Admissibility of Field Sobriety Tests in Drunk Driving Cases

There are three standardized field sobriety tests (FSTs) that make up the Standardized Field Sobriety Test battery. They include the horizontal gaze nystagmus (HGN) test, the "walk and turn" test, and the "one-leg stand" test. The HGN test refers to an involuntary jerking as the eyes gaze toward the side. When intoxicated, a person's smooth and accurate control of his or her eye movements will break down. The walk and turn test and the one-leg stand test are referred to as the "divided attention" tests, which simulate the mental and physical capabilities a driver needs to drive safely. Of the three FSTs, the HGN is considered the most reliable field sobriety test, especially when used in combination with the divided attention tests.

The National Highway Traffic Safety Administration (NHTSA) has maintained that the three tests in combination accurately predict intoxication up to 80 percent of the time. Despite NHTSA's vote of confidence, there has been substantial criticism of the alleged scientific validity of the tests as reliable evidence of intoxication. As a consequence, various courts have approached the admissibility of the test results differently, coming to different conclusions as to the reliability of the FSTs.

Despite the variety of responses, the overwhelming majority of jurisdictions allow the results of FSTs to establish probable cause to arrest and charge a motorist with driving while intoxicated or under the influence. Another conclusion universally reached by state courts is that the tests are not admissible to prove a motorist's specific blood alcohol level. Blood tests or breathalyzer tests are generally used to establish this fact. Where state courts differ is to what extent the results of FSTs, performed in real world situations, are admissible as a valid and reliable predictor of driving impairment. The majority of states, when faced with the admissibility of such evidence, have ruled that the tests are admissible as circumstantial proof of intoxication or impairment. As a result, most courts allow an officer to testify as to the performance of the motorists on the divided attention tests. With respect to the HGN test, however, a number of state courts have viewed the test as a "scientific" technique to which special evidentiary standards may apply. In that event, expert testimony explaining the correlation between alcohol consumption and nystagmus may be required before the admission of the HGN test results into evidence.

In addition to the obvious evidentiary issue, it should be noted that the FSTs, by their nature, are intended to be used in the field. This fact alone introduces potential problems with respect to both their administration and their interpretation. When an officer makes an error in performing the FST, the majority view is to assign the error to the weight to be assigned to the FST and not to its admissibility.


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