What is HGN and how is it related to DUI?

On Behalf of | Apr 30, 2024 | Criminal Defense |

A Virginia police officer must have reasonable cause to make a traffic stop and establish probable cause to make an arrest. Probable cause is evidence based on information and circumstances demonstrating the likelihood that a crime has taken place. One way to establish probable cause for a DUI arrest is to administer a horizontal gaze nystagmus (HGN) test. 

An HGN test is an eye test that police officers may administer during a traffic stop when they suspect driver intoxication, though the driver can decline to take the test. Nystagmus is an erratic shaking or jerking of the eyeball. When the eyeballs shake back and forth (rather than up and down) it is known as horizontal nystagmus. Intoxication often causes horizontal nystagmus in a person’s eyes.  

How does the HGN test work, and when can a DUI arrest occur? 

To administer an HGN test, a police officer typically asks a driver to exit the vehicle, then instructs him or her to track an object (like an ink pen or index finger) using eyes only, without any head movement. The officer will move the object back and forth, then up and down. If horizontal nystagmus occurs, it constitutes test failure and establishes probable cause to make a DUI arrest. 

Failing an HGN test during a Virginia traffic stop does not necessarily mean the court will hand down a conviction. It does not even mean that the case is guaranteed to go to trial. A lot can happen between the time of arrest and a DUI trial. It is always best to secure legal support as soon as possible after an arrest because an experienced defense attorney can help achieve the most positive outcome possible.