If you are pulled over and suspected of drinking and driving or driving while intoxicated, the police officer may request you to submit to a field sobriety test or a preliminary breath test.
The officer will have already gone through two levels of a detection process before requesting a field sobriety test. Their detection process will be called upon for review in the case of a hearing. The detection process begins when they suspect a driver of drinking and driving, and it concludes when they have made an arrest or released the driver.
Understanding the detection process officers are supposed to follow in the case of a suspected drinking and driving situation can help you in understanding what they will be looking for and what actions you may have committed to lead them to suspecting intoxication and impairment. Keep in mind that the detection process is merely a set of proposed steps to follow and an officer can use their best judgement in determining an intoxication level and jump directly to requesting a field sobriety test or preliminary breath test, i.e. in the case of an accident or gross impairment.
Officers observe drivers for swerving, weaving, sporadic braking, and other forms of erratic driving under their first level of detection: the habits of the driver. If you were pulled over during an unrelated traffic stop (for example, speeding) the officer may notice other indications of driving while intoxicated, which is their second phase of detection: face-to-face. These include heavy odors indicating drugs or alcohol or the attempt to cover the scent of drugs or alcohol, paraphernalia or alcoholic beverages, soiled clothing indicating intoxicated behavior, slurred speech, bloodshot eyes, aggressive language, inconsistent responses, and other signs indicating intoxication. Police officers may attempt to divide the attention of the driver during face-to-face detection by asking simultaneous questions to determine their level of impairment. The officer will complete both levels of detection (driving observation and face to face observation) before they conclude whether requesting a field sobriety test is necessary, which is their third level of detection.