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The Controversy Over Cannabis and Driving

In the wake of state-by-state legislation decriminalizing marijuana for recreational and medical use, law enforcement is on heightened alert for drivers who might be under the influence of marijuana. These so-called “marijuana DUI” cases are a relatively new phenomenon. Twenty-five years ago drug DUI cases were scarcely heard of. Most states did have some sort of catch-all legislation on the books that prohibited driving under the influence of any drug or narcotic, but the laws were rarely enforced and prosecutors found it hard to prove who was affected by the drug and who was not. In states where marijuana use has been decriminalized, the police are now looking for drivers who could be impaired by marijuana.

Detecting THC Impairment

It is harder to tell if someone is under the influence of marijuana compared to alcohol. With alcohol there is the obvious order, the slurred speech, the poor balance and coordination. Detecting marijuana intoxication is more difficult. Although marijuana smoke has a distinct odor, marijuana edibles and vaping are on the rise. Unlike 20 years ago when someone would have to make their own edibles, there are now a variety of edible marijuana products such as cookies, lozenges, gummy bears, and infused drinks. Similarly, recreational stores customarily sell vaping products that have no discernable odor. If a law enforcement officer suspects a driver of marijuana DUI, he or she will ask them to perform certain tests. One of the tests is to ask a driver to close their eyes and count to themselves for the duration of 30 seconds. THC can affect a person’s concept of time and an individual who has consumed marijuana may not accurately estimate the time close to 30 seconds. Also, an officer will ask the subject to take an eye test called the gaze nystagmus test which studies the involuntary jerking of the eyes that can be present when marijuana has been consumed. On occasion, an officer will claim that marijuana causes a “green tongue” effect, but that has largely been debunked. See, for example, when a DUI lawyer was interviewed about that subject for the Wall Street Journal. There is no portable testing instrument that can be used to detect and quantify the level of THC in a person’s system.

The Debate Over How Cannabis Affects Driving

Toxicologists all agree that alcohol affects a person’s ability to drive a motor vehicle in a very predictable way. Cannabis, however, is a different story. The influence of cannabis on a person’s ability to drive is dependent on each person’s tolerance level and the individual way each person’s body reacts to THC. Attempts to study the effects of THC on driving have proved to be elusive. In 1973, scientist Harry Klonoff conducted a test on the effects of THC on driving. The studies were conducted in British Columbia and Dr. Klonoff dosed each subject with .7 grams of marijuana. Each driver was directed to inhale the marijuana smoke for 3 seconds and to hold it in their lungs for 15 seconds before exhaling. Dr. Klonoff had a “control group” smoke a placebo which was presumably a marijuana strain with little to no psychoactive substance, similar to hemp, perhaps. Dr. Klonoff then sent the individuals out on a driving course, and then later sent the drivers out on to city streets during rush hour. Dr. Klonoff had each vehicle equipped with “duel controls” similar to the vehicles used to teach drivers education. The result of the study showed that, in general, marijuana did tend to have a negative effect on a person’s ability to drive, but found that, for some test subjects, their driving improved. The doctor attributed the improvement to the tendency of some individuals to over compensate for their intoxication by being particularly cautious.

Laws Today on Cannabis and Driving

Most states today attempt to address the subject of marijuana DUI by setting a per se limit above which a person is legally affected. In many states this level is 5 nanograms of THC per milliliter of blood. The level does vary from state to state, however, and most states impose a level of 0 for drivers under 21.


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