When pulled over by a police officer, a person may be requested to comply with ‘field sobriety tests’ if the police officer suspects intoxication. The results of these tests offer ‘probable cause’ for an officer to arrest a driver on suspicion of intoxication.

The person must complete simple physical or cognitive tests to decide sobriety. The most common requests are to walk a straight line heel-to-toe and twist at the end to return, or stand with feet together and tip the head backwards, or stand on one leg. The officer may move a penlight in front of the person’s eyes and ask that the person follow the light with the eyes. This is the “horizontal gaze nystagmus test,” which is somewhat new to field sobriety tests, and is fairly accurate if administered accurately. The police officer is guessing the angle at which the eye twitches; if this happens at less than 45 degrees, it is a mark of high blood-alcohol concentration.

If through field sobriety tests a driver is assumed of being under the influence of drugs or alcohol, some form of chemical analysis, such as breath test, blood, or urine testing is compulsory. Rebuttal carries a possible sentence such as suspension of driving privileges. A Breathalyzer unit may be used, which is a small hand-held tool that measures the driver’s BAC (blood-alcohol concentration). This may be done roadside, at the detainment center, or at a hospital. Blood or urine tests can also be done at a hospital.

See also Implied Consent Laws.