Is a Citizen Accused of DUI Permitted to Defend Himself with Scientific Facts?

Posted by Lawrence Taylor on August 29th, 2012

I’ve expounded on this blog at great length in recent years on the unreliability and inaccuracy of the various machines used to measure the breath of citizens accused of drunk driving.  See for example,  "Trial by Machine "– But How Good Are the Machines?.  I’ve also blasted the unconstitutional laws of most states which presume the accuracy of these machines.  See If You Can’t Prove It, Make the Defendant Disprove It.   In many cases, as in my home state of California, the defense is even prohibited from offering critical scientifically-accepted evidence of the machine’s defects.  See DUI and the Presumption of Guilt.

A classic example of this law-trumps-science is the banning of evidence of partition ratio.  Simply put, the machines all automatically convert the amount of alcohol tested in the tiny amount of breath taken from the suspect.  The internal computer multiplies the amount by 2100 — using the average ratio of alcohol in blood to alcohol in breath — to estimate the amount of alcohol in the suspect’s blood.  Problem:  We are not all average.  And ratios vary from 1100:1 to 3500:1.  This can cause blood-alcohol test results to vary by as much as .05%.  An accused with a BAC reading of .11% (over the .08% limit), for example, might actually have a true BAC of as little as .06% — well under the limit.

A couple of years ago, the California Supreme Court finally ruled that a defendant should be permitted to defend himself with that widely-accepted scientific fact. (And, I’m proud to say, the Court cited my book California Drunk Driving Defense three times as authority in its decision.)

California Supreme Court Says Defendants Can Challenge 

Breathalyzer Results

The court permits drunk-driving defendants in some cases to question blood-alcohol findings, which are sometimes inaccurate depending on factors such as gender and medical conditions.

Los Angeles Times
.   July 9
  – Accused drunk drivers now have more ammunition for challenging Breathalyzer findings as a result of a unanimous ruling Thursday by the California Supreme Court.

The ruling is expected to make drunk-driving cases more complicated and possibly more difficult to prosecute, lawyers said. Courts in two other states, Arizona and Vermont, have reached similar conclusions.

Under the law, a suspected drunk driver can submit to either a blood test, which measures the amount of alcohol in the blood, or a breath test. Alcohol levels in a breath sample are converted mathematically to derive a blood-alcohol percentage. In California, a person is legally drunk when his or her blood-alcohol level is 0.08% or higher.

The standard formula for converting breath results to blood-alcohol levels is not accurate for everyone, however, and can vary depending on an individual’s medical condition, gender, temperature, the atmospheric pressure and the precision of the measuring device, the court said.

"The question is whether a defendant who has a blood-alcohol concentration of 0.08% or more measured by breath is entitled to rebut that presumption that he was under the influence" in certain cases, Justice Carol A. Corrigan wrote. The court’s answer was yes.

Even though experts say the standard ratio used to derive a blood-alcohol concentration from breath generally approximates or even underestimates the amount of alcohol the driver consumed, they also agree that Breathalyzer results may sometimes overestimate the amount of alcohol in the blood.

Thursday’s ruling permits defendants in some cases to challenge those results based on mathematical ratios.
"Evidence casting doubt on the accuracy of the breath-to-blood conversion ratio is just as relevant as other evidence rebutting the presumption of intoxication from a breath test result, such as evidence that the defendant had a high tolerance for alcohol or performed well in field sobriety testing," Corrigan wrote.

San Bernardino County Deputy Dist. Atty. Mark A. Vos, who prosecuted the case before the court, said the ruling was "going to make DUI trials a little more difficult to put on" because more technical evidence will be permitted.

"The numbers are going to be flying back and forth in DUI trials, so prosecutors are going to have to adapt," Vos said.

Jamie L. Popper, the appellate defense lawyer in the case, said the ruling will ensure that juries have the information they need to judge a case.

"The situation currently is that juries are led to believe that when a person blows into the breath test, the blood-alcohol measure that breath test gives is a fact, when all a breath test is is a measure," Popper said…


Amazing.  It takes a Supreme Court ruling to allow a citizen accused of DUI to defend himself with established scientific truth. 

But….in a typical retreat from logic, the Court limited the admissibility of partition ratio evidence to defending against the charge of driving under the influence — not to the accompanying charge of driving with .08% blood-alcohol concentration (BAC).  So…you can use scientific facts that the BAC reading is faulty to defend yourself against the BAC-based presumption of being under the influence – but not against the charge that your BAC was .08% or higher.


  • DUI-The Constitution Need Not Apply

    I was encouraged that the court had the sense to reference your book in attempting to arrive at a fair ruling…but then I read the last paragraph of your article. A ‘retreat from logic’ is exactly what they’ve done by not allowing the partition ratio evidence to be admissible to the charge of driving over .08%. That’s the only charge to which that evidence is relevant! Would it not be pointless to spend the time and effort building a partition ratio defense against a charge of ‘impaired’ or ‘under the influence’ where the prosecution can us other competent evidence to convict? Once again the state gets to have their cake and eat it too. The deputy DA needn’t complain so much…the court still has his back.

  • Tim

    I think ALL DUI’s should require a blood test only and not a breathalyzer. This would perhaps force police to only follow through on the more serious cases and not for the typical casual social drinker who has a low BAC. The breathalyzer is unreliable and way too convenient for the police and courts.

  • denise sanchez

    Its about time they made more laws for D.W.I People have gotten away with it and thats y they keep doing it over and over again. They r to easy on them and give to many chances.

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