Almost every person arrested for alcohol-related DUI will be charged with both offenses provided under the two subsections of Vehicle Code section 23152, that is, with “driving under the influence” and the separate crime of “driving with a blood alcohol concentration (BAC) of .08% or higher.”  However, while a driver can be convicted of both offenses provided under section 23152, in most circumstancess, the driver may only be sentenced for one. (This is unlike the situation when other DUI-related crimes are charged, which may well result in additional penalties unless they are "lesser included offenses.").  Why, then, are DUI defendants commonly "double charged" under section 23152? The answers are many. 

Because each charge involves separate and different elements, they are considered legally separate offenses, and therefore, charging both does not violate the double jeopardy rule, even though they are based on the same occurence of drunk driving.  So, if prosecutors think they might be able to prove that the defendant violated both provisions, they are free to charge both. 

DUI drivers can easily commit both offenses at the same time.  If the driver was very intoxicated, it’s logical that the intoxication both produced a BAC of greater than .08% and substantially impaired the ability to drive.  By charging both violations separately, when conviction on either is sufficient to trigger the same penalties, the prosecutors makes the case against the driver appear stronger than if just one violation were charged. By overcharging the defendant, prosecutors believe they gain the upper hand in plea bargaining. 

Similarly, prosecutors charge both offences as a safety measure.  If a defense attorney creates reasonable doubt or renders evidence inadmissible as to one charge because of some technical defect in the prosecution's case, the prosecutor hopes to rely on the other charge to support a conviction.  For example, if a defense attorney shows that the blood sample was tainted – effectively killing the per se .08% charge – the prosecutor can still rely on the field sobriety tests, testimony of the arresting officer and any other evidence to show impairment. 

Of course, good defense attorneys will attempt to overcome all charges, taking advantage of technical and other defects in the evidence, making the case that the evidence is insufficient for conviction.