When a client refuses to submit to a urine test the prosecution often relies on the results of standardized field sobriety tests to prove the charge of driving under the influence of cannabis. One of the standardized field sobriety tests recognized by the National Highway Traffic & Safety Administration ("NHTSA") is the Horizontal Gaze Nystagmus ("HGN") test which checks for the involuntary jerking of the eyes. However, when defending a case of driving under the influence of cannabis defense attorneys should pay close attention to the factual findings the Illinois Supreme Court made in the case of People v. McKown, 236 Ill. 2d 278 (2010). The McKown Court noted that Central Nervous System ("CNS") depressant drugs and alcohol affect the neural centers in the brain that control eye movements. Id. at 298. The Court further noted that Horizontal Gaze Nystagmus is a physical manifestation of CNS depression. Id. at 299.
The problem with using the HGN test to prove driving under the influence of cannabis is that Courts in other jurisdictions have concluded that cannabis does not fall in the category of drugs that are considered to be Central Nervous System depressants. See Washington v. Baity, 140 Wn 2d 1, 5; 991 P. 2d 1151; 2000 Wash. LEXIS 81 (1999) (noting that cannabis was not in the category of drugs considered Central Nervous System depressants).
Since the HGN test is only useful in detecting ingestion of Central Nervous System depressants it stands to reason that the test should not be used in cases involving drugs that are not Central Nervous System depressants. Nonetheless, officers continue to assert that motorists are failing the HGN test in DUI cannabis cases. Such a conclusion belies science and legal precedent.