In Iowa, as in other states across the country, law enforcement officers administer a standardized battery of three field sobriety tests to a driver suspected of driving while intoxicated. The tests are the horizontal gaze nystagmus, or HGN test, the walk-and-turn test and the one-leg stand test.
Two of these tests, the walk-and-turn and the one-leg stand, are considered "divided attention" tests. They require suspects to divide their attention between following simple instructions and performing simple physical tasks.
A person who is intoxicated will have trouble dividing their attention between mental and physical tasks simultaneously. Most people can concentrate on a single task, like walking a straight line, when they are intoxicated. But, when a second task is added, they will exhibit signs of intoxication.
In the walk and turn test, these signs or clues of intoxication include losing balance while listening to the instructions, beginning the test too soon, stopping while performing the test, stepping out of the straight line, using their arms to balance themselves and taking the wrong number of steps. In the one-leg stand test, signs of intoxication include putting the raised foot down before the required 30 seconds are up, hopping, using arms to maintain balance and swaying.
There is a significant subjective element in evaluating a person's performance on these tests. Other factors, such as age, weight and medical conditions, can also affect the results. When given without other tests, the walk-and-turn test is only 68 percent accurate in detecting alcohol impairment. The one-leg stand test is only 65 percent accurate when given on its own. Anyone charged with operating while intoxicated on the basis of failing one of these tests may want consider challenging the test result.
Source: Texas District & County Attorneys Association, "Field Sobriety Test Review," accessed Nov. 26, 2016