California Code, Government Code - GOV § 820.25

(a) For purposes of Section 820.2, the decision of a peace officer, as defined in Sections 830.1 and 830.2 of the Penal Code, or a state or local law enforcement official, to render assistance to a motorist who has not been involved in an accident or to leave the scene after rendering assistance, upon learning of a reasonably apparent emergency requiring his immediate attention elsewhere or upon instructions from a superior to assume duties elsewhere, shall be deemed an exercise of discretion.

(b) The provision in subdivision (a) shall not apply if the act or omission occurred pursuant to the performance of a ministerial duty.  For purposes of this section, “ministerial duty” is defined as a plain and mandatory duty involving the execution of a set task and to be performed without the exercise of discretion.