Field Sobriety Tests are Not a Reliable Indicator of Marijuana Intoxication

by | Sep 22, 2017 | DUI |

Massachusetts’ Supreme Court released an opinion this week finding that the standardized field sobriety tests (SFSTs) commonly used by officers to detect alcohol impairment cannot be used to determine marijuana impairment.

South Carolina has not decided this issue nor have we decided whether drug recognition expert (DRE) testimony is admissible in South Carolina courts. It is worth noting, however, that the DRE protocol is based in part on the same SFSTs that Massachusetts has found to be unreliable when applied to marijuana intoxication.

What did the Massachusetts Court Find?

The horizontal gaze nystagmus (HGN) test, one-legged stand test, and walk-and-turn test “were developed specifically to measure alcohol consumption.” They have been researched extensively and standardized by the National Highway and Traffic Safety Administration (NHTSA) with strict rules for how they must be conducted.

The Massachusetts Court found that there “is wide-spread scientific agreement on the existence of a strong correlation between unsatisfactory performance and a blood alcohol level of at least .08%.” On the other hand, “there is as yet no scientific agreement on whether, and, if so, to what extent, these types of tests are indicative of marijuana intoxication.”

The Court notes that there is disagreement among researchers as to whether using marijuana impairs the ability to drive at all. There is further disagreement among researchers as to whether FSTs can indicate marijuana impairment and which FSTs are more reliable:

There is ongoing disagreement among scientists, however, as to whether the FSTs are indicative of marijuana impairment. In recent years, numerous studies have been conducted in an effort to determine whether a person’s performance on the FSTs is a reliable indicator of impairment by marijuana. These studies have produced mixed results.

Ultimately, the Court found that officers can testify as to their observations including erratic driving, moving violations, the driver’s appearance, the odor of marijuana, and the driver’s behavior. The officer can also testify, as a lay witness, as to the defendant’s performance on the FSTs.

What officers cannot do is treat the FSTs as if they were a scientific test that establishes marijuana impairment. Officers cannot testify that FSTs established that a person was under the influence of marijuana, and they cannot testify that a person “passed” or “failed” the FSTs.

What Does This Mean for DUI Defense in South Carolina?

DUI defense lawyers in SC need to challenge any testimony by police officers that FSTs are a reliable indicator of marijuana impairment. There are two situations where this is relevant: 1) Lay testimony (non-expert testimony) by police officers; and 2) DRE testimony by police officers.

Police officers are not usually toxicologists and therefore they cannot testify as to the levels of THC or THC metabolites in a person’s system or the effect that they would have on a person’s impairment.

Because there is no scientific consensus as to the reliability of FSTs in DUI-marijuana cases, the officer should not be permitted to testify that FSTs established intoxication in a DUI-marijuana case.

In cases where an officer is qualified as an expert in DRE, the testimony should still be challenged if the defense has the experts available to testify and make a record as to why DRE is junk science when applied to marijuana. If FSTs are not reliable as an indicator of marijuana impairment, they are not reliable regardless of whether the officer is a certified DRE.

Most of the DRE steps have nothing to do with marijuana impairment. For example, there is no scientific consensus on the application of SFSTs to marijuana impairment. VGN (vertical gaze nystagmus) may indicate the presence of CNS depressants, inhalants, or dissociative anesthetics. Marijuana is none of the above.

As to the other DRE steps, marijuana use does not affect muscle tone and marijuana users do not have injection sites. Marijuana use can result in a larger pupil size, but this alone is not a reliable indicator of marijuana use.

The only step in the DRE protocol that should be considered a reliable indicator of marijuana impairment is the final step where the officer gives the suspect an actual drug test, which requires a toxicologist’s testimony to explain and not a DRE witness…

Charged with DUI Based on Marijuana?

You may have a complex case with multiple issues based on scientific testimony and evidence that may be excluded depending on the facts of your case. The Myrtle Beach DUI lawyers at Coastal Law, LLC, can help. You can schedule a free consultation to discuss the facts of your case by calling (843) 488-5000 or filling out our online form.

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