On behalf of Law Office of Steven Haney on Monday, August 6, 2012

When the police stop the driver of a vehicle on suspicion of driving while intoxicated, the police officer will generally ask the driver to perform some tests, know as field sobriety tests. These tests are designed to allow an officer to detect drivers who are intoxicated.

The National Highway Traffic Safety Administration (NHTSA) helped develop three standardized field sobriety tests (SFST) in the 1980s. NHTSA created this standardization to allow for more consistent identification of intoxicated drivers and to provide better evidence in court for DWI cases.

Walk and Turn

This test has the suspect walk, heel-to-toe nine steps, along a straight line. The results depend on how well the suspect follows the instructions the officer gives and how well they execute the maneuver.

Horizontal Gaze Nystagmus

During this test, the police officer has the suspect’s eyes follow the level movement of an object. Horizontal Gaze Nystagmus (HGN) results from the involuntary movements of the eye as it looks to the sides. When impaired by alcohol, the jerking occurs at lesser angles and is more pronounced.

According to NHTSA, the officer “looks for three indicators of impairment in each eye: if the eye cannot follow a moving object smoothly, if jerking is distinct when the eye is at maximum deviation, and if the angle of onset of jerking is within 45 degrees of center.”

One Leg Stand

This test requires the subject to stand on one leg with the other foot six inches off the ground. They then have to count aloud one-thousand, two-thousand until the officer instructs them to stop. Typically, the test lasts for 30 seconds.

But How Accurate are These Tests?

The NHTSA claims the combination of tests is accurate 91 percent of the time. The problem is they are still subjective tests and some drivers who are not above 0.08 BAC may still fail them. Police officers are human, and thus can make mistakes as the tests are not foolproof.

For example, some individuals may have balance or other medical issues that compromise their ability to pass a test successfully. The SFSTs can also be influenced by the experience level of the officer administering the test.

These are all questions that can be discussed with a DWI defense attorney, because the complex interaction of the traffic stop should be reviewed in its entirety, as there may be many opportunities for a defense attorney to point out behavior that indicates a lack of impairment on behalf of the driver.

Source: NHTSA, “Standardized Field Sobriety Testing”