There are 3 tests developed by the National Highway Safety Administration (NHTSA) that police departments generally use to determine intoxication. They are: the Horizontal Gaze Nystagmus (HGN) test; the walk and turn (W&T) test; and the one leg stand (OLS) test. The HGN test is a medical type evaluation of certain eye muscles (commonly called the pen test). The W&T and OLS are considered divided attention tests. It's very important that your attorney understand how officers are trained to give these tests. If the officer gives the instructions incorrectly it will undoubtably effect the subjects performance of the test.
According to the Federal Government SFSTs are not 100% accurate. Failing a field sobriety test does not automatically prove you guilty. The accuracy of field sobriety tests are as follows:
HGN test.....................77%
Walk and turn.............68%
One leg stand.............65%
* at .10 BAC.
Nystagmus
Nystagmus is an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye. In the impaired driving context, alcohol consumption or consumption of certain other central nervous system depressants, inhalants or phencyclidine, hinders the ability of the brain to correctly control eye muscles, therefore causing the jerk or bounce associated with nystagmus.
Horizontal Gaze Nystagmus
Horizontal Gaze Nystagmus is the most reliable of the field sobriety tests; especially when used in combination with the divided attention tests. In administering the HGN test, the officer has the suspect follow the motion of a small stimulus with the eyes only. The HGN test should always begin with the subject's left eye. Each eye is examined for three specific clues. As a person's blood alcohol concentration increases, the more likely these clues will appear.
1.
As the eye moves from side to side, does it move smoothly or does it jerk noticeably? (As people become impaired by alcohol, their eyes exhibit a lack of smooth pursuit as the move from side to side.)
2.
When the eye moves as far to the side as possible and is kept at that position for several seconds, does it jerk distinctly? (Distinct and sustained nystagmus at maximum deviation is another clue of impairment.)
3.
As the eye moves toward the side, does it start to jerk prior to a 45-degree angle? (Onset of nystagmus prior to 45-degrees is another clue of impairment)
The maximum number of clues that may appear in one eye is three. The maximum number for anyone is six. The original research shows that if four or more clues are evident, it is likely that the suspect's blood alcohol concentration is above 0.10. With four or more clues present, this test is 77 percent accurate.
Vertical Gaze Nystagmus
While Vertical Gaze Nystagmus is not a standard field sobriety test, it is usually addressed in the same battery of the SFSTs. Verticle Nystagmus is an upa nd down jerking of the eyes which occurs when the eyes gaze upward at maximum elevation. Field experience has shown that the presence of Verticle Gaze Nystagmus (VGN) has proven to be a reliable indicator of high doses of alcohol or presence of other drugs.
Horizontal gaze nystagmus (HGN) refers to a lateral or horizontal jerking when the eye gazes to the side. . As the degree of impairment becomes greater, the jerking or bouncing, i.e. the nystagmus, becomes more pronounced. This is assessed in the horizontal gaze nystagmus test. It is assumed that this is an accurate asessment of intoxication when in fact there are 47 other types of Nystagmus that can each be confused with the one "Alcohol Gaze Nystagmus" that is part of the Standard Field Sobriety Testing. Not all courts accept HGN as scientific evidence of intoxication.
Divided Attention Tests
Divided Attention tests are tests that require a subject to concentrate on two seperate things at once. The reason that these tests become important is because driving itself is a divided attention test... focusing on steering, your speed, the A/C, exits, traffic, et cetera. Alcohol and other drugs reduce a person's ability to divide their attention. So the theory is if you can demonstrate a mastery of a divided attention test then you have demonstrated your ability to drive. The tasks that the NHTSA believe exercise the same mental and physical capabilities that person needs to drive safely are information processing, short-term memory, judgment and decision making, balance, steady, sure reactions, clear vision, small muscle control, and coordination of limbs. The following are the two divided attention tests used in the Standard Field Sobriety Test battery.
Walk and Turn
The walk and turn test is probably the most recognizable test in the entire battery of field sobriety testing.In order to properly administer this test, it is important to understand what type of test this is. The physical tasks include balance and coordination while the mental tasks include comprehension of verbal instructions, processing of information and recall of memory. So, It consists of two main stages; the instructions stage and the walking stage.
In the Instructions Stage, the subject must stand with their feet in heel-to -toe position, keep their arms at their sides and listen to the instructions. The Instructions Stage divides the subject's attention betreen a balancing task (standing while maintaining the heel-to-toe position) and an information processing task (listening to and remembering instructions).
In the Walking Stage the subject takes nine heel-to-toe steps back, turn in a prescibed manner, and take nine heel-to-toe steps back, while counting the steps out loud, while watching their feet. During the turn, the subject keeps their front foot on the line, turn in the prescribed manner, and use the other foot ro take several small steps to complete the turn. The Walking Stage divided the subjects attention among a balancing task (walking heel-to-toe and turning); a small muscle control task (counting out loud); and a short-term memory task (recalling the number of steps and the turning instructions).
The Walk and Turn is judged off of several different clues a person can give off while going through the tasks. The officers use these eight clues to evaluate the performance.
1.
can't balance during instructions
2.
starts too soon
3.
stops while walking
4.
doesn't touch heel to toe
5.
steps off the line
6.
uses arms to balance
7.
loses balance on turn or turns incorrectly
8.
takes the wrong number of steps
Inability to complete the Walk and Turn test occurs when the suspect steps off the line three or more times, is in danger of falling, or cannot do the test. Original research shows if a suspect shows two or more clues, or cannot complete the test, then their BAC is likely to be above 0.10. This criterion has been shown to be accurate 68 percent of the time.
One Leg Stand
The One-Leg Stand test also has been validated through the NHTSA's research program. It is a divided attention test consisting of two stages. Instructions Stage, and Balance and Counting Stage.
In the Instructions Stage, the person must stand with their feet together, keep their arms at their sides, and listen to instructions. This will divide their attention between a balancing tast (maintaining a stance) and an information processing task (listening and remembering instructions).
In the Balance and Counting Stage, the subject must raise one of their legs to a point where their foot is approximately six inches off the ground and keep their raised foot parallel to the ground. While looking at the elevated foot, they must count out-loud in the "one thousand and..." manner until they are told to stop. This divides the subject's attention between balancing (standing on one foot) and small muscle control (counting out loud).
The individual must hold their leg up for at least 30 seconds. During the entire test, the officers are looking for four different clues.
1.
swaying while balancing
2
using arms to balance
3.
hopping
4.
putting foot down
Inability to complete the One-Leg-Stand test occurs when the suspect puts their foot down three of more times or cannot do the test. The original research shows that when the suspect produces two or more clues or is unable to complete the test then it is likely that the BAC is above 0.10. This criterion has been shown to be accurate 65 percent of the time.
HGN test.....................77%
Walk and turn.............68%
One leg stand.............65%