California Drunk Driving: The Offense
To begin with, the term "drunk driving" is legally incorrect. "Drunk" is a concept used for the offense commonly known as "public intoxication" or "drunk in public", a violation of California Penal Code section 647(f). The offense consists of being in a public place under the influence of alcohol or drugs to the extent that a person "is unable to exercise care for his or her own safety or the safety of others". Driving is not involved, and the degree of intoxication is considerably greater than that prohibited by "drunk driving" statutes.
The correct term is "driving under the influence", commonly referred to as DUI, and this can be from alcohol, drugs or a combination of alcohol and drugs. The drugs do not have to be illegal; prescribed drugs are included. This offense is described in California Vehicle Code section 23152(a):
"It is unlawful for any person who is under the influence of any alcoholic beverage or drug, or under the combined influence of any alcoholic beverage or drug, to drive a vehicle."
What legally constitutes "under the influence"? The term is generally described as being impaired to the extent that the person is unable to exercise the same care and judgment as a reasonable prudent and sober driver. This is necessarily vague, and is usually proven by such evidence as police evidence concerning erratic driving, physical symptoms (slurred speech, poor balance and coordination, etc. and field sobriety tests. In trial, however, the defendant is legally presumed to be under the influence if a blood or breath test taken within 3 hours of driving indicates a blood-alcohol level (BAC) of .08% or higher; he may offer evidence to rebut this presumption.
A second and related offense is the so-called per se law. This offense consists simply of driving with a blood-alcohol level of .08% or greater. Being impaired or intoxicated is unnecessary and individual tolerance to alcohol is irrelevant: the offense consists solely of having the BAC concentration and driving. The offense is described in California Vehicle Code section 23152(b):
"It is unlawful for any person who has a 0.08% or more, by weight, of alcohol in his or her blood to drive a vehicle"
If the arrested individual takes a blood, breath or urine test indicating .08% BAC or higher, he will be charged with both offenses. Although he can be convicted of both, he cannot be punished for both (the penalties are identical). If there is no evidence of blood-alcohol, her will only be charged with the driving under the influence.