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Assessing Sobriety / Intoxication

A number of formal studies have found it quite difficult to judge mild and moderate levels of intoxication in people who are strangers. A major problem is the enormous differences among people, both in their normal capabilities and their response to alcohol. Some people may appear significantly impaired at blood alcohol concentrations (BAC) well below the legal standard (.08%), whereas other people (often heavy drinkers) may show no noticeable impairment, even when examined by physicians in emergency rooms, at BAC levels of .30% and even .40%.

When a driver is stopped by a police officer, especially late at night, the officer will look for signs of alcohol consumption and impairment. The National Highway Traffic Safety Administration (NHTSA) lists a number of potential clues, including rumpled clothing, odor of alcohol, red or glassy eyes, poor coordination, and loud or slurred speech.

Researchers have also examined indications of intoxication in an attempt to make more accurate judgments. Several general conclusions can be drawn: While individual clues (e.g., red eyes, slurred speech) occur at different BACs on the average, there is wide individual variation, and 2) some people do not show the expected clues. The following chart shows the average BAC of many signs of intoxication (blue bar), along with range (red bar) in which different people show the sign. The error bars show where 68% of people fall. If the error bars reflected a 95% confidence interval, they would be twice as wide. In other words, different people can show dramatically different BAC levels at a given sign.


(C) Steve Rubenzer, 2009. Data is from nearly one thousand drinkers in natural environments, as reported by McKnight A. J., Langston, E. A., Marques, P. R., & Tippetts, A. S. (1997). Estimating blood alcohol from observable signs. Accident Analysis and Prevention,29(2), 247-255.


The difficulty in judging intoxication, combined with the high number of alcohol-related traffic fatalities, led NHTSA to develop a set of standardized sobriety tests. These were introduced in the mid 1980's and consisted of three tests: Horizontal Gaze Nystagmus (HGN), Walk and Turn, and One Leg Stand. HGN involves assessing the driver's eyes for jerkiness (nystagmus) when following a moving object or when held to the side. All three tests are standardized and should be administered, scored, and interpreted the same way each time. Often, however, they are not. This is one major defense in DWI/DUI cases.

However, even if they are conducted correctly, there are many limitations to the tests in determining if someone is intoxicated. A partial listing includes:

  • Lack of norms for how sober people of different ages perform
  • Use of BAC, rather than some index of impairment, as a criterion
  • Limited investigation of the effects of fatigue, anxiety or fear on performance
  • No research on how performance on the tests relates to driving impairment
  • Considerable variability between different officers in administration and scoring of the tests
  • Failure to report data, including unflattering false positive rates

While all these factors are important, the last factor in particular will lead police officers and prosecutors to overvalue the tests. NHTSA training materials do not clearly report false positive rates, and claim that medical conditions that may lead to false arrests are rare, particularly among people that drive. No data or citations are given for this important claim.

FSA certification indicates that your police officer, prosecutor, defense attorney, and judge understands field sobriety testing from a scientific perspective, not just the NHTSA/police science perspective. While NHTSA training helps all parties comprehend the limitations of the evidence when the officer makes major mistakes performing the SFSTs, it provides little assistance when they don't. In these cases, the defense needs to be prepared to challenge the conclusions that are drawn from signs of intoxication and sobriety tests.