The drunk driving case rests heavily upon the subjective opinions of the arresting officer — the abilities of that officer to correctly assess DUI symptoms of intoxication: observations of driving, personal symptoms (slurred speech, flushed face, etc.), answers to questions, performance on field sobriety tests.
Just how expert is the average police officer at judging levels of intoxication in a DUI case?
To answer this question, researchers at Rutger University's Alcohol Behavior Research Laboratory conducted a series of experiments. For purposes of comparison with officers, two groups of non-police witnesses were first tested. In one, 49 lay social drinkers sat in a room as various subjects were brought in one at a time for observation and questioning. Each subject had either consumed varying amounts of alcohol or had consumed nothing; each had been given tests for blood-alcohol levels. Each in turn answered questions from the lay witnesses until all were finished, then got up and left. Each of the 49 witnesses was then asked to judge each subject's state of sobriety or intoxication.
The researchers' conclusion: "The assumption that social drinkers would prove to be accurate judges was not confirmed."
In the second group, 12 bartenders were tested in the setting of a large cocktail lounge. Again, the researchers found that "the bartenders correctly rated a target in only one of four instances".
The researchers then turned to 30 experienced DUI officers from various New Jersey law enforcement agencies. Separated into two groups, the first group of 15 officers were tested under laboratory conditions similar to those in the experiment involving lay social drinkers. The second group of 15 were tested under circumstances commonly encountered in a drunk driving traffic stop — at night, with the subject behind the wheel of a car, who is then asked to step out and conduct a series of DUI field sobriety tests. Results?
When police observers in the laboratory conditions were compared to social drinkers who had experienced an identical procedure, no difference in rating accuracy was found…Officers in the arrest analogue were somewhat more accurate than their colleagues in the laboratory condition but not significantly so.
The scientists concluded that "the results of the three experiments described here are not reassuring. All three of the subject groups studied — social drinkers, bartenders and police officers — correctly judged target levels of intoxication only 25 percent of the time." Langenbrucher and Nathan, "Psychology, Public Policy and the Evidence for Alcohol Intoxication", American Psychologist 1070 (Sept. 1983).