What is the test for determining whether a person is a proximate cause in a motor vehicle accident?

each is a proximate cause ... if that conduct was also a substantial factor contributing to the result...." ' [Citation.]" (Id. at p. 847, italics omitted; see People v. Jennings (2010) 50 Cal.4th 616, 643 [" '[T]he defendant's act must have been a substantial factor contributing to the result, rather than insignificant or merely theoretical.' [Citation.]"].)

We have examined CALCRIM No. 3149 (see ante, at pp. 5-6) and conclude its definition of proximate causation comports with the law. (See People v. Runnion (1994) 30 Cal.App.4th 852, 858 ["[T]he trial court's obligation is to state the law correctly."].) Since a jury is "presumed to understand and follow the court's instructions" (People v. Holt (1997) 15 Cal.4th 619, 662), we cannot conceive a reasonable likelihood it applied CALCRIM No. 3149 in an improper manner. Therefore, we find no instructional error.