Despite the federal Schedule I status of cannabis, more and more states are legalizing marijuana in some form. The nationwide increase in cannabis use means a rise in the number of cannabis-impaired drivers.

Most people know that driving impaired is illegal, regardless of the substance. Yet many people don’t realize how difficult it is to reliably test for cannabis intoxication. Determining actual impairment following marijuana use is far more complex than the simple and reliable tests used to detect alcohol impairment.

When alcohol is consumed, it is readily absorbed into the blood system via the gastrointestinal tract. While factors such as the presence of food may influence this process, it occurs in a consistent manner over time. Peak blood alcohol concentration (BAC) is normally achieved within approximately 20 minutes after one stops drinking. Impairment increases with growing alcohol concentration and wanes with reduced alcohol concentration. The correlation between impairment and alcohol concentration has allowed the use of BAC to infer the individual’s degree of impairment in a reliable and scientifically accepted manner.

What Does “Under the Influence” Mean in Terms of Cannabis Use?

Unlike alcohol, there is great variability among the states in their approach to driving under the influence of drugs (DUID). Fifteen states presently have drug “per se” (zero tolerance) statutes (AZ, DE, GA, ID, IL, IN, IA, MI, MN, NC, RI, SD, UT, VA and WI). Others require a certain threshold of tetrahydrocannabinol (THC) in a person’s system to be considered under the influence, determined by either urine or blood sample. Moreover, a small number of states judge cannabis impairment based on the driver’s behavior, regardless of the actual amount of marijuana in the system.

With alcohol, widely accepted field tests and blood alcohol tests are available to provide the basis for probable cause for arrest. No universal test for cannabis consumption is presently available.

Chemical Tests

In many states, a driver suspected of impaired driving may be arrested and taken to a facility to submit to a blood or urine test. Although there is no standard limit for active THC, several states accept five nanograms of THC as the limit. In states with a per se limit, any amount of THC is grounds for a criminal offense. Blood tests check for THC levels in whole blood and provide grounds for filing charges if the state’s legal limit is exceeded.

This is problematic because THC, which is fat-soluble, can remain in tissue for weeks after ingestion, which can result in positive test results that have no bearing on the individual’s actual level of intoxication. A regular user will likely have measurable THC metabolites regardless of recent ingestion. Moreover, a reliable biomarker of marijuana intoxication has not been identified. Impairment can vary based on (1) the consumed THC concentration, (2) whether the person is a frequent or heavy user, (3) the time elapsed since cannabis ingestion and (4) the ingestion method. A recent study from the American Automobile Association (AAA) found that even blood tests looking only for active THC − excluding residual THC from prior use − are not a reliable indicator for driving fitness.

Recently, some California municipalities and the California Highway Patrol have tested oral swab examinations in place of more complicated blood and urine tests to detect drugs. These tests work by identifying trace amounts of cannabis in a driver’s saliva. “Oral swab testing is still an unproven technology,” Dale Gieringer, director of the California National Organization for the Reform of Marijuana Laws (NORML) said, “there’s no evidence that oral swab testing results have any correlation to impaired driving.” Moreover, trace amounts of cannabis can be present in saliva up to three days after consumption.

Observed Impairment Tests

Many other jurisdictions use driver behavior as well as physical factors such as tongue color and pupil dilation to determine if one is potentially impaired while driving. Common field sobriety tests include the horizontal-gaze nystagmus test, the one-leg stand test, and the walk and turn test.

The horizontal-gaze nystagmus test usually is administered by an officer moving a finger or an object from side to side in front of a driver’s face in order to detect an involuntary jerking of the eye associated with high levels of intoxication. After being strained beyond a 45-degree angle, a person’s eye is believed to jerk naturally. But, if the eye begins to jerk at or before the 45 degree point, this reaction can be referenced as evidence that a driver is impaired. This test is estimated by the National Highway Traffic Safety Administration (NHTSA) as 77 percent reliable. Those results are cold comfort for the 23 percent of sober drivers on the wrong end of a failed test.

During the one-leg stand, the suspect is instructed to raise his or her foot, hold still, count and look down. If hopping, swaying or putting the foot down are observed, the officer has grounds for an arrest. According to NHTSA estimates, this test is effective only 65 percent of the time.

Another divided attention test, the walk and turn test, also referred to as the “walk the line test” splits the attention of a person suspected of driving under the influence of cannabis between mental and physical tasks. The officer provides instructions to the driver and looks for loss of balance, inability to stay on the line, beginning before instructed, breaks in walking and the wrong number of steps. This is estimated by the NHTSA to be effective 68 percent of the time.

Of course, the reliability of observational testing is limited by the skill of the person administering the test, and the results are inherently subjective. Also, it is more difficult than a chemical test to prove in court, coming down to the judgment of the officer who may or may not possess the skills to accurately determine the driver’s impairment.

Problems and Recommendations for DUID Detection Technology

Currently, there are no evidence-based methods to detect marijuana-impaired driving. It is understandable that both the public and lawmakers have a strong desire to create legal limits for impairment in the same way we do with alcohol. “In the case of marijuana, this approach is flawed and not supported by scientific research,” said AAA CEO Marshall Doney. “It’s simply not possible today to determine whether a driver is impaired based solely on the amount of the drug in their body.”

Following a recent study analyzing lab results of drivers arrested for driving under the influence of marijuana, the AAA Foundation for Traffic Safety recommended replacing technology with specifically trained police officers to determine driver impairment, followed by a test for the mere presence of THC, rather than a delineated threshold.

Other organizations have called for states to consider making the presence of THC in the system a traffic violation. Studies supporting this approach show that marijuana-impaired driving roughly doubles the risk of a crash. By comparison, however, talking on a hands-free cell phone while behind the wheel quadruples the risk, and driving with a BAC of .12 − which is about the median amount in drunk driving cases − increases crash risks 15-fold.

According to the NHTSA, scientific studies are consistently finding that marijuana-impaired drivers pose a comparatively nominal accident risk. The largest-ever controlled trial assessing cannabis use and automobile accidents concludes that after controlling for age and gender, marijuana-impaired drivers possess virtually no statistically significant risk of crash compared with drug-free drivers. Still, more data must be collected to support definitive conclusions and policy decisions. In its July 2017 Marijuana-Impaired Driving Report to Congress, the NHTSA recommended that efficient methods for training law enforcement personnel be instituted, including drug recognition experts to increase each officer’s overall knowledge of the general manifestations of marijuana impairment and to be able to recognize these indicators in encounters with drivers.