Field sobriety tests (FSTs), also referred to as standardized field sobriety tests (SFSTs), are a battery of tests used by police officers to determine if a person suspected of impaired driving is intoxicated with alcohol or other drugs. FSTs (and SFSTs) are primarily used in the United States and Canada, to meet "probable cause for arrest" requirements (or the equivalent in either country), necessary to sustain an alcohol-impaired driving (DWI or DUI) conviction based on a chemical blood alcohol test.
Background Impaired driving
Impaired driving, referred to among other terms as driving under the influence (DUI) or driving while intoxicated (DWI), is the crime of driving a motor vehicle while impaired by alcohol or other drugs (including recreational drugs and those prescribed by physicians), to a level that renders the driver incapable of operating a motor vehicle safely. People who receive multiple DUI offenses are often people struggling with alcoholism or alcohol dependence.
Driving under the influence is a significant cause of traffic accidents. The National Highway Traffic Safety Administration found that, between 2014–2023, approximately 11,000 Americans were killed each year in alcohol-related accidents, representing nearly 30% of traffic fatalities in the United States. The Centers for Disease Control and Prevention estimated the monetary cost of alcohol-related traffic fatalities in the United States (including medical costs and the actuarial value of lives lost) as more than $120 billion in 2020. DUI is similarly one of the main causes of mortality for people in Europe aged 15–29.Alonso, Pastor, Montoro & Esteban, 2015
With alcohol, a drunk driver's level of intoxication is typically determined by a measurement of blood alcohol content or BAC; but this can also be expressed as a breath test measurement, often referred to as a BrAC. A BAC or BrAC measurement in excess of the specific threshold level, such as 0.08%, defines the criminal offense with no need to prove impairment. In some jurisdictions, there is an aggravated category of the offense at a higher BAC level, such as 0.12%, 0.15% or 0.20%. In many jurisdictions, police officers can conduct field tests of suspects to look for signs of intoxication. The US state of Colorado has a maximum blood content of THC for drivers who have consumed cannabis.
In most countries, driver's licence suspensions, fines and prison sentences for DUI offenders are used as a deterrent. Anyone who is convicted of driving while under the influence of alcohol or other drugs can be heavily fined and/or given a prison sentence. In some jurisdictions, impaired drivers who injure or kill another person while driving may face heavier penalties. In addition, many countries have prevention campaigns that use advertising to make people aware of the danger of driving while impaired and the potential fines and criminal charges, discourage impaired driving, and encourage drivers to take taxis or public transport home after using alcohol or drugs. In some jurisdictions, the bar that served an impaired driver may face civil liability. In some countries, non-profit advocacy organizations, a well-known example being Mothers Against Drunk Driving (MADD), run their own publicity campaigns against drunk driving.
History In the United States, drunk driving laws were enacted as early as 1906.An Act Defining Motor Vehicles and Providing for the Registration of Same, 1906 N.J. Laws ch. 113, §§ 19, 35, p. 177, 186 (April 12, 1906) However, prior to the early 1980s, drunk driving was regarded as a "folk crime", routinely committed by both good and bad citizens alike, and the crime was rarely prosecuted successfully. The US National Highway Traffic Safety Administration (NHTSA) was formed in 1970. In the early 1970s, Marcelline Burns was writing her Ph.D. thesis in psychology, in California, and she was guided to the idea of researching sobriety tests by Herb Moskowitz, her psychology thesis review professor. The NHTSA had issued several requests for proposals (RFPs). Burns submitted a grant proposal in response to an RFP focused on creating standardized pre-arrest tools for police officers to use to decide which drivers were impaired. The NHTSA funded her proposal, and Burns and Moskowitz began the research in 1975.
