In order to understand the charge of impaired driving one needs to consider the legal definition of “impaired” under the Criminal Code. For many years there was a debate in Canadian courts regarding the standard of proof required for a conviction on a charge of impaired driving. Many courts held that the Crown needed to demonstrate a “marked departure” from that of a normal person. So, if a motorist could drive in a relatively straight line, the Crown’s case would fail even if the driver otherwise exhibited the “classic signs” of impairment such as slurred speech, poor motor coordination and blood shot eyes. This legal debate was put to rest however by the Supreme Court of Canada in the case of R. v. Stellato  2 SCR 478 wherein the court upheld a decision of the Ontario Court of Appeal 1993 CanLII 3375 which states the now authoritative test:
“….if Parliament had intended to proscribe impaired driving only where accompanied by a marked departure from the norm, it also could have done so. With all due respect to those who hold a contrary view, it is my opinion that the interpretation of s. 253(a) which was advanced in Winlaw, Bruhjell and Campbell is the correct one. Specifically, I agree with Mitchell J.A. in Campbell that the Criminal Code does not prescribe any special test for determining impairment. In the words of Mitchell J.A., impairment is an issue of fact which the trial judge must decide on the evidence and the standard of proof is neither more nor less than that required for any other element of a criminal offence: courts should not apply tests which imply a tolerance that does not exist in law.”
Although the Crown likes to rely strictly on that paragraph, the court significantly qualified that observation in the ensuing paragraph:
“In all criminal cases the trial judge must be satisfied as to the accused’s guilt beyond a reasonable doubt before a conviction can be registered. Accordingly, before convicting an accused of impaired driving, the trial judge must be satisfied that the accused’s ability to operate a motor vehicle was impaired by alcohol or a drug. If the evidence of impairment is so frail as to leave the trial judge with a reasonable doubt as to impairment, the accused must be acquitted. If the evidence of impairment establishes any degree of impairment ranging from slight to great, the offence has been made out.”
This is the key consideration regarding the defence of an impaired driving charge: the ultimate question to be addressed is not impairment generally but impairment specifically in relation to one’s ability to operate a motor vehicle. This point was forcefully brought home by the Alberta Court of Appeal in Andrews 1996 ABCA 23 (CanLII):
“The courts must not fail to recognize the fine but crucial distinction between “slight impairment” generally, and “slight impairment of one’s ability to operate a motor vehicle”. Every time a person has a drink, his or her ability to drive is not necessarily impaired. It may well be that one drink would impair one’s ability to do brain surgery, or one’s ability to thread a needle. The question is not whether the individual’s functional ability is impaired to any degree. The question is whether the person’s ability to drive is impaired to any degree by alcohol or a drug. In considering this question, judges must be careful not to assume that, where a person’s functional ability is affected in some respects by consumption of alcohol, his or her ability to drive is also automatically impaired.”
In many cases involving impaired driving there may be a factual debate between the police who attribute bad driving and poor coordination to the accused who denies same. The most helpful evidence in this regard is generally the videotaped activities of the accused on the night in question and one of the priorities in defending this type of case is to demand the full videotaped activities of the accused at the scene and the station.
See for instance the case of R. v. H.P.  O.J. No. 7211 where, in acquitting the defendant, the judge referenced the defence’s use of the police video to undermine their testimony:
“I must consider the evidence that Mr. P was swaying on the roadside. I do not accept this evidence from any of the officers. I have looked carefully at the video exhibits that show Mr. Prado in the cruiser, at the booking-in and in the breath room. Mr. P is at all times physically composed while standing or sitting. I did not observe any type of sway. There was no video of the interaction outside of the cruiser, on scene. However, I do not accept that there was any swaying at this time given the proximity in time of the video evidence I have seen.”
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