Experienced traffic patrol officers are familiar with a phenomenon which is sometimes referred to as "black-and-white fever". That phenomenon is simply the normal reaction of most drivers to being followed by a marked police car (painted, in many jurisdictions, black and white).
As soon as the motorist becomes aware that a police car is following him, he becomes understandably apprehensive’and focuses his attention increasingly on the rear view mirror. As the officer continues to follow, the driver becomes tense, worried, and his concentration on driving is broken: He keeps his eyes more on the mirror and less on the road ahead. Each time the driver brings his eyes back to the road, he finds that he has drifted and must correct the course of the car back to the center of the lane.
The result: weaving’and, possibly, erratic movements such as sudden increases or decreases in speed (tension can cause the foot to depress the accelerator). And, of course, these are the most commonly encountered symptoms of a drunk driver on the highway. In other words, it is the very presence of the officer which tends to create the probable cause for suspecting a DUI. And after the officer pulls the driver over, he gets out and approaches the car with the very human preconception that the driver is probably intoxicated.
And, as we know from psychological studies, we tend to see what we expect to see: normally veined eyes appear "bloodshot", normally but stressed speech sounds "slurred", normal pink complexions appear "flushed", etc. These observations are quickly followed by the notoriously subjective field sobriety tests, difficult to perform under the best of conditions. Followed in turn by an arrest for DUI.