A dispute over the admissibility of police “opinions” at drugged-driving trials lands at the Supreme Court of Canada Thursday in a case spotlighting an emerging legal dilemma over the hazy science of marijuana impairment.
With federal legislation to legalize recreational marijuana use expected next spring, the case of Carson Bingley of Ottawa could decide if, without a scientific standard, police officers’ opinions about whether motorists were high while driving should be automatically accepted as expert evidence at trial.
Unlike alcohol impairment, there is no legal blood-concentration driving limit for the active ingredients in marijuana and other drugs, nor is there an approved instrument for police to take readings that will be accepted in court.
A scientific advisory committee is reviewing the international literature to determine if there is consensus on appropriate blood-level limits for THC, the major psychoactive component of marijuana. But new research suggests a blood-cannabis concentration is not a reliable predictor of how impaired a person is.
This forces courts to rely on the opinions of police officers trained to spot drug impairment. But opinions are typically only allowed as evidence at criminal trials if they are made by qualified experts. The question of admissibility is usually decided during a voir dire hearing before the presiding judge.