Whenever a police officer initiates a traffic stop and suspects the driver might be impaired by alcohol and/or drugs, they will ask the driver to perform field sobreity tests (FST's). In Washington, law enforcement is required to obtain consent from the driver and the manner they do this (or are supposed to do this) is by informing the driver that the tests are voluntary. If a driver is ever asked to perform these voluntary roadside agility tests, it is best to politely decline. In my view, these tests are designed for failure and a driver should never perform them.
The National Highway Traffic Safety Administration (NHTSA) has developed 3 "standardized" field tests for law enforcement to ulitilze in DUI detection. No, one of them is not reciting the alphabet backwards. These three tests are supposed to be administered exactly as prescrived by NHTSA and if an officer deviates from the proper way, the validity of the results are jeopardized. See NHTSA FST Manual.
The first test is the horizontal gaze nystagmus (HGN). Nystagmus is the involuntary jerking of the eyes. An officer will administer this test with either a pen or his/her finger. They begin by doing a preliminary check to make sure there is equal pupil size and equal tracking. They will also check to make sure there is no "resting" nystagmus. Then the officer will go back and forth looking for three things. First, the officer will check to see if there is smooth pursuit of the eyes (meaning the eyes go back and forth in a fluid motion). Next, the oficer will go all the way out to either side and check to see if there nystagmus (jerking) at maximum deviation. Third, the officer will go to an approximate 45 degree angle on either side to see if there is nystagmus at or prior to that angle. After checking for those three things, the officer usually performs a vertical nystagmus check to see if the eyes are jerking while going up and down (vertical nystagmus is very uncommon and either means a high level of intoxication or the person is on PCP).
The second test is the Walk and Turn test. The officer will have a person stand on either a painted or imaginary line and instruct the driver to put his/her right foot on the line with the left in front and with heel touching toe. The officer will instruct the driver to maintain that position until instructed to begin the test (the driver is actually being tested to see if they can keep that unnatural position and not begin too soon). After the instructions, the driver is supposed to walk 9 steps forward. While walking, the driver is to touch heel to toe on all steps, never step off line, never stop walking, keep their arms to their sides, count each step out loud and watch their feet. When the driver reaches step 9, they are supposed to make a turn by taking a series of small steps counter-clockwise while keeping their lead foot on the line. Then the driver is supposed to take 9 steps back while doing everything as required on the first 9 steps.
The third test is the One Leg Stand test. This test is performed by the driver picking either the left or right foot approximately 6 inches off the ground and with their toes pointing upwards. The driver is supposed to count "one-one thousand, two-one thousand....." until instructed to stop. During the test, the driver is not to sway, to use their arms for balance, to drop their foot or to hop.
Law enforcement and prosecutors rely on studies that claim a very high probability of impairment when a person fails these tests. There are many reasons a person might not perform well on these tests and the reason is not impairment. For example, a person might issues with their legs, back, knees or feet that hinders their performance. Often times, a driver will be on the shoulder of a busy street or highway with vehicles passing only feet away that might impact their performance. The area of the tests might be at a slope or severe weather could be present. It is common for a person to have extreme anxiety or fear being investigated for a DUI and that too could impact their coordination.
There are additional tests law enforcement occassionally use during their DUI investigation including a backwards count (counting from 53 to 36), reciting the alphabet without singing, a finger to nose test and/or a balance test where a person tilts their head backwards and closes their eyes and estimates the passage of 30 seconds. Also, in Washington, it is very common for the last test to be a preliminary or portable breath test (PBT). Law enforcement is required to advise the driver that the test is voluntary and does not replace the evidentiary value of any other test (meaning it does not replace the breath test at the station).
Once again, these tests are voluntary, and if a Washington driver is ever asked to perform them, it is in their best interest to politely decline.