Since the late 1970s, police officers throughout the United States have been using the three “standardized” field sobriety tests (FSTs) – the one-leg stand, walk-and-turn, and horizontal gaze nystagmus – to determine whether a driver is too impaired or intoxicated to operate his or her vehicle. The results are typically the basis of an officer’s probable cause for a DUI or DWI arrest.
However, how reliable are these tests? While law enforcement officials and prosecutors depend on FST results for gauging a driver’s level of intoxication, many criminal defense lawyers feel these tests are often inaccurate.
National Highway Traffic Safety Administration (NHTSA) Studies
The NHTSA conducted a variety of studies where police officers gave FSTs to people with known BACs. The purpose was to determine how well law enforcement officials could properly identify an individual with a BAC of .1% or higher.
The following are the results of the NHTSA-sponsored tests:
- The accuracy rate of the one-leg stand is 65%
- The accuracy rate of the walk-and-turn is 68%
- The accuracy rate of the horizontal gaze nystagmus is 77%
- The accuracy rate when all three were used together is 82%
Unfortunately for people subjected to these tests in the real world, the NHTSA knows that officer’s error rate for determining BAC level was only 47% in the study.
Factors That Impact Reliability
There are many factors which can impact FST reliability. For example, an officer not following NHTSA guidelines when administering an FST can result in inaccuracies.
In addition, some individuals have mental and physical disabilities or injuries which may hinder their performance on these tests. An elderly person may have difficulties standing on one leg, someone with glaucoma can have difficulties performing in the horizontal gaze nystagmus.
Criminal defense attorneys typically challenge FST results based on these types of errors. On the other hand, prosecutors might attempt to minimize circumstances that made how an arresting officer administered or interpreted an FST that wasn’t perfect.