DUI - Presumptions of intoxication

All 50 states have a law in effect that allows for a presumption to be drawn at a predetermined amount of blood alcohol level (BAC). In Tennessee the current BAC to trigger this presumption is .08% BAC.  The purpose of this presumption is to ease the burden on the State to prove that the person was actually “under the influence”. 

So in layman's terms if a person submits to a chemical test and that test was administered correctly the jury is to presume the defendant was under the influence if the BAC is .08% or greater. Many legal scholars are concerned with the wording of such laws in fear that any “presumption” other than the presumption of innocence is improper.  Many courts use the term “inference” when charging a jury on this issue. 

In any event, the presumptions created by the statute are rebuttable. This means that if you can show that the test was somehow faulty or that the person was not “under the influence” then the jury is free to disregard this presumption. 

For example, if during cross examination the person who took the blood from the defendant could not remember where he stored the blood for several hours,a jury could reject the presumption that the blood was an accurate account of the defendant’s BAC. A much more difficult rebuttal to the presumption would be to prove that even though the BAC was over .08% the defendant was not under the influence. In fact, there is some reliable evidence that a person who is .10% BAC will have no impairment in reaction time.(See Lane Baylor’s article - Effects of Ethanol on Human Fractionated Response Times)

If the defendant submits to a chemical test and the results are above the legal limit, the State will normally charge DUI as well as a DUI per se. These are two separate charges but if a defendant is convicted then the charges will merge into one conviction.  BUT since the two charges contain different elements to prove, you can actually be found NOT GUILTY of DUI and have a hung jury on the DUI per se and then be retried on the DUI per se without any type of double jeopardy implications.