The Supreme Court of Canada last week ruled to exclude scientific evidence from trials that involving driving under the influence of alcohol (DUI). The case considered the accuracy of the breathalyzer readings used to convict two motorists.

Robert Albert Gibson had produced a breathalyzer reading that estimated his blood alcohol content (BAC) at .12 and then .10 in a subsequent test. Martin Foster MacDonald blew a .146. At the time the breathalyzer test was administered, each was over the .08 limit. However, because it takes time for the body to absorb the alcohol and cause impairment, the defendants argued that it was possible that they were sober at the time they had been behind the wheel.

Expert scientific witnesses backed up the defendants with evidence based on the average absorption and elimination rate of alcohol. At the time Gibson and MacDonald were actually driving, their BAC could have been as low as .40 and as high as .10 -- the range straddled the .08 limit. Their convictions were upheld because the court ruled that the possibility that the BAC could have been lower than .08 raises no reasonable doubt.

"Straddle evidence merely confirms that the accused falls into the category of drivers targeted by parliament," Justice Louise Charron wrote for the majority. "It is therefore not enough to show, based on evidence about the accused's pattern of consumption of alcohol during the relevant time period, that the accused consumed enough alcohol to exceed the legal limit, albeit in a quantity that would place him within a range that may be somewhat different from that which could be extrapolated from the breathalyzer reading."

MacDonald went a step further to present his defense. He went to a lab that measured precisely how fast his body eliminated alcohol instead of assuming average rates based on factors such as age, height, weight and sex. This testing showed that his BAC would have been .071 at the time he was driving -- within the legal limit. Even though breathalyzer tests themselves are based on broad assumptions about absorption rates applicable to the general population, the court rejected the specific testing results in this case.

"Furthermore, because it is scientifically undisputed that absorption and elimination rates can vary from time to time, nothing is really gained by post‑offence testing of an accused's elimination rate," Justice Charron wrote.

The ruling made clear that the goal of the legislature is not to create means for the accused to escape conviction.

"Because absorption and elimination rates continually vary, it is readily apparent that a breathalyzer reading of [.095], for example, may not reflect the actual concentration of alcohol in the accused's blood at the time of the alleged offence -- it would depend on the rate at which the particular accused is metabolizing the alcohol during the relevant time period on the day in question," the high court ruled. "Yet, it can be no defense for an accused to say that the actual alcohol concentration at the material time may have been less than the legal limit based on this variable alone. To admit such a defense would obviously fly in the face of the presumption itself. It is because of these inherent variations in absorption and elimination rates that the presumption of identity is needed in the first place. In order to facilitate proof of the offence, the presumption treats all persons as one person with a fixed rate of elimination and absorption."

California DUI attorney Lawrence Taylor said the ruling sets troubling precedent mirrors several US decisions.

"In other words, the defendant is not permitted to question the Breathlayzer results -- because it interferes with efforts to combat drunk driving," Taylor said. "Think about that."

A full copy of the decision is available in a 140k PDF file at the source link below.