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How reliable are field sobriety tests?

The consequences of an operating while intoxicated conviction in Iowa are serious. Even a first-time OWI can result in driver's license suspension, significant fines and a jail sentence. Given the severe penalties it may make sense to challenge the prosecution's evidence of intoxication whenever possible.

Iowa police use a standard battery of three field sobriety tests to obtain evidence of intoxication and establish probable cause to make an arrest. These three tests are the walk and turn, the one-leg stand, and the horizontal gaze nystagmus or HGN test, in which the suspect is asked to follow a pen or flashlight with their eyes.

Police and prosecutors treat field sobriety test results as reliable indicators of intoxication. But are they really that reliable? According to a report by the National Highway Transportation Safety Administration, 79 percent of suspects who fail two or more of the indicators in the walk and turn test are legally intoxicated, that is, they have a blood alcohol content of 0.08 or more. The NHTSA report also concludes that the one-leg stand test will identify legally intoxicated individuals 83 percent of the time, and the HGN test will identify those individuals 88 percent of the time.

Assuming the NHTSA report is accurate, this means that 21 percent of suspects who fail the walk and turn, 17 percent who fail the one-leg stand, and 12 percent who fail the HGN test are not legally intoxicated. When one considers the number of OWI arrests made every year in Iowa, that's a significant number of people incorrectly identified as intoxicated by field sobriety test results.

Individuals charged with OWI in Iowa have the right to an attorney and the right to contest the prosecution's evidence in court. Challenging the accuracy of field sobriety tests is one way to fight the charges.

Source:, "Development of a Standardized Field Sobriety Test," accessed May 25, 2015

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