What is the legal test for substituting a real name for a "John Doe" defendant in an action?

The issue of "Doe" defendants most often arises in the context of amending the style of cause and pleadings in an action to reflect the real name of an individual. In Jackson v. Bubela Bull J.A. discussed the criteria for substituting a real name for that of "John Doe", in the context of a claim arising from a motor vehicle accident. When the action was commenced against the "Doe" defendant the limitation period was near expiry and the name of the driver was not known, although the name of the owner of the car and the time and place of the accident were pleaded. When the name of the driver became known after the limitation period had expired and the plaintiff sought to amend the writ to insert the real name in place of "John Doe", objection was made that the plaintiff was seeking to add a defendant. In his reasons holding that the substitution of the name was a correction of a misnomer, albeit an advertent misnomer, and not a substitution or addition of a party, Bull J.A. reviewed the use of "Doe" in naming the parties. He said at p. 82-84: The words "John Doe" to my mind are not restricted in connotation to a "fictitious" person or one not in existence. Traditionally the words were used in that limited sense in early ejectment suits, but for generations they have come to be accepted, used and understood, both in legal and common parlance as indicating a real person existing and identifiable but whose name is not known or available to the person referring to him. That is the situation here. . . . [The plaintiff’s] litigating finger was pointed at that driver and no one else, but she did not know his name.