A limited field assessment of a standardized field sobriety test battery was conducted in a three-phase study. During Phase I, all police officers collected baseline data;​ in Phase II, half the officers were trained to use the field sobriety test battery;​ and during Phase III, all police officers used the field sobriety test battery. They filled out data forms for traffic stops made during the three-month study. An observer rode with each officer during each phase to collect feasibility data and anonymous breath samples from stopped drivers. The data suggested that careful application of the field sobriety test battery improved officers' estimates of blood alcohol concentration (BACs) for stopped drivers suspected of driving under the influence of alcohol. Many drivers with borderline BACs were not given sobriety tests. A standardized test battery can only be effective on this population of drivers if it is used.