The accuracy of the subjective tests is more/less binary; above or below legal limit. The visual acuity test adds nuance to the extent to which the subject is intoxicated in excess of the legal limit. I’m not suggesting “some beat cop” can beat you or any physician at a physical exam. I’m also not suggesting anyone rely on this test alone…when a police officer pulls someone over after 12:30 AM, detects an odor of alcohol and slurred speech, and puts the driver through the visual acuity test (which also requires them to stand in front of the officer and allows them to observe the subjects balance, respiration, as well as their response to the test itself) they’ve got a pretty good justification to make an arrest for suspicion of OUI.