Driving under the influence of marijuana is a tricky issue, in large part because it is not clear what impact marijuana has on driving and because determining such influence is so difficult. A study on the effects of driving under the influence of marijuana showed that drunk driving was much more dangerous than stoned driving. Marijuana impairment reduced the driver’s peripheral vision and made the driver slightly more likely to weave within the lane. Alcohol, on the other hand, made it more likely for drivers to actually swerve out of their lane. Another study showed that marijuana use could reduce a driver’s capacity to manage distractions but not with the same severity of alcohol.
Even assuming that marijuana impairment significantly inhibits one’s ability to drive, there is no truly effective method to test for impairment. Officers use drug tests to determine whether a driver is under the influence of drugs. Drug tests identify metabolites from cannabis that can remain in a person’s body for up to 30 days. These tests do not indicate when a person consumed cannabis or whether that person is impaired; they simply measure the amount of THC in the body at that time. In many states, drivers can be arrested for stoned driving weeks after they actually consumed marijuana.
States have taken a variety of approaches in creating “drugged driving laws.” Some use a Zero-Tolerance Per Se Cannabis limit. If a driver has any trace of marijuana, he or she is guilty. Under this test, the actual impairment of the driver is irrelevant.
Per se marijuana laws are similar to the Zero-Tolerance methods, but instead each state sets a THC limit akin to a Blood Alcohol Content threshold. If the driver exceeds this limit he or she will face DUI charges, regardless of impairment.
Colorado uses Permissive Inference laws where impairment is inferred, but not defined, by the amount of THC in blood levels. This means that a person who has a certain level THC in his or her blood stream may be subject to a DUI charge, but guilt is not automatically presumed.
Finally, many states use Effect-Based Laws where evidence of impairment due to recent consumption of marijuana must be established. These laws focus on the behavioral impairment of the driver, rather than a pre-determined THC threshold.
The private sector is attempting to cash in on the need to measure impairment. An app has been developed to help drivers determine if they are “good to drive.” The app uses tests to assess memory, reaction time, balance, and time perception. Users should first take these tests sober to set a baseline to which they can compare themselves after consuming cannabis. This app is being marketed directly to consumers, not to law enforcement.
A more dubious product is the “marijuana breathalyzer,” which would measure amounts of marijuana on a person’s breath. Proponents argue this can be used to determine whether a person consumed marijuana recently because marijuana remains on a person’s breath only for a short time. Taking this argument further, a breathalyzer may be a better indicator of whether that person is impaired, because it determines whether someone consumed recently. However, as of now, the idea of a marijuana breathalyzer is nothing more than that, an idea. Companies are vying to be the first to develop a functional breathalyzer, but at this point none has delivered the goods. Additionally, this type of breathalyzer could end up being just as incapable of testing for impairment as the drug tests.
States should consider whether their drugged driving laws are an effective way to keep their roads safe. At this point, the best we can tell you is to stay abreast of your own states drugged driving laws and act accordingly.