The One Legged Stand is the second Standardized Field Sobriety Test (SFST). In the One-Leg Stand test, the driver is instructed to stand with one foot approximately six inches off the ground and count aloud by thousands (One thousand-one, one thousand-two, etc.) until told to put the foot down.
OLS Test Instructions
The officer times the subject for 30 seconds. The officer looks for four indicators of impairment, including swaying while balancing, using arms to balance, hopping to maintain balance, and putting the foot down. NHTSA research indicates that 83 percent of individuals who exhibit two or more such indicators in the performance of the test will have a BAC of 0.08 of greater.
The problem with the one-legged-stand is that it is often performed on uneven pavement. In addition, drivers sometimes have unsuitable shoes - high heels or sandals - that may make it more difficult for them to perform the One Legged Stand properly.
Test Interpretation and Limitations
Moreover, unlike the HGN, which is (somewhat) scientific, the One Legged Stand test is very subjective. For instance, an officer may mark down that the person was swaying (which is one clue of impairment) even though "swaying" is in the eye of the beholder. In addition, certain people may be naturally unbalanced, uncoordinated or out of shape.
Since the SFST and the One-Legged-Stand test are one-size-fits all tests, a football player with excellent coordination or a ballerina may do really well on this test, while someone who is not in good shape or in good practice might fail miserably, even though the football player may be drunk and the out-of-shape person may be perfectly sober.