California Code, Government Code - GOV § 53153

For purposes of this article, a person is under the influence of an alcoholic beverage or any drug, or the combined influence of an alcoholic beverage and any drug, when as a result of drinking an alcoholic beverage or using a drug, or both, his or her physical or mental abilities are impaired to a degree that he or she no longer has the ability to operate a motor vehicle, boat or vessel, or aircraft with the caution characteristic of a sober person of ordinary prudence under the same or similar circumstances. For purposes of this article, the presumptions described in Sections 23152 and 23155 of the Vehicle Code shall apply.