Burns began by conducting a literature survey. No researchers in the US, including her, had any significant background in roadside testing. Her survey did find research by Penttilä, Tenhu, and Kataja, who did a retrospective study of the 15 tests then in use by Finnish law enforcement. Burns also examined officer training manuals and went on ride-alongs with the DUI or special enforcement teams of several police departments. Burns observed numerous tests which had been devised, adopted, and modified by officers, with no records of origin or validation of the tests. Burns also observed inconsistent practices, such as some departments not using tests at all. Burns compiled a list of around 15 to 20 tests. She conducted a series of pilot studies, statistical analyses, and practical considerations, and reduced this list to three "recommended" tests: the one-leg stand, walk-and-turn, and alcohol gaze nystagmus.M. Burns & H. Moskowitz, Psychophysical Tests for DWI Arrest, NHTSA report DOT HS 802 424, Jun-1977, (Contract No. DOT-HS-5-01242) (alt. link) By 1981, officers in the United States began using this battery of standardized sobriety tests to help make decisions about whether to arrest suspected impaired drivers.V. Tharp, M. Burns, H. Moskowitz, Development and field test of psychophysical tests for DWI arrest, NHTSA report DOT HS-805 864, Mar-1981 (Contract No. DOT-HS-8-01970) (alt. link) As the Los Angeles Police Department was among the first to use these field tests, the law enforcement community sometimes referred to them as the "California tests". The tests were used in real-world conditions and reported as being able to determine intoxication above the then-effective blood alcohol concentration (BAC) limit of .10 grams per deciliter (g/dL) of blood. After some US states began lowering their legal BAC limits to .08 g/dL, other studies reported that the battery could also be used to detect BACs at or above .08 g/dL and above and below .04 g/dL.J. Stuster, M. Burns, Validation of the standardized field sobriety test battery at BACs below 0.10 percent, NHTSA report DOT HS-808 839, Aug-1998 (Contract No. DTNH22-95-C-05192) (alt. link, photocopy)
Use and purpose
The National Highway Traffic Safety Administration (NHTSA) has developed a model system for managing standardized field sobriety test (SFST) training. They have published several training manuals associated with FSTs. FSTs and SFSTs are promoted as "used to determine whether a subject is impaired", but FST tests are widely regarded having, as their primary purpose, establishing tangible evidence of "probable cause for arrest" ("reasonable grounds" in Canada). A secondary purpose is to provide supporting corroborative tangible evidence for use against the suspect for use at trial. Probable cause is necessary under US law (4th Amendment) to sustain an arrest and invocation of the implied consent law.
Similar considerations apply under the Canadian requirement to establish "reasonable grounds" for making an approved instrument demand, by establishing that there is reasonable and probable cause which lies at the "point where credibly-based probability replaces suspicion".Hunter et al. v. Southam Inc at p. 114, 115"Improperly or Illegally Obtained Evidence: The Exclusionary Evidence Rule in Canada". . International Centre for Criminal Law Reform and Criminal Justice Policy. December 2005. It is likely that, if FSTs are being used, some equivalent to probable cause is necessary to sustain a conviction based on a demand for a chemical test.
While the primary purpose of FSTs is to document probable cause or the equivalent, in some jurisdictions, FST performance can be introduced as corroborating evidence of impairment."Admissibility of Horizontal Gaze Nystagmus Evidence". . American Prosecutors Research Institute. May 2003.
Testing During a traffic stop, upon suspicion of DUI, the officer will administer one or more field sobriety tests. FSTs are considered "divided attention tests" that test the suspect's ability to perform the type of mental and physical multitasking that is required to operate an automobile. According to the NHTSA, a suspect does not "pass" or "fail" a field sobriety test, but rather the police determine whether "clues" are observed during the test. Nevertheless, some of the literature will still include comments that a suspect "fails" one or more of these tests.
Standardized field sobriety tests The three tests chosen to constitute the standardized field sobriety tests (SFSTs), which have been validated by NHTSA, are: The horizontal gaze nystagmus test, which involves following an object with the eyes (such as a pen) to determine characteristic eye movement reaction. The walk-and-turn test (heel-to-toe in a straight line). This test is designed to measure a person's ability to follow directions and remember a series of steps while dividing attention between physical and mental tasks. The one-leg-stand test
Most law enforcement agencies use this three-test battery on all DUI traffic stops.
On the subject of standardization, Burns stated that the tests must be administered in a standardized way in order to have meaning as objective measures. That is, the instructions must be given properly, and the critical elements of the test must be preserved, for the scientific studies to meaningfully validate the results of the tests. Up through 2009, NHTSA manuals stated: (emphasis and capitalization as originally supplied)
Horizontal gaze nystagmus test (HGN) The first test that is typically administered is the horizontal gaze nystagmus or HGN test, which is administered by the police officer checking the test subject's eyes. During this test, the officer looks for involuntary jerking of the suspect's eyes as they gaze toward the side. The officer checks for three clues in each eye, which gives six clues for this test. The clues are: lack of smooth pursuit of the eyes, distinct and sustained nystagmus at the eyes' maximum deviation and nystagmus starting before the eyes reach 45 degrees.
While the original research indicated that 6 out of 6 clues (or cues) meant that a person was more likely above 0.08% at the time of the test, subsequent research conducted by the NHTSA has indicated that a "Hit" occurred when the number of reported signs for a given BAC fell within the range: a > 0.06% at 4–6 clues; a 0.05 – 0.059% at 2–4 clues; a 0.03 – 0.049% at 0–4 clues and a < 0.03% at 0–2 cues or clues."The Robustness of the Horizontal Gaze Nystagmus Test". . September 2007. The police may also then check for Vertical Gaze Nystagmus, which is used to test for high blood alcohol levels and/or the presence of certain drugs.
While the purpose is obtaining probable cause support for an arrest and possibly screening, in some jurisdictions, the HGN test may be used as corroborating evidence at the trial stage. US jurisdictions differ on whether trial use of the HGN test requires that an expert establish a reliable foundation, as required under the Daubert standard.See also Frye standard for test results "generally accepted as reliable".
Walk and turn test (WAT)