R. v. Murray, 2010 ABQB 784 involved an impaired driving charge. A group of angry men were threatening and the accused, though impaired, claimed he had to drive away. The defence of necessity was accepted and the court gave a helpful summary of the law:
The Law on the Defence of Necessity
 In R. v. Perka, 1984 CanLII 23 (SCC),  2 S.C.R. 232, Dickson J. explained at p. 248 that the defence of necessity rests on a realistic assessment of human weakness, recognizing that a liberal and humane criminal law cannot hold people to the strict obedience of laws in emergency situations where normal human instincts, whether of self‑preservation or of altruism, overwhelmingly impel disobedience. Such acts are still wrongful, but are excusable in the circumstances. He further stated at pp. 257-258:
Although necessity is spoken of as a defence, in the sense that it is raised by the accused, the Crown always bears the burden of proving a voluntary act. The prosecution must prove every element of the crime charged. One such element is the voluntariness of the act. Normally, voluntariness can be presumed, but if the accused places before the Court, through his own witnesses or through cross‑examination of Crown witnesses, evidence sufficient to raise an issue that the situation created by external forces was so emergent that failure to act could endanger life or health and upon any reasonable view of the facts, compliance with the law was impossible, then the Crown must be prepared to meet that issue. There is no onus of proof on the accused.
 The Supreme Court in R. v. Latimer, 2001 SCC 1 (CanLII),  1 S.C.R. 3, at paras. 26 to 34, confirmed that the defence of necessity is restricted to those rare cases in which true "involuntariness" is present, and that the defence of necessity requires: 1) a requirement of imminent peril or danger; 2) no reasonable legal alternative to the course of action he or she undertook; and 3) proportionality between the harm inflicted and the harm avoided.
 The peril must be on the verge of transpiring and virtually certain to occur, such that normal human instincts cry out for action and make a counsel of patience unreasonable. The second requirement of no reasonable legal alternative involves a realistic appreciation of the alternatives open to a person; the accused need not be placed in the last resort imaginable.
 The first two criteria are assessed on a modified objective basis. This is an objective evaluation, taking into account the situation and characteristics of the particular accused person, including his or her reasonable perceptions of the existence of alternative courses of action. Proportionality is measured on an objective standard.