets out in part (3) that Schedule I drugs (such as heroin and mescaline) are per se impairing, so that any amount at all found in the driver’s blood or urine proves impairment. However, marijuana is not a Schedule I drug but a Schedule IV drug, so impairment must be proven under part (1) of the DWI statute.
The problem with proving impairment from marijuana is that the blood and urine tests that are used to test for other drugs do not give a good indication of impairment from marijuana. Because alcohol has only one intoxicant and because of the way that alcohol is metabolized by the body, the blood alcohol content is a good indicator of impairment from alcohol. However, not only does marijuana have more than one intoxicant, but THC (the major psychoactive constituent of marijuana) is treated very differently by the body and can show up in a person’s blood days after that person has smoked or ingested the marijuana.
According to the National Highway Traffic Safety Administration fact sheet on marijuana, which summarizes various studies conducted on marijuana, blood tests are not a good indicator of impairment because there is not a good relationship between the amount of THC in a person’s system and the impairment of the person. People who are chronic users of marijuana can have high levels of THC in their blood even 12 hours after using marijuana. The fact sheet concludes that “It is inadvisable to try and predict effects based on blood THC concentrations alone.”
Another way to test for marijuana use is through urine tests, but the National Highway Traffic Safety Administration fact sheet concludes that this also is not a good indicator of impairment. Urine tests detect a THC metabolite, which only indicates prior use of marijuana. This metabolite can take as long as 4 hours after marijuana use to appear in a urine test, although impairment typically lasts about 2 hours after marijuana use. A positive urine test will usually indicate marijuana use within a few days, but for chronic marijuana users, the metabolite can appear even weeks later.
Similarly, the field tests that police officers typically employ to test for impairment from alcohol are not as reliable for indicating impairment from marijuana. Colorado, where the recreational use of marijuana is legal, has recently trained police officers in a variety of tests that are better indicators of impairment from marijuana. Oregon has also trained certain police officers to better detect impairment from marijuana, but most states have not spent the resources conducting this type of training.
Some states have dealt with the difficulty of detecting impairment from marijuana by providing that any amount found in the blood or urine is per se impairment. This is the same way that North Carolina treats Schedule I drugs. The rationale behind this policy is that there is a zero tolerance policy for the use of illegal drugs. Curiously, there are a couple of states that have legalized the medical use of marijuana which also employ a per se impairment doctrine for marijuana.
Would there be a different analysis if North Carolina were to legalize the use of marijuana? In both Colorado and Washington, where the recreational use of marijuana is legal, there is a blood test limit. It is illegal to drive where THC levels are greater than or equal to 5 nanograms per milliliter of blood. However, this limit has been controversial and has been argued that it is too high. Based on studies conducted in Europe, many impaired drivers would go undetected with the current limit, and limits as low as 1 nanogram per milliliter of blood have been recommended. However, because of the way that THC stays in the blood of chronic users, a lower limit could make chronic marijuana users violate the limit long after being high.
After all this discussion, should we even worry about people driving stoned? Various studies have found that driving while stoned is not as dangerous as driving while drunk. However, the National Highway Traffic Safety Administration fact sheet did conclude that impairment did result from marijuana use, especially in difficult or unpredictable tasks. Also, the impairment from the use of marijuana and alcohol together is more severe than the impairment from either one by itself. For now, under the current North Carolina statute impairment from marijuana alone will remain difficult to prove.