Law enforcement agencies in Texas and around the country use a standardized set of field sobriety tests developed in the 1980s by the National Highway Traffic Safety Administration to determine whether or not motorists are operating their vehicles while impaired by alcohol. Failing one of these tests cannot be used to prove intoxication in court, but it would give a police officer the probable cause they need to make a drunk driving arrest. Even the NHTSA concedes that its field sobriety tests are only reliable about 80% of the time, and there are several reasons why a sober motorist could fail.
Properly calibrated breath test devices can accurately determine a motorist’s blood alcohol concentration, but the results of a field sobriety test are based on the subjective conclusions of a police officer, which could be influenced by bias. If you fail a field sobriety test and believe that the police officer did not treat you fairly, you could challenge the results. You could also cast doubt on the test failure and ensuing DWI arrest if the police officer deviated from NHTSA procedure by asking you to perform tasks not included in the standardized test, such as reciting the alphabet from Z to A.
Motorists who suffer from certain medical conditions would find the standardized field sobriety test difficult or impossible to pass even if they had not consumed a drop of alcohol. Drivers with a condition like an inner ear infection that could affect their ability to maintain balance would find the one-leg stand extremely difficult, and a neurological injury could render the horizontal gaze nystagmus test unreliable. Completing the walk-and-turn test would probably not be challenging for a young driver, but an older motorist with arthritis could find it all but impossible to pass.
Why challenge evidence that cannot be used in court?
The results of a field sobriety test cannot be used in court to prove that a driver had a blood alcohol concentration above the legal limit, but it does provide a police officer with probable cause to arrest a motorist. If that probable cause is found to be lacking, the arrest would be invalid.