Field sobriety tests (FST) are in no way a scientific method of determining intoxication. Conducting multiple tests and administering them correctly greatly increases the chances of reliability. They are still by no means 100% accurate. Each test is riddled with its own imperfections. There are three recognized field sobriety tests; walk and turn, stand on one leg, and horizontal gaze nystagmus (HGN).
All of the tests expect a level of physical health that many drivers do not comprise. They share that common weakness, but for different reasons. Let’s examine each test and fully understand their strengths and weaknesses.
Horizontal Gaze Nystagmus
This test requires the suspect to stare at an object (often the police officer’s pen). The object is moved back and forth while the officer detects a lack of smooth pursuit in the driver’s eyes. In other words, the eyes don’t move smoothly. While it is true alcohol and other illegal substances cause this occurrence, it’s also caused by an onslaught of other conditions. Multiple sclerosis, stoke, natural occurrence, and sedatives are all known causes of nystagmus, which is nothing more than an involuntary jerking of the eyes.
Walk and Turn
The walk and turn tests require the suspect to take nine steps, heel to toe. They are then instructed by the police officer to turn and walk back while staying in a straight line and maintaining balance. Other than intoxication, these are causes for poor results:
• Excess weight/obesity
• Over 65 years of age
• Awkward shoes
• Any physical condition affecting balance
• Limited site in one eye
Stand on One Leg
The driver is instructed to stand on one leg for 30 seconds while maintaining balance. If any hopping, wobbling, or arm rising is observed, the suspect may be arrested. Reasons for test failure include many all the way from the walk and turn test to an uneven or wet/slippery surface to poor lighting.
The walk and turn and stand on one leg test should be demonstrated by the officer if the suspect is having trouble understanding the request.
The HGN test is arguable the most accurate with regard to determining an excess level of blood alcohol content (BAC). The others are approximately only accurate just over half the time. The probability of accuracy increases with the number of tests given. FSTs are one tool for law enforcement to detect intoxication. Used alone they are highly ineffective and easy for a strong defense attorney to poke holes through.