I’m often cynical of police methods for detecting impaired driving, but I am cynical with reliable science on my side. The legalization of cannabis brought attention to the problem of identifying people driving while impaired by cannabis. For our practice, understanding the scientific limitations of current detection methods is essential, because it directly impacts how we defend cannabis impaired driving cases.
BC traffic officers typically rely on three components to determine suspected impairment: observation of driving behaviour, driver observation often during the discussion they have once the driver is stopped, and performance on Field Sobriety Tests (FSTs). If the detaining officer’s opinion is that the driver is impaired by a drug in their ability to drive, further steps will follow leading to a Drug Recognition Expert (DRE) typically performing a lengthy evaluation, following which compelled toxicology samples (typically blood).
Field Sobriety Tests (FSTs) – Designed for Alcohol, Not Cannabis
FSTs were developed in the US in the 1970s, standardizing the approach to determining impairment, but they were originally designed to detect blood alcohol concentration (BAC) levels, not driving impairment. Importantly, they were never developed or validated to determine cannabis impairment.
A significant knowledge gap exists regarding how well non-intoxicated individuals perform on FSTs. Studies supporting FST efficacy with sober individuals have major limitations, such as participants receiving FST training before the study or using police officers as “controls” who were likely familiar with the tests and whose drug status was known to the evaluators, introducing confirmation bias. The more you dig, the more you realize that the majority of studies of impairment related to driving by alcohol or drugs are infused with bias.
More recent, blinded studies reveal alarming false positives: one controlled trial found that 48.5% of placebo participants performed less than expected on FSTs. Similarly, “lack of convergence” (eyes not converging when an object is moved toward the nose), though cited as rare (5%) in the general population, was found in 45% of placebo individuals and 34.9% of control participants in studies. Simply put, innocent people have the same symptoms as people who were identified as impaired by cannabis.
Drug Recognition Experts (DREs) – The “Police Science” Challenge
DRE evaluations involve a 12-step process comprising additional tests, examination of pupillary function, vital signs, and other factors. The DRE methods have been increasingly criticized for its lack of any scientific rigor, often being termed “police science” rather than genuinely science-based. It’s important to note that DREs use were never designed to detect impairment. Many DRE indicators are subjective and unreliable:
- Muscle Tone: Evaluated by an officer squeezing limbs to determine if they feel “normal,” “rigid,” or “flaccid,” a method not used or recognized by healthcare professionals.
- Vital Signs: High blood pressure and elevated pulse rates are cited as indicators, but about half of the adult population has high blood pressure when sober, and both can be accounted for by the stress of a roadside or police station encounter.
- Psychiatric Diagnosis: DREs claim to make on-the-spot psychiatric diagnoses like paranoia, a skill even board-certified psychiatrists do not claim.
- Other Subjective Observations: Even “increased appetite” (the munchies) is listed as an indicator, alongside “normal” readings for pupil size, body temperature, and eye tracking, which are then interpreted as signs of impairment. These flaws lead to inconsistent and unreliable results. A 1998 controlled laboratory study found DRE protocols produced false negatives or false positives 45.5% of the time, suggesting results not much better than a coin toss. Inter-DRE agreement is poor; a 2002 study found only 69% agreement between two groups of DREs on cannabis impairment, and significant disagreement (34.3%) on whether any impairing drugs were present when the subject used none.
Toxicology – Presence Does Not Equal Impairment
Canada took the approach of criminalizing blood-THC concentrations. Unlike alcohol, blood concentrations of THC do not reliably relate to impairment. Cannabis compounds (cannabinoids) are lipid-soluble and act through metabolites, meaning they can remain detectable in blood or oral fluid long after any impairing effects have subsided. This fundamental difference makes parallels to alcohol breathalyzers completely misleading. Laws that rely on per se blood THC limits for impairment are scientifically unsupportable, as the mere presence of cannabinoids provides “little, if any, information concerning behavioural impairment.”
Pervasive Confirmation Bias
A significant risk across all these methods is confirmation bias, a common phenomenon where even trained professionals, unknowingly seek or interpret evidence that supports their existing beliefs. For example, an officer might interpret a driver’s poor FST performance as being due to cannabis if they already suspect the driver is impaired by cannabis, even if the performance falls within normal variation for a sober individual.
Implications for BC DUI / impaired driving lawyers
I have been complaining about much of this for years. Increasingly attention to these issues are showing that I’ve been right to complain.
Some U.S. courts no longer accept DRE testimony. Minnesota now forbids DRE police officers from calling themselves “experts.” In Canada, we seem to have unfounded confidence in the RCMP. Despite relying on an entire testing scheme developed in the U.S. and now largely discredited in the U.S., Canadian police unquestionably adopt this unreliable testing scheme.
The current tools for cannabis impaired detection suffer from critical scientific gaps and reliability issues. Canadian lawyers need to be prepared to challenge the scientific validity and reliability of FST and DRE findings, especially given high false positive rates and documented inconsistencies.
Despite the per se legal limits in Canada, lawyers need to push back with Charter challenges because the mere presence of THC does not prove impairment and is little more than confirmation of the presence of associated chemicals in the blood. Canadian lawyers need to continue to highlight the potential for confirmation bias in officer observations and evaluations.
If you’ve been stopped for impaired driving either by alcohol or drugs anywhere in British Columbia, reach out to me as soon as possible. In addition to all the known defences in law, I also use the science to successfully defend my clients.
