Whether in a so-called "per se" case (driving with over .08% blood alcohol) or a traditional DUI situation (impaired driving due to alcohol), the prosecutor will usually introduce chemical evidence of the defendant’s blood alcohol level. Of course, by itself, this is irrelevant: It reflects the blood alcohol level of the defendant at the time of testing — not at the time of driving. And, of course, it is not a crime to be under the influence of alcohol or over .08% in a police station or hospital.
So…the prosecutor will usually introduce evidence — usually through expert witnesses such as crime lab forensic toxicologists — of retrograde extrapolation. Put simply, retrograde extrapolation is the computation backwards in time of the blood alcohol level — that is, the estimation of the level at the time of driving. Or, as many would define it, "guessing" backwards.
In many states, this effort is made easier by a statutory presumption that the blood alcohol concentration at the time of testing was the same as at the time of driving. Of course, this is almost never the actual truth, as blood alcohol levels are constantly changing — and are almost certainly not the same when tested an hour or two after driving. As so often happens in DUI cases, the laws are changed to ignore scientific facts to make it easier to convict the accused.
To combat this, the defense has only two tools. First, such statutes generally require that the breath or blood test be administered within a given period of time after driving for the presumption to take effect — usually two or three hours.
Second, most states have statutes that make this presumption a rebuttable one. That is, the defense is permitted to produce its own evidence to contradict this evidence — through cross-examination of the prosecution’s expert witness or with testimony of other witnesses or the defendant — to "erase" this presumption.
In effect, then, the burden of proof is shifted from the prosecution to the defendant: confronted with a legal presumption, the defendant must prove his innocence or be convicted.
Is retrograde extrapolation so scientifically accepted that we are prepared to presume such important facts as whether an accused was, in fact, intoxicated at the time of driving? Consider the following opinion of the most well-known and respected expert in the field, Dr. Kurt Dubowski, a professor at the University of Oklahoma:
It is unusual for enough reliable information to be available in a given case to permit a meaningful and fair value to be obtained by retrograde extrapolation. If attempted, it must be based upon assumptions of uncertain validity, or the answer must be given in terms of a range of possible values so wide that it is rarely of any use. If retrograde extrapolation is based upon breath analysis the difficulty is compounded.
In the continuing effort to facilitate drunk driving convictions, laws have been passed which change questionable evidence into legally-presumed scientific fact — thus forcing the defendant to prove his blood-alcohol concentration at the time of driving or be convicted of drunk driving. And exactly how does he go about doing that?