Standardized field sobriety tests are often believed to effectively predict actual blood alcohol concentrations. However, those studies have been called into question.
Several years ago the National Highway Traffic Safety Administration (NHTSA) paid a research institute to conduct studies to determine which of the field sobriety tests used by police were most effective. The results of the test would help them develop standardized tests.
The study found that a test of walk-and-turn, one-leg-stand and nystagmus provided a strong correlation with breath test results. One study reported a 91% correlation with these tests and officers effectively being able to predict blood alcohol levels at over .08%.
However a subsequent article pointed out that the subjects of these studies were often already heavily intoxicated. Which would make obvious a high BAC level regardless of sobriety test performance. The study also did not eliminate other indicators of drunkenness that officers may have observed such as erratic driving, slurred speech, odor of alcohol, flushed face, and admissions of having consumed alcohol.
One huge defect with the study was that officers had portable breath testing devices. So officers already had good estimates of actual blood alcohol concentrations.
Furthermore, officers relying on field sobriety tests were far more likely to overestimate BACs than to underestimate. This was especially true for suspects with very low BACs.
In cases were persons were at or under the legal limit officers were not able to effectively gauge BAC based on field sobriety tests. This is particularly dangerous in terms of false charges because officers can easily determine when people with high BACs are driving, but it shows that field sobriety tests can not accurately point to when a person is just below the legal limit.