Well, it's a pretty good indication of impairment, but it only works about two-thirds of the time. Even so, officials consider those odds good enough to make it one of the three tests that make up the Standardized Field Sobriety Test battery administered by officers here in California and across the country. In addition to the one-leg stand, the horizontal gaze nystagmus and the walk and turn tests round out the three tests that make up the SFST.
The primary issue with all three of these tests is that they require a subjective opinion from the officer administering them. To show how much the opinion of law enforcement officers can vary, several of them watched the same video of someone taking these tests. Some believed that the person in the video was impaired and some did not.