Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent VALIDATION OF THE STANDARDIZED FIELD SOBRIETY TEST

This study evaluated the accuracy of the Standardized Field Sobriety Test (SFST) Battery to assist officers in making arrest decisions for DWI at blood alcohol concentrations (BACs) below 0.10 percent. NHTSA's SFST battery was validated at 0.10 percent BAC in 1981. The trend to reduce statutory DWI limits to 0.08 percent BAC prompted this research project. The research was composed of several project tasks, including planning, site-selection, training, data entry, and data analysis, in addition to the actual conduct of a major field study. The City of San Diego, California, was selected as the site. Seven officers of the San Diego Police Department's alcohol enforcement unit were trained in the administration and modified scoring of NHTSA's SFST battery (i.e., Horizontal Gaze Nystagmus-HGN, Walk and Turn, and One Leg Stand). SFST scoring was adjusted: the observation of four HGN clues indicated a BAC >_0.08 percent (rather than four clues indicating a BAC >_0.10 percent), and the observation of two HGN clues indicated a BAC >_0.04 percent. During routine patrols, the participating officers followed study procedures in administering SFSTs and completing a data collection form for each test administered. The officers' final step in each case was the administration of an evidentiary breath alcohol test. Data analysis found the SFSTs to be extremely accurate in discriminating between BACs above and below 0.08 percent. The mean estimated and measured BACs of the 297 motorists tested were 0.117 and 0.122 , respectively; the difference between the means (0.005 percent BAC) is very small and operationally irrelevant. Further, analyses found the HGN test to be the most predictive of the three components of the SFST battery (r=0.65), however a higher correlation was obtained when the results of all three tests were combined (r=0.69). Decision analyses found that officers' estimates of whether a motorist's BAC was above or below 0.08 or 0. 04 percent were extremely accurate. Estimates at the 0.08 level were accurate in 91 percent of the cases, or as high as 94 percent if explanations for some of the false positives are accepted. Officers' estimates of whether a motorist's BAC was above 0.04 percent but lower than 0.08 percent were accurate in 94 percent of the decisions to arrest and in 80 percent of cases overall. Also, the officers and prosecutors who were interviewed about the SFSTs found the test battery to be acceptable for field use to establish probable cause for DWI …