What is the test for driving under the influence of methamphetamine?

The relevant statutes do not proscribe the operation of a vehicle with a specific measurable amount of methamphetamine in the blood; rather, they require proof that defendant was driving "under the influence" of methamphetamine. (Pen.Code, 192, subd. (c)(3); Veh.Code, 23152.) A person is under the influence within the meaning of the statutes when, as a result of using methamphetamine, his physical or mental abilities are impaired to such a degree that he no longer has the ability to drive his vehicle with the caution characteristic of a sober person of ordinary prudence under the same or similar circumstances. (People v. Enriquez (1996) 42 Cal.App.4th 661, 665, 49 Cal.Rptr.2d 710.)