Proving drunk driving: What do the numbers really mean?

Ask motorists in Bowling Green, Kentucky, to describe the significance of the number 0.08, and most of them will probably tell you that it represents the legal limit for driving while intoxicated in Kentucky. Not as clear in the minds of many people is how a blood alcohol content level of 0.08 percent works as evidence against a driver facing a DUI charge.

A breath test or blood test of a driver that shows a BAC level of 0.08 percent or more may be introduced as evidence in support of drunk driving charges. Prosecutors do not have to offer evidence of erratic or unsafe driving to prove that the motorist was driving under the influence under the state illegal per se law. The test results on their own are enough proof of intoxication for prosecutors to get a conviction.

Tests that reveal levels lower than illegal per se may still be used in court as evidence of driving while intoxicated, but it must be combined with other proof that the driver’s abilities to operate the motor vehicle were adversely affected. This is when the police officer’s testimony of what he or she observed will be important in proving a DUI charge.

Defense attorneys representing motorists on drunk driving charges may also benefit from blood test or breath test results. Measurements of BAC levels below 0.05 percent are presumed, under Kentucky law, to show that the driver was not under the influence of alcohol. This places a heavy burden on prosecutors to come forward with evidence to refute the presumption.

Tests to measure a driver’s blood alcohol concentration must be administered following strict guidelines. For example, the law requires that the tests be given within two hours of the motorist being stopped by the police. Failure to do so could mean that the results cannot be used as evidence against the driver.

Because of the technical nature of BAC level evidence, a motorist facing DUI charges might benefit from the services of a criminal defense attorney who is skilled and knowledgeable in the law and the rules of evidence in drunk driving cases.