Field Sobriety Tests

Definition: The Standardized Field Sobriety Test (SFST) is a battery of 3 tests performed by a police officer during a traffic stop in order to determine if a person suspected of impaired driving is intoxicated with alcohol or drugs. The 3 tests that make up the SFST are the horizontal gaze nystagmus (HGN), the walk-and-turn, and the one-leg stand tests. Developed in the 1970s, these tests are claimed to be scientifically validated, and are admissible as evidence in court in a majority of states including California.

Other sobriety tests which have not been standardized by the National Highway Transportation Safety Administration (NHTSA) can be included by the police officer, such as finger-to-nose, alphabet recitation,  DUI Rhomberg balance test (eyes-closed, head tilted back, estimate passage of 30 seconds), etc.

The intended purpose of the tests

The tests are intended to help officers make decisions about whether or not to arrest suspected impaired drivers.  However, in reality, the police often use these tests for the purpose of providing “evidence” to support the officer’s opinion of intoxication.

The tests are optional

California drivers are not legally required to take a Field Sobriety Test.  Taking the test is optional and you can politely tell the officer, “No thank you, I do not want to take the test”. However, the arresting police officer is unlikely to inform you of this fact.

How reliable are these tests?

Evidence of reliability

The National Highway Traffic Safety Administration (NHTSA) sponsored research beginning in 1979 that lead to the development of the Standardized Field Sobriety Test (SFST). The tests were designed to help officers make arrest decisions at and above the 0.10 percent blood alcohol concentration (BAC).

Since then, many states, including California, have lowered their BAC limits to .08 percent and further studies have been conducted to validate the tests.  See footnote for some detail. As a result, police and court officials have confidence in these tests.

Evidence of unreliability and invalidity

While police like using the tests, some critics say the tests are unreliable. Dr Sturgeon Cole, a retired professor from Clemson University, says the tests are designed to fail.  He is concerned that “There are no norms and there is no average score.  We have no idea what an average person can do on one leg doing heel to toe.”  He also says that even when conducted correctly, the tests give officers an indication of intoxication which is only 26% better than chance (or randomly guessing). After studying these tests for decades, he is convinced they are neither valid nor reliable.

See for details of a study titled “FIELD SOBRIETY TESTS: ARE THEY DESIGNED FOR FAILURE?” In this study, it was found that “Even without [any] alcohol, the number of errors made by individuals performing the field sobriety tests was sufficient for officers to judge that the [completely sober] individuals had had too much to drink.”

Whatever your opinion on the reliability of the tests, bear in mind that the 3 tests comprising the Standardized Field Sobriety Test are considered reliable by judges and are admissible as evidence in court in California.

The following 40-second CNN video shows some live Field Sobriety Tests being performed including doing an eye test (following the light with their eyes) walking the line, and standing on one foot.


See also:-


In 2006, Anacapa Sciences, Inc. of Santa Barbara, California was commissioned to conduct a study to validate the accuracy of the SFST battery to discriminate above or below 0.08 and above and below 0.04 percent blood alcohol concentrations. (BAC of 0.04 is the national standard for Commercial Driver License holders).  Their study was based on the collection of data by seven experienced officers of the San Diego Police Department’s alcohol enforcement unit.  As documented on this NHTSA page, the results were as follows:

  • Overall, officers’ decisions were correct in more than 91% of the cases at the 0.08% BAC level
  • At 0.04 BAC or above, the officers’ estimates were accurate in 80 percent of cases overall

Excluded from this report were BAC levels below 0.04.


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