Field sobriety testing
Field sobriety tests are a battery of tests used by police officers to determine if a person suspected of impaired driving is intoxicated with alcohol or drugs. The US National Highway Traffic Safety Administration (NHTSA) began research in 1975 on how to test suspects for impaired driving. The NHTSA developed a series of tests that police officers could use when evaluating suspected impaired drivers.  By 1981, officers in the United States began using the organization's battery of standardized sobriety tests to help make decisions about whether to arrest suspected impaired drivers. The tests were designed to indicate intoxication associated with a blood alcohol concentration (BAC) of 0.10%.
After some US states began lowering their BAC limits to 0.08%, a study was done to see if the battery could be used to detect BACs at or above 0.08% and above and below 0.04%. This was done to deal with the changes in the laws that led to lower legal BAC limits across the US.
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.
Traffic accidents are predominantly caused by driving under the influence; for people in Europe between the age of 15 and 29, DUI is one of the main causes of mortality. According to the National Highway Traffic Safety Administration alcohol-related crashes cause approximately $37 billion in damages annually.DUI and alcohol-related crashes produce an estimated $45 billion in damages every year.
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.
Horizontal Gaze Nystagmus Test (HGN)
The first test that is typically administered is the Horizontal Gaze Nystagmus test. 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 cues in each eye, which gives six cues for this test. The cues 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 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 cues or clues; a .05% – .059% at 2 - 4 cues or clues; a 0.03% – 0.049% at 0 - 4 cues or clues and a < 0.03% at 0 – 2 cues or clues.  Officers 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.
Walk & Turn Test (WAT)
The second test that is usually administered is the Walk and Turn Test, or WAT Test. This test is a measures the suspect's ability to have divided attention. It also measures balance. It requires the suspect to walk from heel-to-toe on a line. The suspect is also given other instructions.
One Leg Stand Test (OLS)
The other standardized test is the One Leg Stand (OLS). The OLS test requires the suspect to stand on one leg for 30 seconds and also measures balance, coordination, and similar to the WAT test, divides the suspect's attention. The officer is looking for any of the four possible clues: Sways while balancing, uses arms for balance, hopping and puts their foot down.
To determine impairment in countries such as Australia, a simple breath or urine test is often taken. If police suspect that a driver is under the influence of a substance such as alcohol, then the driver will undergo a breath test. If over the legal limit of 0.05g per 100 milliltres of blood, then a second breath test will be taken and used as evidence against the driver when charged with the offence. If a person is suspected to be under the influence of an illegal drug, they will be required to supply a urine sample. If the urine sample is positive, then the urine is sent for more testing to determine the exact drug taken (confirmation of being illegal or prescribed). A similar process to being over the legal BAC level is undertaken using the evidence to penalise the user.
- "Standardized Field Sobriety Test (SFST) Validated at BACS Below 0.10 Percent". US Department of Transportation, National Highway Traffic Safety Administration. Retrieved 10 March 2016. This article incorporates text from this source, which is in the public domain.
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- "Impaired Driving | National Highway Traffic Safety Administration (NHTSA)". www.nhtsa.gov. Retrieved 2015-09-29.
- "Between bail, court, insurance and attorney's fees, the true cost of DUI is more like $10,000". ABCActionNews.com.
- "The Robustness of the Horizontal Gaze Nystagmus Test" (September, 2007).
- "Blood alcohol concentration". DrugInfo. Australian Drug Foundation. 2014. Retrieved 2015-04-16.
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