It is unlawful to have an excessive blood-alcohol concentration (BAC) at the time of DRIVING -- not at the time of being TESTED. Since it takes between 45 minutes and 3 hours for alcohol to be absorbed into the system, an individual's BAC may continue to rise for some time after he is stopped and arrested.

Commonly, it is an hour or more after the stop when the blood, breath or urine test is given to the suspect. Assume that the result is .12%. If the suspect has continued to absorb alcohol since he was stopped, his BAC at the time he was driving may have been only .08%. In other words, the test result shows a blood-alcohol concentration above the legal limit -- but his actual BAC AT THE TIME OF DRIVING was below.