Many things happen when cops stop motorists on suspicion of drunk driving. The entire stop can be an unnerving experience for anyone, even those who are fully sober.
At some point during a traffic stop, a law enforcement officer may ask the driver to take a field sobriety test. These tests come in a variety of forms, but not all of them are able to accurately gauge whether a driver is intoxicated.
Some tests aren't standardized
Of all of the field sobriety tests, there is a grouping of three that are considered the standardized field sobriety test battery. The three tests -- the one-leg stand, the walk-and-turn, and the horizontal gaze nystagmus test -- are endorsed by the National Highway Traffic Safety Administration because they have a good rate of successfully showing police officers the level of a driver's impairment.
It is possible for officers to do tests that aren't in the standardized field sobriety test battery. These other tests, which can include asking a driver to count backwards, aren't standardized and have a higher rate of incorrect conclusions about intoxication.
Evaluation of the results of any field sobriety test, even the standardized version, is a subjective undertaking. Take the horizontal gaze nystagmus test for example. It requires that the officer look for a specific jerking pattern in the eyes while the person is moving the eye to look at an object. Police officers might mistake the effects of eye fatigue or the impact of oncoming lights for intoxication.
There are some instances in which a driver wouldn't be able to complete the test, even if he or she is fully sober. For example, a person with a back problem or brain injury might have trouble standing on one leg or walking in the heel-to-toe manner that the walk-and-turn test requires. A brain or spinal cord injury could even have an impact on how the person's eyes function, which could affect the results of the horizontal gaze nystagmus test.
Defense options are possible
It is possible to use factors in your field sobriety test as part of the defense plan. You can address the issues encountered during the test to fight the charges.
In some DUI cases, a defense attorney can attack the prosecution's evidence if it is based on subjective testing. In jury trials, some test-focused defenses could cause jurors to question the prosecutor's case against you, and that doubt can lead to the conclusion that the case doesn't meet the common standard of "beyond a reasonable doubt."