When an Iowa police officer suspects a driver is intoxicated, the officer will ask the driver to perform field sobriety tests. These tests may include the one-leg stand, the walk-and-turn, and the horizontal gaze nystagmus or HGN test. In the HGN test, the officer has the driver follow a pen or small flashlight with their eyes. If the driver's eyes jerk involuntarily, it is considered evidence of intoxication.
Courts around the country have generally ruled that the HGN test is scientifically reliable as a means of detecting intoxication. There are ways to challenge HGN test results in court, however.
HGN test results can be challenged on the basis of officer training and failure to follow test protocols. Before an officer testifies about HGN test results, the prosecution will generally be required to establish that the officer has been properly trained in the test and that they conducted the test in strict compliance with established test procedures. If the prosecution fails to do so, or if the defense can successfully challenge the evidence, the judge may refuse to allow the testimony.
It is also important to note that courts have consistently ruled that HGN test results can only be used to support an opinion that the driver was intoxicated. They cannot be used as evidence of a specific blood alcohol content.
Challenging the state's evidence in an OWI trial requires skill and experience. Working with an experienced OWI lawyer can improve a defendant's odds of successfully fighting drunk driving charges.
Source: tdcaa.com, "DWI Detection and Standardized Field Sobriety Testing," pp. 110-24, accessed November 28, 2015