Officer's often have bias towards seeing indications of a crime when they just simply are not there. They want to see them and so they do. They want red eyes to indicate intoxication so they don't allow for any other possibility.
Confirmation bias is defined as: "the tendency to interpret new evidence as confirmation of one's existing beliefs or theories." In criminal investigations this is the tendency of law enforcement to interpret new information in a way that conforms with their pre-existing beliefs about the case. Confirmation bias will causes investigators to ignore evidence that that conflicts with or challenges their pre-existing theory of the case, to discount the importance of new information unless it conforms with their beliefs, fail to properly investigate leads that they believe are unnecessary, eliminate suspects without proper investigation because they like the individual or believe someone else is responsible, and accuse suspects without sufficient evidence because they dislike the individual or have a "gut feeling" about their guilt.
Conformation bias occurs to some degree in nearly every criminal investigation. An experienced DUI defense attorney will work to expose any police bias in regard to your case.
Take a look at my blog article on this area for more information: Confirmation Bias.