June 22, 2024

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Are Field Sobriety Tests Effective in Identifying Marijuana-Influenced Drivers?

Are Field Sobriety Tests Effective in Identifying Marijuana-Influenced Drivers?


Are Field Sobriety Tests Effective in Identifying Marijuana-Influenced Drivers?

Scientists at UC San Diego evaluated the accuracy of field sobriety tests administered by law enforcement officers in measuring marijuana exposure and impairment.

In an era of increasing legalization of marijuana, road safety is a critical issue. Cannabis is known to impair reaction time, decision-making, coordination and perception skills necessary for safe driving. The number of fatal crashes involving drug-related injuries in California has increased 62 percent over the past three years.


Are Field Sobriety Tests Effective in Identifying Marijuana-Influenced Drivers?



Challenges of Measuring THC Damage

Interestingly, there was no correlation between blood alcohol levels and impairment between blood levels of tetrahydrocannabinol (THC) and driving performance.

Therefore, law enforcement officers must rely on behavioral tests to determine the extent of a driver’s impairment.

However, these field sobriety tests are primarily validated against alcohol consumption, so their effectiveness in detecting marijuana impairment remains uncertain.




Field Sobriety Tests and Cannabis Impairment

In a study published August 2, 2023 in JAMA Psychiatry, researchers at the Center for Medicinal Cannabis Research at UC San Diego conducted a double-blind, placebo-controlled Randomized clinical trial to assess the accuracy of field sobriety tests in identifying motorists under the influence of THC.

The results showed that tests conducted by law enforcement officials could distinguish between people who had consumed THC at certain points in time and those who had not.

Still, the overall accuracy of the test may not be sufficient to indicate damage from THC alone.



Are Field Sobriety Tests Effective in Identifying Marijuana-Influenced Drivers?

Marijuana is the most common illicit drug found in the blood of drivers involved in motor vehicle crashes, including fatal ones.


“Driving is a complex task that requires undivided attention strength and motor skills to be safe. While marijuana can cause harm to the human body, the effects vary from person to person.  Therefore, public health needs to confirm that the assessment of impairment is valid and unbiased, and this study is toward that An important step towards one goal.”



Research Methods and Results

The study included 184 adult marijuana users between the ages of 21 and 55. During the experiment, 63 participants smoked placebo marijuana and 121 smoked THC marijuana.

Participants who smoked THC reported a median “high” of 64 (on a scale of 0 to 100), indicating that they had reached a level of overt intoxication.


Trained law enforcement officers then administered field sobriety tests to check for balance, coordination, distraction and eye movement, among other abilities.

These tests included walking and turning, standing on one leg, touching the nose, lack of convergence, and the modified Langberg test.

The tests were performed at four different time intervals, approximately one, two, three and four hours after smoking.


The results showed that officers classified participants in the THC group as impaired based on field sobriety tests at three of the four time points measured compared with the placebo group.

For example, they classified 98 participants (81%) in the THC group as alcohol-impaired based on their performance one hour after smoking, compared with 31 participants (49%) in the placebo group.

But regardless of their actual allocation (THC vs. placebo), officers suspect that 99 percent of those who failed the test were vaping THC.




Simulated driving and on-site sobriety testing

In addition to field sobriety tests, study participants also performed simulated driving and found significant correlations between simulated driving and selected field sobriety test results. However, the officers were not aware of this information.


Existing field sobriety tests may be sensitive enough to detect people under the influence of marijuana, the researchers concluded.

However, there was a substantial overlap in poor performance on the test between the placebo and THC groups, and officers suspect this was due to the high frequency of THC ingestion, suggesting that field sobriety tests alone may Not enough to identify the effects of THC on driving.




Future Impact and Research

The authors note that at the scene, officers can gain additional information by questioning drivers and observing their ability to drive, so combining field sobriety tests with this additional information may be more successful in fully determining whether a driver’s ability to drive is impaired .


“Field sobriety testing is a useful addition to a driver’s overall assessment, but by itself it is not sufficient to accurately determine whether THC is impaired,” said Marcotte. “New and effective measures to identify cannabis impairment are needed to ensure safe driving for all drivers.” “.


The Center for Medicinal Cannabis Research at UC San Diego has now partnered with the California Department of Motor Vehicles and the California Highway Patrol on a follow-up study to test various methods of detecting cannabis-impaired driving.

The study aims to enroll 300 participants and is scheduled to begin in late summer 2023.





Are Field Sobriety Tests Effective in Identifying Marijuana-Influenced Drivers?

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