In California, drunk driving (Driving Under the Influence or "DUI") laws address impaired driving whether it is presumed by law or a person is in fact too impaired to drive safely. DUI laws also address whether the impairment is caused by alcohol or drugs (legal or illegal). I'll address the different ways
1 Act of Drunk Driving Can Be Charged in 2 Ways
1. Vehicle Code Section 23152(a) - Driving Under the Influence of Alcohol and/or Drugs ("a count")
In California, it is illegal for a person to drive a car will enough alcohol or drugs in his/her body that he/she cannot drive as a safely as a sober person. Under this charge, a person must be driving at the time he/she is driving dangerously due to alcohol and/or drug. A person can be prosecuted for DUI and be UNDER THE LEGAL LIMIT OF .08% FOR ALCOHOL as long as the person is DRIVING MORE DANGEROUSLY THAN A SOBER PERSON. If a person has a drug in his/her system, the DRUG DOES NOT HAVE TO BE AN ILLEGAL SUBSTANCE as long as the person is DRIVING MORE DANGEROUSLY THAN A SOBER PERSON.
For example, if a person took Ambien, a legally prescribed sleeping medication, and drives poorly (such as weaving, nearly colliding into other cars), the he/she could be charged with DUI under the "a count." Even though the drug/medication is legally prescribed, it is illegal to drive while under the influence of the substance when a person drives more dangerously than a sober person. The same holds true for illegal drugs.
In order to be convicted, there must be a showing of "under the influence," District Attorneys will try to suggest under the influence through poor driving, poor performance on Field Sobriety Tests ("FST's"), trouble standing, slurring speech, red & watery eyes, or through testimony of a Criminalist suggesting impairment based on the amount of drugs in the body. Can a Criminalist really know the level of impairment based on a lab result by itself? If the lab results show a very high amount of alcohol/drugs, perhaps. If low amounts, I think not, that to testify as to impairment is speculative and best, pure guesswork at worst.
2. Vehicle Code Section 23152(b) - Driving With Blood Alcohol of .08% or More ("b count")
The second way to charge drunk driving involves ALCOHOL ONLY. In the "b count," the District Attorney is NOT REQUIRED TO SHOW A PERSON IS IMPAIRED while driving, but merely AT THE TIME OF DRIVING, A PERSON'S BLOOD ALCOHOL LEVEL WAS .08% OR MORE. No poor driving is required. No bad FST's are required. The law also provides that if a BLOOD TEST IS PERFORMED WITHIN A COUPLE HOURS OF DRIVING, THAT IS THE BLOOD ALCOHOL LEVEL AT THE TIME OF DRIVING. That is a REBUTTABLE PRESUMPTION, meaning the Defense has the burden of proving to a jury that the presumption of blood alcohol level at the time of driving is wrong. Fair? As a public defender, I don't think so.
For the "b count," the prosecution is not required to show a person was driving impaired or driving more dangerously than a sober person. The prosecution is required to show that 1) a person was driving and 2) at the time of driving, the person's blood alcohol was .08% or higher.