Are Standardized Field Sobriety Tests effective in predicting cannabis impairment?
Police officers use standardized field sobriety tests (SFST) at the roadside when deciding whether or not a suspected impaired driver should be taken back to the station for further breath or blood tests. While approved roadside screening devices are widely used across Canada to detect alcohol impairment, drug screening devices are not as common. While the government approved the Draeger DrugTest 5000 for police use, the portable devices, which test saliva for the presence of THC, are not always reliable. As a result, some police forces opted not to use them and many still rely on the good old-fashioned SFST in drug-impaired driving investigations.
SFSTs, in case you don’t know, are a battery of physical tests designed to identify when a person is impaired by alcohol or a drug. There are a few different tests but the most commonly used are the horizontal gaze nystagmus (HGN), the one leg stand (OLS) and the walk and turn (WAT). During the HGN test, the officer observes the driver’s eyes as they follow a moving object – usually a pen – from side to side. For the OLS, the subject has to stand on one leg and count aloud starting from 1,000 for 30 seconds. For the WAT, the subject must take nine steps heel-to-toe along a straight line. In each test the officer looks for indicia of impairment, for example, difficulty maintaining balance.
The search for a reliable method for detecting cannabis impairment at the roadside has been going for decades. Most recently, following the legalization of recreational marijuana in 2018, people were worried about the prospect of an army of stoned drivers suddenly on the roads and SFSTs were back in the spotlight. But just how reliable are SFSTs? Well, one study carried out by the Canadian Centre on Substance Abuse sheds some light on this subject.
The study, An Evaluation of the Validity of the Standardized Field Sobriety Test in Detecting Drug Impairment Using Data from the Drug Evaluation and Classification Program, by Amy J. Porath-Waller and Douglas J. Beirness, sought to discover whether or not SFSTs are a valid indicator of drug impairment. It took data from more than 2,000 Drug Evaluation and Classification (DEC) reports from the RCMP.
The DEC reports included the opinion of the Drug Recognition Expert (DRE) on the category and class of the suspected drug as well as the results of toxicological tests carried out on suspects’ bodily fluid samples.
So, in effect, the study assessed how accurate standardized field sobriety tests were by comparing them to the results of the bodily fluid tests. As a side note, I wonder whether the RCMP routinely assesses the accuracy of its own DEC evaluations. It would be simple enough to find out how often the DREs prediction matched the toxicology report, if not.
The study looked at how effective HGN, OLS and WAT tests were in predicting the category and class of drug causing the impairment. There were four different categories of drugs included in the study: central nervous system (CNS) stimulants, CNS depressants, narcotic analgesics and cannabis. Cases where the opinion of the DRE was that no drug was present and no drug was found in the bodily fluid sample analysis were also included.
The results of the study showed that less than half of all cases (42.2 percent) were correctly classified based on the HGN test. Although the HGN correctly classified nearly all CNS stimulants (94.6 percent) the correct classification rate for narcotic analgesics was zero and for cannabis it was a measly one percent.
The study showed the OLS test was more effective in classifying cannabis. It correctly predicted the presence of cannabis in 55.4 percent of cases. The correct classification for CNS stimulants based on the OLS was 59.9 percent, whereas it was only 10.6 percent for narcotic analgesics and no CNS depressant cases were correctly classified.
The overall classification rate for the WAT test was 42.8 percent. It correctly identified 72.2 percent of CNS stimulant cases, 39.7 percent of cannabis, nine percent of CNS depressants and 3.5 percent of narcotic analgesics.
According to the study, the results supported for the validity of using standardized field sobriety tests to identify when someone is impaired by a drug other than alcohol. However, it also found that “cannabis adversely affected performance on the OLS test but not the WAT and HGN tests”. According to the DEC program cannabis does not produce observable result in the HGN test.
The study suggests the reason WAT and HGN tests are ineffective in predicting cannabis impairment is that cannabis users develop a tolerance to the effects of THC. It states: “It is conceivable that the cannabis users in the current study developed tolerance to the impairing effects of THC as well, which may have affected their performance in the WAT test.”
While standardized field sobriety tests are a screening tool and are not used to convict someone of impaired driving, it is still worrying that the correct classification rate of two SFSTs is so low for cannabis. Although nearly two years have passed since legalization, a lot of people are still worried it will lead to an increase in impaired drivers. SFSTs were put forward as an answer to these fears but as this study shows, they still have their flaws.
