An officer makes the decision to arrest a citizen when he believes there is “probable cause” for the arrest. This is a subjective belief on the part of the officer and to survive a challenge in court, it must be shown that there were sufficient facts to predicate the conclusion that the driver was under the influence. The rest is usually based upon observations of driving, field sobriety tests, a “portable breath test,” statements made by the driver, and general observations of the drivers demeanor including speech, balance, and odor of alcohol.
A DUI arrest typically occurs after the officer has seen something that triggers him to make the initial detention of the vehicle. This may be an infraction such as speeding, weaving, or something innocuous such as a burned-out taillight. (In many cases, an officer will stop a vehicle for a minor traffic infraction at night when the same officer would not stop the same vehicle for the same infraction in the middle of the day.)