Police use many different tests during the motor vehicle stop to help them determine whether they believe a suspect is under the influence of liquor or drugs. While some of these tests fall under the category of “Standardized Field Sobriety Tests,” some of the tests are non-standardized and have not been studied to determine the how reliably they can predict intoxication.

Standardized Field Sobriety Testing

In the mid-1970s, the National Highway Transportation Safety Administration (NHTSA) began sponsoring scientific research studies to determine which of the roadside field sobriety tests already used by police officers in the investigation of DWI were the most accurate. It was determined that three of these tests, when administered in a standardized manner, were an accurate and reliable battery of tests for determining blood alcohol content over .10%. (It should be noted that the current legal limit is .08%.) Those three tests were:

  1. Horizontal Gaze Nystagmus (HGN) – 77% accurate, by itself
  2. Walk-and-Turn (WAT) – 68% accurate, by itself
  3. One-Leg Stand (OLS) – 65% accurate, by itself.

When combined, the accuracy of the HGN and WAT together was determined to be 80%. Field validation studies were conducted in the late 1990s in Colorado, Florida, and San Diego.

1).Horizontal Gaze Nystagmus

Nystagmus is the involuntary jerking of the eyes. Alcohol and some drugs cause Horizontal Gaze Nystagmus. When administering this SFST, an officer first checks for equal pupil size, resting nystagmus, and equal tracking (which means that the eyes can follow and object together). The officer will then ask the suspect to follow a stimulus (generally a pen or a finger) using only his or her eyes while the officer moves the stimulus back and forth. The clues the officer is looking for are threefold: (1) lack of smooth pursuit, meaning a jerking or bouncing of the eyes as they follow a smoothly moving stimulus; (2) distinct and sustained nystagmus at maximum deviation, meaning that a nystagmus is present when the eye is held at maximum deviation for a minimum of four seconds; and (3) onset of nystagmus prior to 45 degrees, meaning that the eyes begin jerking prior to 45 degrees.

It must be noted that while the HGN test is utilized by nearly all police agencies and that the New Jersey State Police even give out certifications to officers trained in this procedure, its reliability has never been the subject of a scientific reliability hearing in any New Jersey court and thus, the test itself is not admissible as an indicia of guilt.


During the WAT test, an officer will ask the suspect to stand in a specified starting position while giving the test instructions. The officer will then instruct the suspect to take 9 heel-to-toe steps forward in a straight line, while holding her arms at her sides, looking at her feet, and counting her steps out loud. She is then instructed to execute a turn and walk 9 heel-to-toe steps back in the opposite direction, again holding her arms at her sides, looking at her feet and counting her steps out loud, to complete the test. Some of the clues of impairment that the officer will be looking for are as follows:

  • Suspect cannot keep balance while listening to the instructions
  • Suspect starts the test before the instructions are finished
  • Suspect pauses while walking
  • Suspect does not touch heel-to-toe
  • Suspect steps off the line
  • Suspect uses arms for balance
  • Suspect turns improperly
  • Suspect completes the wrong number of steps

According to the research, when a suspect exhibits two or more clues or fails to complete the test, the suspects BAC will be over .10% approximately 68% of the time.

This test is supposed to be given on a flat, dry surface that is clear of any debris, in any area where there is sufficient room to complete the test. Additionally, suspects who are over 65 years of age or who had back, leg or inner ear problems had difficulty completing the test.

3).One-Leg Stand

This test, like the WAT, tests a suspect’s balance. During the OLS, the suspect will be told by the officer to stand with her feet together and arms at her sides. She will then be told to lift the leg of her choosing approximately 6 inches off the ground and hold it there while counting out loud by thousands (one-thousand-one, one-thousand-two, etc.), keeping her arms at her sides while looking at her raised foot. She will be told to continue counting until told to stop (while the officer measures the passage of 30 seconds). She should also be told that if she puts her foot down at any point, she should pick it back up and begin counting again where she left off.

The clues of impairment that an officer is looking for during this test are:

  • Suspect sways while balancing
  • Suspect uses arms for balance
  • Hopping
  • Suspect puts foot down

According to the research, a person with a BAC of .10% can maintain their balance for up to 25 seconds, but rarely for the entire 30 seconds, which is why the officer is timing the passage of 30 seconds. Officers are trained that the presence of two or more of these clues, or failure to complete the test, indicates a BAC of .10% or higher about 65% of the time.

This test requires the same testing conditions as the WAT.

Other Field Sobriety Tests

As noted above, there are several other field sobriety tests which are used by police officers in New Jersey. Some of these are:

  • Alphabet test: In this test, the suspect is asked to recite the alphabet from A to Z without singing it. In some variations of the test, the suspect will be asked to only recite the alphabet from a specific letter to another specific letter (D to T, for example). The officer will note whether the suspect’s speech is slurred and whether letters are repeated or omitted.
  • Counting backwards: In this test, the officer will ask the suspect to count backwards out loud from some number to some other number, and will note whether the suspect repeats numbers or skips numbers, or whether the suspect can perform the test at all.
  • Sway test: In this test, the suspect is asked to lean his head backwards with his eyes closed and estimate the passage of 30 seconds. The officer is looking at whether the suspect is swaying while leaning his head back, and whether he correctly estimates the passage of 30 seconds.
  • Portable Breath Test (PBT): A PBT is a device used by some police departments at the side of the road to help them determine whether someone’s BAC is over the legal limit. Many police departments tell suspects that the PBT only demonstrates the presence of alcohol in a person’s system, but that is not true. The PBT has a screen which shows a number to the officer. Most officers will not reveal this number to the suspect. PBTs are not admissible in court because their reliability has never been demonstrated, and PBTs are never calibrated by the police departments, so their evidential value is suspect anyway.