How Accurate Are Field Sobriety Tests In California


One of the first things an officer will ask a driver to do when he or she suspects the driver is under the influence is to perform field sobriety tests or FSTs for short. In California, drivers are not required to submit to these tests but the officer is not required to inform the driver that the tests are not required. Consequently, most drivers will submit to the tests, either thinking they must or because they are afraid to say no, or because they believe they can pass the FSTs and be on their merry way (that’s a false assumption). FSTs are almost designed to be failed—even many entirely sober people will “fail” the tests—and the only real purpose of FSTs is to give the officer a reason to arrest the driver and to provide the prosecution with evidence.

But how accurate are FSTs anyway? The Standardized Field Sobriety Test, which is endorsed by the National Highway Traffic and Safety Administration (NHTSA), and used in California, consists of three separate tests: The Horizontal Gaze Nystagmus (HGN), the Walk and Turn (WAT). and the One-Leg Stand (OLS). The HGN measures eye movements in which the officer is looking for three indicators that suggest alcohol impairment. The WAT is the one most people associate with roadside drunk driving tests: the driver is asked to walk heel-to-toe in a straight line and to return in the same manner. The OLS requires the driver to stand with one foot approximately six inches off the ground and to count to 30. Any swaying, loss of balance, or inability to stay on one foot for 30 seconds may indicate alcohol impairment. According to the NHTSA, these three tests accurately detect alcohol impairment in 91% of all cases. According to the NHTSA, non-standard FSTs are unreliable.

It is important to remember that whether a person “passes” a field sobriety test or not is a subjective determination by the officer. Despite the NHTSA’s claim, many independent studies have been conducted that question the accuracy and reliability of FSTs. Indeed, even the NHTSA’s studies and results have been criticized. The critics maintain that the study designs were seriously flawed. Apparently, a number of the study subjects had blood alcohol limits far over the legal limit so that their poor performance on the FSTs was a given. Even more astounding is that the officers who were part of the study were asked to estimate the subjects’ blood alcohol content after viewing the subjects’ performance of the FSTs but they also were allowed to consider the subject’s breathalyzer results. That is not an objective study!

As you can imagine, a perfectly sober person could fail to walk in a straight line heel-to-toe or stand on one foot for 30 seconds, especially if the person is older, or weak, or simply not very coordinated. In fact, studies have been conducted where completely sober subjects have “failed” FSTs.

If you are ever asked by an officer to perform FSTs, your best strategy is to politely decline, even if you are 100% sober.

William Weinberg has almost 25 years of experience in defending drivers charged with driving under the influence. You may consult with him about your DUI matter by contacting him at his Irvine office at 949-474-8008 or emailing him at