If an officer suspects that a driver is driving under the influence, they
will administer what is known as a field sobriety test. A field sobriety test
is a preliminary test conducted by the officer on-scene to give the officer a
better indication of the driver's condition. The United States National Highway
Traffic Safety Administration has scientifically determined that three field
sobriety tests are statistically reliable in detecting impaired drivers. These
three standardized tests are the "Walk and Turn" test, in which the
law enforcement officer observes the person walking heel-to-toe on a straight
like, the "One-leg Stand", in which the law enforcement officer
observes the person standing and balancing (or attempting to) on one leg, and
"Horizontal Gaze Nystagmus" in which the law enforcement officer
observes the discrete movements of a person's eyes when tracking a stimulus
across their field of vision. Other tests may include having the driver recite
parts of the alphabet as a whole or portions of the alphabet.
If arrested, the driver is brought to the police station and given one or
more chemical tests: breath, urine, and/or blood. Breath test results are
usually available immediately and are sometimes given before the actual arrest
takes place. Urine and blood samples are sent to a lab to determine the blood
alcohol content. In some jurisdictions, refusing to take a breathalyzer test is
an offense in itself, often creating an automatic assumption of guilt under the
law. Chemical tests are better at determining the driver's blood alcohol
content than they are at estimating the level of impairment, but their accuracy
is disputed by some. Tests can only determine the blood alcohol content at the
time the test is taken, which sometimes can be either higher or lower than when
the vehicle was actually operated..