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Marijuana Driving Impairment and the Standard Field Sobriety Tests

This column will update readers on developments in marijuana legalization

the stop and/or the Drug Recognition Expert (DRE)1

I who conducted a drug

evaluation. Te Drug Evaluation and Classification Program (DEC) focuses on law enforcement to recognize when an individual has been driving under the influence of drugs and to identify the type of drug causing impairment. Te DEC Program was developed by the National Highway Traffic Safety Administration (NHTSA). Upon completion of their initial training, officers are certified as Drug Recognition Experts (DREs).2 Among observations like the condition of the eyes (dilated pupils, tremors) and whether there was slurred speech or swaying, these examinations include standard field sobriety tests (SFSTs) that detect categories and or specific drug use and go to the ultimate issue of impairment. If these tests are performed, they usually are documented in a law enforcement report. Tese reports not only state the physical observations by the law enforcement officer or DRE and the results of the SFSTs, they also render an opinion as to whether they believed that the defendant was impaired at the time of driving. Tese reports along with confirmed drug testing, if a sample was collected and properly handled and tested, are used to form part of the basis of the toxicologist’s ultimate opinion on impairment.

54 datia focus

n the majority of marijuana driving cases, there is testimony from the law enforcement officer who made

Te NHTSA developed the SFSTs for

law enforcement to determine alcohol driving impairment in 1975 and they were implemented in 1981.3

Te SFSTs are

composed of the horizontal gaze nystagmus test, the walk and turn and the one-leg stand. Tere are other tests like the Romberg test4 (subject stands erect with feet together and eyes closed-sometimes the added feature of finger to nose touch) but this test has not been sanctioned by NHTSA to date. Although the science backing the use of SFSTs for alcohol impairment is reasonably strong, it still remains somewhat controversial.5

Te admissibility of the

science related to the horizontal gaze nystagmus test itself has met varying court opinions.6

However, courts mostly admit all

of the SFSTs with some courts even taking judicial notice of their scientific reliability and acceptance in the scientific community for alcohol impairment. Te general acceptance in the scientific

community of the correlation between marijuana impairment and SFSTs is not totally clear. Based upon some toxicologist’s opinions and studies, some believe that the horizontal gaze nystagmus test is inapplicable to THC impairment. According to the 1993 NHTSA Manual, horizontal gaze nystagmus test can recognize central nervous system depressants, “PCP,” and “Inhalants” because they affect the same neural centers as alcohol. It cannot detect central nervous system stimulants, hallucinogens, narcotic analgesics, or cannabis.7

Drugs that depress

the central nervous system, such as inhalants or phencyclidine affect the brain’s ability to properly control the eye musculature.8


law enforcement itself seems to be aware of this proposition.9 Tere are other scientific studies that

question the significance of all of the SFSTs to marijuana impairment. A 2009 study

assessed which signs of the Drug Evaluation and Classifications predicted various drug categories (including cannabis), and, at best, showed that the one-leg stand contributed significantly to the prediction and the horizontal gaze nystagmus test and “walk and turn” did not.10

Additionally, a 2012

study stated that “in general, the present data indicates that SFSTs were mildly sensitive to the effects of marijuana depending on dose and cannabis use history.”11

NHTSA is in

the process of further studying SFSTs as they relate to drugged driving in general.12 However a 2004 study identified a positive relationship between the dose of THC administered and impairment based on the SFSTs.13

Tere was also a 2012 Australian

study that stated that SFSTs may be “moderately accurate for marijuana.”14 Te most recent study seems to support

the use of SFSTs for marijuana detection. Te study compared 302 cannabis and 302 non-cannabis driving cases from 2009–2014. Not surprisingly they found an increase in pulse rate, blood pressure and pupil rebound dilation. Tey also found that finger to nose was the best predictor with three misses, eyelid tremors, two clues on the WAT and two clues on the one-leg stand. Te remarkable finding was that there was no difference in the clues for marijuana impairment under and over 5 nano grams per milliliter of blood.15


recent study supports these findings regarding the SFSTs and states that “. . . drivers with THC concentrations below 5 ng/mL are just as likely as those with higher THC concentrations to show signs and symptoms consistent with cannabis use and impairment.16

❚ References 1 Drug-Evaluation-and-Classification-Program.aspx#. VwAd24-cGM8

2 current/Standardized+Field+Sobriety+Test+(SFST)+Validate d+at+BACS+Below+0.10+Percent

summer 2016

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