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Just How Reliable Are Field Sobriety Tests?

Assuming the machine is properly serviced and calibrated, breathalyzer tests are fairly accurate. Blood draws are even more accurate, since they go straight to the source. Most DUI charges result in a guilty plea precisely because BAC tests are so difficult to challenge. 

But what about the infamous field sobriety test?

The evaluation seems very subjective, especially since the results are dependent upon the officer's judgment. Even so, the primary function of the Standardized Field Sobriety Test is not to collective conclusive evidence but rather to establish probable cause for arrest (NHTSA).

For those of you not intimately familiar with the test, it's made up of three main components:

  1. Horizontal Gaze Nystagmus (HGN)
  2. Walk-and-Turn (WAT)
  3. One-Leg Stand (OLS)

The test for HGN, a natural jerking of the eye, is conducted with a flashlight. The officer asks the driver to follow the light with his or her eyes (keeping the head still) as it veers to the side, looking for exaggerated jerking of the eye that may indicate intoxication. Tracking objects with one's eyes also becomes difficult when intoxicated.

NHSTA research claims that this test is 88 percent effective, but may also indicate the influence of seizure medications.

The Walk-and-Turn and One-Leg Stand tests are self-explanatory and usually quite easy to perform under normal circumstances. When you're drunk? Not so much.

Taken as a whole, at least according to NHSTA (citing its source of the research), the three components are quite effective:

When the component tests of the SFST battery are combined, officers are accurate in 91 percent of cases, overall, and in 94 percent of cases if explanations for some of the false positives are accepted (Stuster and Burns, 1998). 

Still, the results of any sobriety test are merely evidence before trial, which an Illinois DUI lawyer may or may not try to challenge.