In a 6-1 decision, the justices say the trial judge was right to acquit Donald Boudreault, who was convicted in 2009 after the taxi driver he'd summoned for a ride home arrived to find him asleep in the driver's seat.
The justices say the Quebec Court of Appeal erred in setting aside the lower court ruling and convicting the man of having care or control of a vehicle while impaired.
Writing for the majority, Justice Morris Fish says the trial judge was right to conclude that there was no realistic risk that the man, from Jonquiere, Que., would drive the vehicle while impaired.
He had come up with a plan to get home safely with a taxi service and was waiting with the motor running to stave off the -15 C temperature.
In dissent, Justice Thomas Cromwell argued that risk is not an element of the offence.
The case nails down that risk of danger should be considered in such cases.
The majority found that "care or control" must include "circumstances that create a realistic risk, as opposed to a remote possibility, of danger to persons or property."
"The Crown submits that risk of danger is not an element of 'care or control' under s. 253 (1) of the code," Fish wrote. "The trial judge found that it is. With respect, I agree with the trial judge."
Fish said anyone found in that situation would almost invariably be convicted.
"It hardly follows, however, that a conviction in these circumstances is, or should be, 'automatic'. A conviction will be neither appropriate nor inevitable absent a realistic risk of danger in the particular circumstances of the case."
He pointed to a key element in his decision: "One of the factors of particular relevance in this case is that the accused took care to arrange what some courts have called an 'alternate plan' to ensure his safe transportation home."
Finally, Fish pointed out that the trial judge's decision was a matter of fact, not a matter of law alone.
"This finding of fact, however unsatisfactory or unreasonable it may appear to others, was not reviewable on an appeal by the Crown.
"I therefore feel bound, as a matter of law, to allow Mr. Boudreault’s appeal, to set aside the judgment of the Court of Appeal and to restore his acquittals at trial."