Convicting the “Average” DUI Suspect

Posted by Lawrence Taylor on November 11th, 2004

One of the greatest sources of error in breath-alcohol testing is the consistently recurring fallacy that the individual tested is perfectly average in certain critical physiological traits. Put another way, obtaining an accurate blood-alcohol reading in a DUI case is completely dependent on the validity of a number of assumptions. Unfortunately for the person being tested, these assumptions are usually incorrect: The person tested is rarely “average” in even one of these critical characteristics, let alone in all of them.

For example, all breath testing devices depend on the assumption that the ratio between alcohol in the exhaled breath and alcohol in the blood is 1 to 2100. In fact, the machine is designed to produce a reading based on that assumption; the accuracy of the reading is directly tied to the accuracy of the presumption. Yet, the actual ratio in any given individual can vary from less than 1:1300 to more than 1:3000. So a DUI suspect with a true blood-alcohol level of .08 but a breath-to-blood ratio of, say, 1:1700 would have a .10 reading on an “accurate” breath testing instrument.

Put simply, these machines do not test individuals. Rather, they test the same “average suspect” over and over again, but using the individual subject’s breath.

Another example of the assumption of “averageness” can be found in urinalysis. When a DUI suspect’s urine is analyzed for blood-alcohol, a presumption exists that there are 1.3 parts of alcohol in the bladder’s urine for every 1 part of alcohol in the blood. This 1:1.3 ratio is as fallacious as the 1:2100 ratio’that is, it is based entirely on the ratio found in the average person. In fact, however, the actual ratio found in any given individual can vary greatly. And as the ratio is in error, so will be the final blood-alcohol reading.

Another example of this constant reliance on averages shows itself when the prosecutor in a DUI trial offers evidence of so-called “retrograde extrapolation”, or guessing backwards. The blood-alcohol level at the time of testing is not relevant to the charge, of course, and so the state will offer evidence to show what the level was when the defendant was driving. This is commonly done by “extrapolating” backward’that is, computing the earlier blood-alcohol level by estimating how much alcohol had been eliminated or “burned off” in the period between driving and testing. But this requires two assumptions: The blood-alcohol level was declining and the rate of elimination is known. This second assumption involves the further assumption that the “burn-off” rate was .015 percent per hour (sometimes the assumed rate is .02 percent). How does the prosecution know that the defendant was eliminating (assuming he was eliminating rather than absorbing) at that rate and not at .005 percent or .3 percent? Quite simply, the prosecution does not know: It merely assumes that the defendant eliminates at the average rate. And, of course, error in such an assumption translates into error in the extrapolation.

This ever-present “average person” in the DUI arena is not limited to chemical analysis. We even find him with the arresting officer in the field. When the officer administers the “nystagmus'’ test (“follow my finger with your eyes”) as part of the battery of field sobriety tests, he operates on the assumption that the suspect is “Mr. Average.” The officer has been trained to “read” at what angle the suspect’s eyes begin jerking. If it begins before 45 degrees, the suspect fails. And where does the magic figure of 45 come from? The average person.

Don Nichols, a pioneer among DUI defense attorneys, would point out to juries that his client is female, Chinese and deceased’despite obvious evidence to the contrary. He then explains that statistically there are more women than men in the world, more Chinese than any other nationality and more dead human beings than living ones. Statistically, then, the average person is female, Chinese and deceased’and so, according to the prosecution, must be his client. He also asks the jury how many of them have 2.3 children’the average in the United States.

So why does the state presume facts that are clearly untrue? Simple. It is convenient: it makes prosecution and conviction much easier.


    One important corrective note is that breath machines do not assume any partition ratio for calculating BrAC.  They report whatever result they get back out to however many litres of breath that state’s legal per se limits require them to.  There is no assumed 2100/2300/whatever blood to breath ratio in breath testing instruments.
    Additionally, most driver impariment studies that the 0.08 impairment level is based on, is using breath machines.  So one could more correctly argue that someone who’s blood alcohol level is 0.08 may or may not be as impaired as someone who had a measured BrAC level of 0.08 – based upon the correctly stated variable blood/breath ratio.
    Also, state witnesses should always use a range of values when doing retrograde extrapolations.  For just the reason mentioned, individuals elimination rates vary.  “Professional drunk drivers” elmination rates can be above 0.03, which is very high indeed.

  • RichardAlan

    September of 2007 I was stopped for allegedly going 60 mph in a 45. My true speed was 47 mph. I have (had) a Class A commercial license a perfect driving record i.e. no accidents, tickets, actions, or FTA’s. Safe driving awards from United Parcel Service, and a perfect safety record as noted and in regards to all my colleagues in the transportation/ mobile television industry.

    I handle and transport million dollar mobile television equipment around the nation with perfect safety, I’m the best value money can buy.

    After my stop for this alleged speeding, I was approached by a Law Enforcement officer as he approached my window and before I could roll it down he’s yelling at me; “you can put the water bottle down, nothing is going to help you now” and with that I recognized Malfeasance by this officer and did not say one word. I was asked questions, I said no, I refuse to talk, I want a lawyer.

    I refused both the FST, and the Breath test, why ? the officer already told me; “nothing was going to help me now” so why then would I do the voodoo science monkey tricks for him? To audition for my freedom? which he already made himself clear, “was not going to help me now”.

    I was arrested for an alleged DUI. I called an attorney the same day of my arrest and hired him, it cost $14,000.

    We demanded the patrol cars video which would have shown my “real” speed and proved the Malfeasance of the officer , i.e., “nothing is going to help you now”

    We demanded a jury trial, it took 7 months, the D.A. kept postponing my trial. The officer did not show, and the video evidence
    somehow was not available for both the court and the DMV hearing.

    My case was dismissed this past April, all Alcohol related charges were dropped. I plead to a reckless driving, and a $150 ticket.

    My license/property was taken for one year without due process and absent a conviction.

    Because I was a freelance transportation coordinator I did not lose my job, just my career, which included medical benefits, pension, seniority, my ability to put food in my mouth and provide for my family…
    this is devastating.

    of course my credit is now ruined and at 43 years old my life and ability to stay current on my bills is lost, I have to file bankruptcy.

    I am now a burden of the state.

    All of my colleagues in mobile television are desperately awaiting for me to get my license back. I still get job offers to drive.

    I’m amazed at the respect I’m getting for standing up for the Constitution of the United States of America in regards to a bogus dui allegation or I mean speeding in a 45 mph zone.

    In the last 3 months I’ve lost $15,000 in potential earnings.

    I was the safest professional driver on the highway, and now I can’t drive professionally, my right to work in California has been taken away.

    I would do it all over again for the integrity of our United States Constitution.

    If there is ever a class action lawsuit against the State I would be apart of it.

    “No Reasonable Cause” the Movie is up at youtube. search No Reasonable Cause.

    I hope there is someone out there that is interested in my story.

  • ruReadyMyFriend

    Variations of this story are told all the time. Driving is a privilege not a right and it is the tool law enforcement uses to destroy many lives including my own.

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