The law
This offence includes the operation or care or control of a motor vehicle, vessel, or aircraft while one's ability to operate is 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 proved.
Impairment is an issue of fact, which the trial judge must decide on the evidence. The prosecutor must prove the case (the accused does not have to prove that he was not impaired). The standard or degree of proof required before a judge will convict is the same as in all criminal cases. That is, the judge must be convinced "beyond a reasonable doubt" that the charge has been proved.
The prosecutions case
To prove this charge, the prosecutor must call evidence that shows signs of impairment. No breath test is required to prove impaired driving.
The charge is mainly proved through observations of the accused and is most often given by a police officer. This normally includes evidence of:
The prosecution will argue that all of the above symptoms are evidence of impairment.
Challenging the prosecutions case
While the points listed above may be evidence of impairment that is not necessarily the case. Evidence, which may appear at first blush to support an impaired driving charge, may be as consistent with innocence as with guilt, or as consistent with normal but imperfect driving, and not necessarily impairment.
There can be other explanations for the police officers' observations. For example:
The case for the defence
In addition to challenging the prosecutions case through skilled cross-examination, your defence lawyer may be able to lead evidence of your sobriety. There may be witnesses who will testify that you drank moderately and behaved normally.
The bottom line
Nothing can be taken for granted. If you are charged with impaired driving, all is not lost. An experienced impaired driving lawyer should conduct a detailed examination of the circumstances, which led up to the charge.
In addition to challenging the evidence against you, the prosecutions case may have procedural problems which, if challenged, can result in a finding of not guilty.