For DWI investigations, what are standardized field sobriety tests and are they reliable for showing impairment by alcohol or marijuana? Do they predict Blood Alcohol Content or levels of THC in the blood?
When you’re pulled over on suspicion of driving while intoxicated in New Mexico, if officers can develop Reasonable Suspicion of DWI, they earn the right to ask you to get out of the car and ask you to perform standardized field sobriety tests (SFSTs) check out Did the police violate my rights when they started my DWI investigation? – Max Pines Law. These roadside evaluations have become a cornerstone of DWI enforcement, but how reliable are they really? Can they accurately predict your blood alcohol concentration (BAC) or detect marijuana impairment?
What Are Standardized Field Sobriety Tests?
First, what are SFSTs? Field sobriety tests are physical and cognitive exercises designed to help law enforcement officers determine whether a driver is impaired by alcohol or drugs. They are supposed to measure divided attention which is the capacity to focus on one thing while performing other acts. Divided attention is what you use to drive because you focus on the road while physically operating your car. The National Highway Traffic Safety Administration (NHTSA) has validated three specific tests that make up the standardized battery:
- Horizontal Gaze Nystagmus (HGN) Test: The officer asks you to follow an object (like a pen or flashlight) with your eyes while keeping your head still. They’re looking for involuntary jerking movements in your eyes, which can indicate alcohol impairment.
- Walk-and-Turn Test: You’re instructed to take nine heel-to-toe steps in a straight line, turn around using small steps, and return in the same manner. The officer watches for signs like losing balance, stepping off the line, or inability to follow instructions.
- One-Leg Stand Test: You must stand on one leg with the other foot raised about six inches off the ground while counting aloud for 30 seconds. Officers look for swaying, hopping, or putting your foot down.
These tests are designed to evaluate divided attention; the tasks strain your ability to perform both mental and physical tasks simultaneously, which alcohol can impair.
There are alternative field sobriety tests which can also be administered in the case of an injury or disability. These tests involve counting, reciting the alphabet and using your fingers in special prescribed ways.
These tests are important for the police to obtain probable cause for an arrest. To understand the limits of how police can act in an investigation, please read this blog: https://maxpineslaw.com/did-the-police-violate-my-rights-when-they-started-my-dwi-investigation/
The Reliability Question: What the Research Really Shows
NHTSA’s Claims vs. Scientific Reality
NHTSA often touts impressive accuracy rates for these tests. According to their own studies, when all three standard tests are administered together, they claim an overall accuracy rate of approximately 91% for detecting drivers with a BAC of 0.08% or higher. 197439NCJRS.pdf Individual test accuracy rates are reported as:
- Horizontal Gaze Nystagmus: 77-88% accurate
- Walk-and-Turn: 68-79% accurate
- One-Leg Stand: 65-83% accurate
These sound impressive and they certainly have some good use. However, these numbers don’t tell the complete story.
The False Positive Problem
The most concerning issue with field sobriety tests is their high false positive rate—meaning they incorrectly identify sober people as impaired. A 2014 study found that 26% of completely sober individuals failed standardized field sobriety tests. Validation of the standardized field sobriety test battery at 0.08% blood alcohol concentration – PubMed
Statistical Manipulation and Study Limitations
The most significant problems with how NHTSA presents these statistics is that the accuracy rates are “heavily weighted by the large number of subjects with very high BAC levels” Statistical evaluation of standardized field sobriety tests – PubMed
Do Field Sobriety Tests Predict BAC Levels?
The simple answer is yes, but not accurately enough to bank on in a close call. The tests are “likely to be mainly useful in identifying subjects with a BAC substantially greater than 0.08%” Statistical evaluation of standardized field sobriety tests – PubMed Statistical analysis shows “the accuracy of the SFSTs depends on the BAC level and is much poorer than that indicated” in NHTSA studies. Id. Importantly, these tests don’t measure impairment—they attempt to predict BAC levels, which may or may not correlate with actual driving ability.
Field Sobriety Tests and Marijuana: The Fundamental Problem
Given the legality of marijuana today, the lack of reliable testing to detect impairment by weed is troubling. Field sobriety tests were validated specifically for alcohol detection, not marijuana impairment. This creates significant problems:
- Unlike alcohol, THC blood concentrations do not correlate with driving performance
- There’s no simple “limit” for marijuana like the 0.08% BAC standard for alcohol
- The tests cannot reliably distinguish between recent use and impairment
Recent Research Findings
A comprehensive 2023 study published in JAMA Psychiatry (Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial – PMC) examined how well field sobriety tests detect marijuana impairment:
Key Findings:
- Officers classified 81.0% of THC users and 49.2% of placebo users as impaired
- 49.2% of completely sober participants were incorrectly identified as impaired
- Officers suspected 99.2% of those who “failed” the tests had used THC, regardless of actual substance use
- The placebo group failed to complete a median of 8 out of the FST components as instructed
The study’s conclusion was telling: “FSTs, absent other indicators, may be insufficient to denote THC-specific impairment in drivers”.
Factors That Can Cause False Positives
Field sobriety tests can be affected by numerous factors unrelated to alcohol or drug use:
Medical Conditions
- Vertigo and balance disorders
- Knee, back, and joint problems
- Neurological conditions
- Inner ear infections
- Eye disorders or injuries
- Fatigue or illness
- Injuries or trauma due to a car crash
- Age and weight factors
Environmental Factors
- Uneven or slippery surfaces
- Poor lighting conditions
- Inclement weather
- Flashing police lights causing distraction
- Traffic noise and distractions
Psychological Factors
- Anxiety and nervousness during traffic stops
- Stress and fear
- Language barriers
- Confusion about instructions
Your Rights in New Mexico
In New Mexico, field sobriety tests are voluntary. You have the right to refuse these tests but in that case, the prosecution may argue that you refused them because you believed you would do poorly because you knew you were impaired; lawyers call this consciousness of guilt and it’s a powerful argument at trial. Officers may still request a breathalyzer test, which is not optional under New Mexico’s implied consent law, Refusing chemical testing (breath or blood) results in automatic license suspension: What is “Implied Consent” to give a breath test? – Max Pines Law
Challenging Field Sobriety Test Evidence
At Max Pines Law, we understand that field sobriety tests are far from foolproof. We regularly challenge these tests in court based on:
Improper Administration
- Officers failing to follow standardized procedures
- Incorrect instructions or demonstrations
- Improper scoring of test results
- Unsuitable testing conditions
Medical and Physical Factors
- Documenting relevant medical conditions
- Physical limitations that affect performance
- Medications that can impact test results
- Age, weight, and injury considerations
Scientific Limitations
- High false positive rates
- Lack of correlation with actual impairment
- Subjectivity in officer interpretation
- Inadequate validation for drug detection
Drinking and driving or driving while high is dangerous and should always be avoided. DWI is a serious indicator of a substance problem and we advocate for smart and effective treatment. But the problem is that an innocent person can be painted guilty if the evidence is allowed to be presented without a thorough vetting. And innocent or not, the Constitution of the United States puts the burden on the government to prove guilt beyond a reasonable doubt. So a vigorous and learned defense informed by science is crucial.
