Field sobriety testing

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.

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.

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.

In the United States, drunk driving laws were enacted as early as 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. 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. 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.

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.