What is the test for establishing proximate cause in a motor vehicle accident?

[54 Cal.3d 1049] One of the concepts included in the term proximate cause is cause in fact, also referred to as actual cause. ³ Indeed, for purposes of BAJI No. 3.75, "so far as a jury is concerned 'proximate cause' only relates to causation in fact." (Com. to BAJI No. 3.75, italics added.) ⁴ "There are two widely recognized tests for establishing cause in fact. The 'but for' or 'sine qua non' rule, unfortunately labeled 'proximate cause' in BAJI No. 3.75, asks whether the injury would not have occurred but for the defendant's conduct. The other test, labeled 'legal cause' in BAJI No. 3.76, asks whether the defendant's conduct was a substantial factor in bringing about the injury." (Maupin v. Widling (1987) 192 Cal.App.3d 568, 574, 237 Cal.Rptr. 521.)