What is the legal test for the necessity defense?

"By definition, the necessity defense is founded upon public policy and provides a justification distinct from the elements required to prove the crime. [Citation.] The situation presented to the defendant must be of an emergency nature, threatening physical harm, and lacking an alternative, legal course of action. [Citation.] The defense involves a determination that the harm or evil sought to be avoided by such conduct is greater than that sought to be prevented by the law defining the offense charged. [Citation.] Necessity does not negate any element of the crime, but represents a public policy decision not to punish such an individual despite proof of the crime. [Citations.] [] An important factor of the necessity defense involves the balancing of the harm to be avoided as opposed to the costs of the criminal conduct. [Citation.] Unlike duress, the threatened harm is in the immediate future, which contemplates the defendant having time to balance alternative courses of conduct. [Citation.]" (People v. Heath (1989) 207 Cal.App.3d 892, 900-901.)