That includes both the consequences—such as accidents, sometimes fatal—and the detection of drivers who may be impaired..
There are no reliable or accepted means of testing for driving under the influence of marijuana. One study found biases that distorted the results of the three studies most often cited by law enforcement to validate existing tests.
The problem: marijuana is not like alcohol.
While alcohol in the blood is a good measure of intoxication, THC in the blood is not. Because THC is stored in the fat, traces of it in the body long outlast their euphoric or impairing effects. Depending on which test is used and how often the driver uses marijuana, all you may be able to determine is if the driver used marijuana in the past few hours, days or even weeks.
Even so, law enforcement is exploring multiple alternative tests:
- Marijuana breathalyzers. At least two are under development from Cannabix and Hound Labs.
- Roadside saliva tests. Again there are two competing products in the works: the Draeger DrugTest 5000 and the Alere DDS2.
- Lab analysis of blood, urine, and mouth swabs, such as used for DNA tests, at the police station.
- The DRUID (DRiving Under the Influence of Drugs) app. Using a tablet, the driver performs simple tasks to measure reaction time, hand-eye coordination and balance.
- Drug Recognition Experts (DREs). Police officers are trained in a 12-step method to determine intoxication.
- Standard Field Sobriety Tests (SFSTs). Basically the same type of drunk test given for alcohol intoxication, measuring things such as eye movements, pupil dilation and the ability (or lack thereof) to walk a straight line.
The problem with all of the chemical tests for how much THC is in the body—breath, blood, saliva, urine—is that they are not accurate measures of the driver’s impairment.
The problem with DREs and SFSTs is that they are subjective. At least six states—including California where the test originated—don’t allow DREs to testify as experts in court.
In contrast, some police departments—including in Georgia’s Cobb County— trust the results of their DREs above scientific evidence. Even if there is no trace of marijuana in the bodily fluids—and these can last for days or weeks after last use—they still believe their DRE’s evaluation. (Polygraph proponents have a similar confidence in their methods, but many courts don’t accept them as evidence either.)
Another reason not to trust marijuana tests is false (or at least misleading) positives.
Cannabidiol (CBD) is a non-euphoric component of marijuana that is believed by many to have medical uses—maybe most or all of the benefits associated with medical marijuana—such as reducing chronic pain, anxiety, and preventing epileptic fits in children.
The last led to Food and Drug Administration approval for the CBD drug Epidiolex.
CBD cannot get you high, but its legal status, aside from Epidiolex, is hazy. Some states that haven’t approved medical marijuana have nonetheless approved CBD. According to some sources, CBD is legal under federal law as well and under UN Conventions.
Sometimes it isn’t considered a drug at all, only an herbal remedy like many sold in health food stores and unregulated. It is sometimes found in other places, too, such as gas stations and marijuana dispensaries.
The CBD sold in marijuana dispensaries likely has better quality control. You’re more likely to get what you’re asking for. In less regulated settings, it could contain more THC than claimed or even synthetic marijuana, also known as Spice or K2.
The problem with CBD and drug testing is that since CBD does contain at least a trace amount of THC—though no more than 0.3 percent—a test for marijuana may detect that tiny, tiny amount.
It’s not clear how necessary it is to test for marijuana, however. While some reports from states that have legalized recreational marijuana claim there is a large increase in the number of accidents, fatal and otherwise, since legalization, and of drivers with THC detected at the time of their accidents or impairment, they do not control for all variables and are not always accepted as legitimate.
The National Highway Traffic Safety Administration (NHTSA) did try to control all variables for a “crash-risk” study—“the first large-scale study in the United States to include drugs other than alcohol”—included in its July 2017 Marijuana-Impaired Driving: A Report to Congress.
The study—for which “research teams responded to crashes 24 hours a day, seven days a week over a 20-month period” in Virginia Beach, Virginia—controlled for the presence of alcohol and other drugs, “the demographic variable of age, gender, and race/ethnicity,” plus location and time of day.
Its conclusion: “There was no increased risk of crash involvement found over alcohol or drug-free drivers.”
Much more common was the risk of crash involvement when the driver tested positive for a combination of prescription drugs or alcohol and marijuana.
The NHTSA study isn’t perfect. Virginia is not a state that allows recreational cannabis use. It’s possible that where it is allowed, greater use results in greater problems. So far it doesn’t seem to be a major problem in California, however; nor is marijuana dependency a strain on the state’s top-rated addiction treatment centers.
An alarmist news story warning of possibly increasing cannabis-related auto accidents in the Bay area admitted that there have been no deaths there due to traffic collisions involving a “cannabis-only DUI arrest.”
Los Angeles County in California did have two convictions for fatal wrecks in which marijuana was the only substance detected since 2016, but deaths from alcohol-only DUIs are far higher.
Marijuana impairment tends to have the opposite effect of alcohol. Drivers tend to be more cautious, go slower, keep more distance between the car ahead of them, and don’t try to overtake them. Even at higher speeds, though lane control is poorer than with sober drivers, they still recognize road signs and can change lanes and respond to road hazards. In fact, according to one analysis, “low concentrations of THC do not increase the rate of accidents, and may even decrease them”.
However, high concentrations of THC or, when it is used in conjunction with alcohol, result “in impairment even at doses that would be insignificant were they of either drug alone.”
Another study—Driving While Stoned: Issues and Policy Options, from De Gruyter’s Journal of Drug Policy Analysis—concluded that “The maximum risk for cannabis intoxication alone, unmixed with alcohol or other drugs, appears to be more comparable to risks such as talking on a hands-free cellphone”.
The authors recommend “criminalizing only combination use”—that is, when marijuana is combined with prescription pills, alcohol, or other intoxicants—“while treating driving under the influence of cannabis… as a traffic offense, like speeding”.
No one suggests law enforcement should look the other way when a stoned driver crosses their path, and let’s hope a reliable and accurate test is developed, but that’s no reason to oppose marijuana legalization.
So far it looks like it is less of a concern than those opponents would like us to believe.
Stephen Bitsoli writes on drug policy, addiction, and related subjects for several websites. A former journalist and a lifelong reader, Stephen loves learning and sharing what he’s learned. He welcomes readers’ comments.