Field tests are less reliable than chemical tests
The best indicator of possible driving impairment appears to be a reliable blood alcohol test. Chemical tests can objectively and accurately determine the blood alcohol concentration (BAC) of the driver within the margin of error of the testing method and instrument. From this analysis, an opinion can be given on driving impairment based on scientific studies.
Field sobriety (non-chemical) tests are less reliable, and often are a poor indicator of driving impairment or sobriety. One of the reasons for this is the inherent error in subjective evaluations of an individual whose normal performance is unknown.
Nevertheless, field sobriety tests are still used as a tool to:
- Enable an officer to develop probable cause for the stop or arrest.
- Assist in establishing possible physical or mental impairment at a time close to driving.
- Establish the general causal relationship between alcohol and driving behavior.
Types of tests
A wide variety of field sobriety tests have been used, including:
- Asking distracting/interrupting questions.
- Finger to nose.
- Finger count test.
- Reciting or writing the alphabet, with signature and time and date.
- Counting backwards from 100.
- Tracing (a paper-and-pencil exercise).
- Romberg balance test.
- Romberg balance test combined with finger to nose.
- Nystagmus (“HGN”).
- One leg stand.
- Line walk.
- Hand pat.
- Picking up coins.
- Recitation of date and time.
- Standing heel to toe.
Standardized and non-standardized tests
The United States Department of Transportation, National Highway Traffic Safety Administration (NHTSA) studied several field sobriety tests starting in 1975, with the results being published in 1977.
Three field sobriety tests were determined to have a good correlation with impairment when used together (provided that they are administered in strict compliance with the testing protocols). The three tests have come to be referred to as Standardized Field Sobriety Tests or SFSTs. The three SFSTs the research validated were:
In actual practice police officers regularly offer a battery of tests that include exercises that have not been validated.
Not all tests have been studied
Some of the tests have been explored for their degree of reliability. For example, there is considerable data on the nystagmus (HGN) test and the Romberg test, since these tests have clinical origins.
However, not all of the various field exercises have been studied for reliability, or have been validated as meaningful indicators of alcohol impairment. Some tests are used even without proper validation or any scientific study to support their use by officers. Some tests merely look for changes in divided attention ability.