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  • Writer's pictureDavid A. Cmelik Law PLC

Iowa DUI Lawyer: Field sobriety tests not 100% accurate

The National Highway Traffic Safety Administration, or, NHTSA, acknowledges that the standardized field sobriety tests are not 100% accurate at determining whether a motorist is under the influence at the roadside.

For example, in some of the original validation testing that occurred—federal bureaucrats are quick to point out that they have organized follow up tests that unsurprisingly boost the percentages—no single field sobriety test is greater than 77% effective at correlating so-called “clues,” or signs of impairment identified on the tests, with a presumptively intoxicated breath alcohol level.

For example, the Horizontal Gaze Nystagmus, or, HGN, is described by some law enforcement officers as the “gold standard” because eye movements are completely involuntary and no amount of concentration and tensing can fake a steady gaze. Nevertheless, the National Highway Traffic Safety Administration, or, NHTSA, which is largely responsible for federalizing field sobriety testing, acknowledges that the HGN was validated in original validation studies at just 77% effective at correlating four clues, or, signs of impairment including:

Failure to smoothly pursue the stimulus, e.g., pen, finger, or flashlight, in one or both eyes; and a distinct and sustained nystagmus in one or both eyes at rest within a 180 degree field of vision, e.g., at approximately 45 degrees before maximum deviation and near maximum deviation, or the edges of peripheral vision, in one or both eyes.

If the HGN is the “gold standard,” how did the other field sobriety tests do? Not so well, it turns out.

The Walk and Turn was just 68% effective at correlating two or more clues, or, signs of impairment in original validation testing to a presumptive level of intoxication. In other words, over 32% of those who took and “failed” the Walk and Turn were not presumptively intoxicated.

The One Leg Stand was no better—just 65% effective at correlating two or more clues of impairment in the original validation studies to an intoxicated subject.

So what does this mean for a DUI defendant? Well, a couple things really. First, law enforcement officers emphasize that the tests should be read together not individually. That can cut both ways. If a test subject scores poorly on one test but does not reach a so-called “decision point” on the other two, the question arises whether the officer has achieved the sum total grounds necessary to (1) request a preliminary breath test, or, preliminary screen using a handheld device; and (2) invoke implied consent and temporarily detain the motorist for further testing at the law enforcement center using a machine like the Datamaster DMT evidentiary breath test machine.

Second, as stated above, officers need only reasonable grounds to believe a motorist is operating a motor vehicle under the influence to invoke implied consent and request further evidentiary testing. That’s a lower bar than proof beyond a reasonable doubt and even probable cause. The question is whether a judge would agree that the sum total grounds were sufficient to request a PBT and, later, an evidentiary breath, blood, or urine test. If the subject refused the breath test, standardized field sobriety test performance becomes more central to what a jury would conclude. However, if the officer has requested and obtained an evidentiary breath, blood, or urine test, field sobriety test performance is not usually a fighting issue in the jury trial. It becomes more central to pretrial evidence gatekeeping.

In short, it requires a seasoned, trained professional to examine all of these factors. The firm principal at David A. Cmelik Law PLC has been practicing criminal law for 21 years with over 5,000 cases completed. He has secured verdicts of acquittal in OWI and vehicular homicide.

If you or a loved one have been arrested for DUI, called Operating While Intoxicated, or, OWI, in the State of Iowa, and you are searching for ‘best Iowa DUI lawyers’ or ‘best DUI lawyers,’ we know best means something different to everyone. We want to be on your list of qualified candidates to represent you in this legal time of need. Contact us.


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