When you have been arrested on the suspicion of driving under the influence, you are only being charged with the potential of having committed an offense. This does not mean you are convicted. Before you can be convicted, the government must prove that you are guilty beyond a reasonable doubt, only then can you be found guilty of the offense.
The prosecutor must prove each element beyond a reasonable doubt. For a person to be found guilty of driving under the influence, they must be guilty of two elements; they must be found to be driving, and driving while impaired.
When a person has consumed alcohol and are driving, the Court has a guideline which they can follow to determine impairment. The government has found that if a person has a blood alcohol level of .08 or higher, they are reasonably impaired enough to the point where their driving may be impacted.