How Field Sobriety Tests Work — and Why They’re Not Always Reliable

When a police officer in Massachusetts suspects that a driver may be under the influence of alcohol or drugs, one of the first steps they take is to conduct field sobriety tests (FSTs). These roadside tests are designed to assess coordination, balance, and mental focus to help determine whether someone is impaired. While they’re widely used across the country, these tests are far from perfect. In fact, mistakes, medical conditions, or even weather conditions can lead to false results. If you’ve been arrested after taking a field sobriety test, consulting a skilled Massachusetts DUI defense lawyer can help you understand your options and challenge the validity of the officer’s conclusions.

The Purpose of Field Sobriety Tests

Field sobriety tests give law enforcement officers a quick, visual method to evaluate potential impairment before making an arrest. These tests help establish probable cause—a legal threshold that allows police to arrest someone and request further chemical testing, such as a breathalyzer or blood test.

However, it’s important to understand that these tests do not measure blood alcohol content (BAC). Instead, they rely on the officer’s subjective interpretation of your physical and mental performance—an interpretation that can be influenced by many external factors.

The Three Standardized Field Sobriety Tests

The National Highway Traffic Safety Administration (NHTSA) has approved three standardized tests for use across the U.S.:

Horizontal Gaze Nystagmus (HGN) Test – The officer moves a pen or small object in front of your eyes while watching for involuntary jerking movements. Excessive jerking is thought to indicate intoxication.

Walk-and-Turn Test – You are asked to take nine heel-to-toe steps along a straight line, turn, and walk back while counting your steps aloud.

One-Leg Stand Test – You must stand on one leg while counting for about 30 seconds without swaying, hopping, or using your arms for balance.

While these tests sound simple, even a sober person may have difficulty performing them correctly—especially under stressful roadside conditions.

Why Field Sobriety Tests Aren’t Always Reliable

Many factors unrelated to alcohol or drugs can affect test results. For instance:

Environmental conditions – Uneven pavement, poor lighting, traffic noise, or bad weather can impact balance and focus.

Health and age – Certain medical conditions, inner ear problems, vision issues, or age-related balance difficulties can mimic “signs of impairment.”

Footwear and clothing – High heels, boots, or tight clothing can make physical coordination tests more difficult.

Officer bias or misinterpretation – Fatigue, stress, or preconceived assumptions may influence how officers judge performance.

Scientific studies have also shown that even under ideal conditions, standardized FSTs can be inaccurate in up to one-third of cases.

A massachusetts dui defense lawyer can cross-examine the officer about these factors, expose inconsistencies in their observations, and challenge whether probable cause truly existed for the arrest.

How an Attorney Can Challenge Field Sobriety Test Evidence

Field sobriety tests are often used to justify an arrest—but they’re not conclusive proof of impairment. Your attorney can use several strategies to undermine the prosecution’s reliance on FST results, including:

Questioning the officer’s training and certification to conduct the tests.

Highlighting improper instructions or deviations from standardized NHTSA procedures.

Introducing medical or expert testimony explaining why your performance may have been affected by non-alcohol factors.

Pointing out lack of video evidence, if no dashcam or bodycam footage was recorded.

When done effectively, these challenges can weaken the state’s case and even lead to dismissal of the charges.

What to Remember If You’re Asked to Perform FSTs

In Massachusetts, you are not legally required to perform field sobriety tests. Refusing them cannot be used as direct evidence of guilt in court. However, the officer may still arrest you based on other observations, such as odor of alcohol or erratic driving.

If you do take the tests and are later charged, don’t assume failure means conviction. An attorney can review the evidence, identify procedural flaws, and protect your rights from the very beginning.

Conclusion

Field sobriety tests are designed to help officers detect impaired drivers—but they’re not scientific, and they’re far from foolproof. Environmental conditions, medical issues, and human error can all play a role in false “failures.”

If you’ve been charged with OUI after taking these tests, remember: the results are not the final word. With the help of a knowledgeable attorney, you can challenge the evidence, protect your license, and fight for a fair outcome.