The NHTSA manual breaks an encounter into three steps or phases. Phase I is observing the vehicle in motion. Officers are to look for traffic code violations, equipment violations or unsafe driving. If he observes any of these then he has reasonable suspicion to stop the driver and moves to the next phase.
Phase II is the initial observation of the driver. Officers are taught to rely on their senses to determine if there is probable cause to suspect the driver is intoxicated. Officers look for signs of intoxication: bloodshot or glassy eyes, fumbling for paperwork and trouble maintaining balance. They use their sense of smell to detect the odor of an alcoholic beverage. The officer listens for slurred or thick speech, for admissions of alcohol consumption or for drivers who have trouble answering questions. If the officer detects any of these he moves to the final phase.
Phase III is pre-arrest screening. In Phase III, the officer administers and interprets a series of field sobriety tests, what I prefer to call police coordination exercises. The officer may also ask the driver to blow into a portable breath test machine.
Only after all three phases have been completed has the officer gathered enough evidence to make an arrest decision.
But, as those of us who litigate these matters are well aware, the decision to arrest tends to be made upon the smell of an alcoholic beverage and a driver's admission that he has been drinking. How else to explain an arrest when the driver refuses to perform the coordination exercises -- or when the driver performs well on them?
An effective method of attacking a DWI arrest is to challenge the objectivity of the arresting officer -- in other words, to demonstrate that the arresting officer made up his mind to arrest the driver before he had a chance to observe all available clues and signs.
If you've been charged with driving while intoxicated, contact my office.