An officer needs probable cause that a person is under the influence of alcohol before he or she can request them to submit to a breath test. Phase III of DUI Detection or the Pre-Arrest Screening Phase is where an officer administers three scientifically validated field sobriety tests to see if he has probable cause for an arrest.
There are only three Standardized Field Sobriety Tests (FSTs) scientifically validated: (1) Horizontal Gaze Nystagmus (HGN) Test; (2) Walk and Turn Test; and (3) the One Legged Stand Test.
An officer will use all three tests to build a case against the subject. The HGN test is often the most difficult test to explain to a jury and will not show well on the cruiser’s dash-cam. Nystagumas is an involuntary jerking of the eye and the officer will ask the subject to perform several different procedures in order to see if the subject has nystagmus. Check out this video for more information: https://www.youtube.com/watch?v=vjPR5WvYCx0.
The officer will then ask the subject to perform to physical tests: (1) the walk and turn, and (2) the one legged stand. Defendants often say “they don’t look bad,” but officers are trained to identify slight clues of impairment. For example, during the walk and turn test, officers note that the subject (a) improperly turns, (b) uses arms to balance, (c) does not touch heel to toe, (d) stops while walking, and (e) starts before the instructions are finished. Similarly, officers make note of slight clues of impairment during the one-legged stand test, including: (a) suspect sways while balancing and (b) uses arms for balance.
If an officer finds that the subject failed two of three tests SFSTs he will likely ask the subject to submit to a breath test.