What is the legal test for reasonable suspicion of impaired driving?

The legitimate state objective addressed by the statutory provision in question is the removal of potentially dangerous drivers from the highways; the legislative motive is the reduction of deaths and injuries caused by such drivers. There can be no doubt that the motive and purpose are legitimate and reasonable. It is the means chosen to achieve the purpose that is, in my view, unreasonable. The social evil against which the legislation is directed is not clearly more dangerous to the fabric of society than are other common serious crimes. Yet in regard to such crimes, including the crime of impaired driving, Canadian legal and political traditions point to a higher standard than mere suspicion. I refer to the provisions of ss. 449(1)(a) and 450(1)(a) and (b) of the Criminal Code, which I have already quoted. The standards used in these formulations — i.e., either finding a person actually committing an offence, or actual belief, on reasonable and probable grounds, that the person being arrested has committed an offence — reflect the same balancing of interests as was discussed in the context of searches and seizures by Dickson C.J.C. in Hunter v. Southam. At pp. 167-68 S.C.R. he said: The state’s interest in detecting and preventing crime begins to prevail over the individual’s interest in being left alone at the point where credibly-based probability replaces suspicion. History has confirmed the appropriateness of this requirement as the threshhold [sic] for subordinating the expectation of privacy to the needs of law enforcement. To continue the adaptation of Dickson C.J.C.’s reasoning: In cases like the present, reasonable and probable grounds to believe in the existence of a state of facts constitute the minimum standard. I would go further and say that belief by the police officer in that state of facts at the time is also part of the minimum standard.