A much-anticipated Illinois Supreme Court opinion held that the HGN, one of three standardized field sobriety tests in Illinois DUI cases, was a valid indicator of alcohol impaired driving. What this means in practical terms that the state will not have to provide expert testimony that the HGN is some evidence of alcohol impairment every time it wishes to use the HGN in an Illinois DUI prosecution.
While this decision is helpful to the state, the Supreme Court placed limits on its use. First of all, the state cannot present the test as proof of intoxication. It will be treated like the other standardized field sobriety tests, or like an open beer can in your car, or an odor of alcohol on your breath, or your admission that you had been drinking. None of these things, standing alone, proves that you are drunk. They just tend to make it more likely that you had consumed alcohol.
The Court also held that any police officer who administers the HGN must be trained according to the student manual of the National Traffic Safety Highway Administration (NHTSA).