What is the test for impaired driving in a motor vehicle case?

If the issue is whether the ability of the driver to operate his vehicle is impaired, the manner in which the vehicle is driven is evidence which tends to prove or disprove that fact. The guilt of an accused may be established by circumstantial as well as by direct evidence, and the right to draw proper inferences from the evidence is a function of the jury. (People v. McGinnis (1953) 123 Cal.App.2d Supp. 945, 947, 267 P.2d 458.)