What is the test for determining probable cause in a motor vehicle accident?

Although we conclude that the trial court erred in leaving the probable cause decision to the jury, in fairness to the trial court we must recognize [47 Cal.3d 877] that the court's error was in large part a product of long-standing confusion in the case law over both the substantive content of the probable cause standard and the underlying facts which are relevant to the probable cause determination. While, as we have just discussed, the probable cause determination has always been considered a question of law for the court, the cases have also made clear that if the facts upon which the defendant acted in bringing the prior action "are controverted, they must be passed upon by the jury before the court can determine the issue of probable cause.... 'What facts and circumstances amount to probable cause is a pure question of law. Whether they exist or not in any particular case is a pure question of fact. The former is exclusively for the court, the latter for the jury.' " (Ball v. Rawles, supra, 93 Cal. 222, 227, 28 P. 937. See Rest.2d Torts, 681B, subd. (2)(a).)