The modern field sobriety test has as much as a 21% margin of error
A field sobriety test is one of the ways police officers verify intoxication when pulling over motorists. It typically involves motor and balance evaluations like standing on one leg for a time. However, data from the NHTSA suggests the testing is flawed compared to blood alcohol concentration (BAC) measuring devices like a breathalyzer.
NHTSA data suggests every part of a field sobriety test could be off by over 15%
The National Highway Traffic Safety Administration (NHTSA) evaluates all things safety on American motorways. After all, it’s the same agency behind the star safety ratings for vehicle crashworthiness. However, the NHTSA also publishes data and findings based on safety measures like Standardized Field Sobriety Testing (SFST).
Now, a field sobriety test is one of the first tools police officers use when evaluating a driver for intoxication. The SFSTs are typically broken up into three parts: the horizontal gaze nystagmus test (HGN), the walk-and-turn (WAT) test, and the one-leg stand (OLS) test. While those tests might not seem like the most complicated maneuvers, they have a pretty sobering margin of error.
For example, the walk-and-turn test involves nine steps, heel-to-toe. Then the subject turns on one foot and returns in the same manner. According to the NHTSA, the WAT was 79% accurate in supporting “arrest decisions at above or below 0.08 [BAC]” for California law enforcement. That leaves the balance-oriented test with a shocking 21% inaccuracy rate.
In addition to the WAT, the one-leg stand test has an 83% accuracy rate. Like the WAT, the OLS test is balance-oriented. As a result, individuals with poor balance, equilibrium issues, or even sinus infections can fail the tests. On the other hand, the horizontal gaze nystagmus test’s 88% accuracy presents a much more reliable means of detecting inebriation.
Predictably, physical testing is much less effective compared to alcohol measurement devices like a breathalyzer. For instance, while more expensive than instructing a driver to perform tasks, a breath test device is quicker, more effective for drivers with high tolerances, and better for crash scenes where suspects might be injured and unable to perform an SFST.
