After pulling a vehicle over, an officer may develop a suspicion that the driver has had too much to drink. To confirm this, the officer may request that the driver perform field sobriety tests.
According to AAA, the tests do have a high level of success rate at detecting impairment, but they are far from accurate.
Balance and coordination
Alcohol can affect coordination, so the officer will watch for signs that the driver is off balance while walking heel-to-toe along a straight line. Pausing, putting the arms out to regain balance or stepping off the line can indicate impairment.
The one-leg stand test requires the driver to maintain balance on one foot while counting aloud. Swaying, hopping or putting the arms out to regain balance are signs the driver has had too much to drink.
Many other things can affect balance, though. An inner ear condition, back problems, medication and other health issues may cause the same symptoms as drunkenness during these tests.
Concentration and reaction
As blood alcohol content increases, the ability to concentrate and react normally decreases. At several points while administering the tests, the officer will watch to see if the driver can listen with enough focus to follow the directions to the letter. Failing to wait for the officer to stop talking or being unable to remember the instructions long enough to carry them out indicates the driver may be drunk. Losing track of numbers while counting aloud may also signal impairment.
Nervousness, mental health issues and many other factors may affect concentration and reaction, as well. As part of the proper administration of the test, the officer should ask whether the driver has any issues that may affect the ability to complete the activities.