More than half of fatally injured drivers were driving while they had alcohol or other drugs in their body. In total, driving under the influence (DUI) or driving while intoxicated (DWI) cost the US and Canada's economy $150 Billion annually.
Not only alcohol consumption can cause impaired driving, but also the use of recreational drugs such as cannabis products such as marijuana or hashish, as well as prescription drugs such as opioids and benzodiazepines may have a serious effect.
Several studies show drugs, including alcohol, have a profound effect on human eye movement. This is why one of the main field sobriety tests being performed by authorities in DUI cases is the evaluation of eyes which generally includes tests of equal eye size, convergence, nystagmus, and smooth pursuit.
Driving Under the Influence
In 2014 there were 1.1 million DUI violations in the USA
Every day, 28 people in the USA die in motor vechile crahes that involve an imparied driver. (NHTSA)
DUI's cost the USA $132 billion dollars a year?
The Horizontal Gaze Nystagmus (HGN) test is a standard and the most scientifically reliable field sobriety test that police administer to find whether a driver is under the influence of drug or alcohol with the reliability rate of 77%. The officer instructs the driver to follow with his/her eyes a stimulus to the left and the right while tries to record the angle at which the pupil starts to exhibit nystagmus, which is an involuntary jerking of the eye. An early onset of this jerky movement at or before a 45-degree angle is a clue associated with DUI.
However, the reliability of the HGN test can be affected by errors that may happen in the way the test is being performed. For example, estimation of a 45-degree angle is difficult, and improper positioning may tarnish the conclusion. The officer should place the stimulus within 12-15 inches from the eyes, and his/her eyes should be above suspected driver eye-level while administration of the test. Lighting conditions may vary significantly when HGN test is performed on different subjects. This can limit drivers' eyes reaction to the light stimuli, and limit officers' ability in spotting abnormalities.