Judge: High BAC Readings Don’t Necessarily Prove DWI

David J. Hanson, Ph.D.

Alcohol breath tests are subject to many sources of error and their results vary at least 15% from actual blood alcohol concentration (BAC). At least 23% (about one of every four) individuals tested will have a BAC reading higher than their actual BAC.

Therefore, many people convicted of DUI/DWI simply on the basis of a breath test results alone will be innocent drivers who are falsely convicted.

In addition, about one quarter (24%) of alcoholics or people who are alcohol dependent show no clinical signs of impairment at a BAC of .10, which is 25% higher than the current limit of .08.

So there are two obvious problems with convicting any and every driver who has an alcohol breath test reading of .08.

  1. Many people will have an actual BAC much less than their Breathalyzer readings.
  2. Many alcoholics and others who have developed a high tolerance for alcohol will show no signs of impairment with an alcohol breath test reading of .08. However, they’ll be convicted of impairment.

Because of these facts, Judge Ian O’Flaherty of Virginia believes that the state’s DWI law is unconstitutional. That law says that anyone with a BAC of .08 is presumed to be driving under the influence of alcohol regardless of any evidence to the contrary. The burden of proof is on drivers to prove that they weren’t intoxicated.

Judge O’Flaherty, who’s known as a tough judge, has ruled that the law is unconstitutional because under the US Constitution the burden of proof always rests with the prosecution, whereas the Virginia law puts the burden on the defendant. He cites Francis v. Franklin, a Supreme Court case in which the high court held that all elements of a crime must be proven beyond a reasonable doubt.

The Virginia law also denies defendants the ability to present evidence demonstrating the fact that different individuals are intoxicated at different blood alcohol concentrations and that BAC can rise after driving. This means that their BAC can be below .08 while driving but then rise to .08 or higher at the time the alcohol breath test is given. This denial of evidence makes it even more difficult for defendants to prove their innocence.

The law was actually passed with the explicit goal of obtaining higher conviction rates. It begins by assuming the accused to be guilty and then prohibits them from presenting evidence to the contrary to prove their innocence.

In western democracies the legal system presumes people to be innocent and the state’s prosecutor must prove them to be guilty; the burden of proof is on the government. But in dictatorships and police states people are assumed to be guilty and they must prove their innocence. That’s all the difference in the world and we shouldn’t make exceptions for those accused on DWI/DUI.

It’s important that drunk drivers, and especially hard core repeat offenders be convicted and/or receive appropriate treatment. But it’s also important that innocent drivers not be falsely convicted of DWI/DUI by a law that intentionally stacks the deck heavily against them.


  • See Breath Analyzer Accuracy. One veteran of the courts says that the breath tester is “a piece of crap” (Wright, Gary , Alexander, Ames, and Mellnik, Ted. “Judges are not following the law.” Charlotte Observer, August 9, 2004).
  • Judge: High blood-alcohol levels don’t necessarily prove DWI. Azcentral.com, August 12, 2005.
  • Judge rules parts of DWI laws unconstitutional. Washington Times, August 12, 2005

Filed Under: Drinking and Driving