Field Sobriety Tests and DREs Are Not Science. They Are Guesswork—And Bad Guesswork at That.
In the fight to protect our communities from intoxicated drivers, the science behind how we determine impairment must be reliable. Unfortunately, when it comes to marijuana and DWI, it simply isn’t. Two recent and highly credible scientific articles—one by legal scholar William McNichol and another by leading researchers Thomas Marcotte and Robert Fitzgerald—lay bare what many of us DWI defense lawyers have known for years: Field Sobriety Tests (FSTs) and Drug Recognition Expert (DRE) evaluations are fundamentally flawed tools for detecting THC-related impairment.
If you’re facing a DWI involving marijuana, HB 393 (Bentley’s Law), or a Child Support on Intoxication Manslaughter Case, the stakes could not be higher. These flawed protocols can cost someone their freedom, their license, their livelihood—and in some cases, their future as a parent.
As a Galveston DWI Lawyer with decades of courtroom experience and a law professor who teaches Texas criminal procedure, let me explain exactly why this pseudoscience is being exposed—and what it means for you.
The Zendeh Del & Associates, PLLC Team
FSTs and DRE Protocols: Scientifically Invalid and Dangerous in Court
McNichol’s article calls it like it is: the methods police use to detect marijuana impairment are “police science”—not real science. The Drug Recognition Expert protocol, which is widely used across the country, was developed not by scientists or physicians, but by police officers. And despite being dressed up in pseudo-medical jargon, the techniques are laughable when viewed through a scientific lens.
For example, one supposed “indicator” of THC impairment is having the munchies. That’s right—an open bag of chips in your car could become evidence. Officers are trained to squeeze your limbs and decide whether you feel “flaccid” or “rigid.” That’s not medicine. It’s theater.
More importantly, DREs are stunningly unreliable. In a controlled study cited by McNichol, DRE evaluations were wrong nearly half the time—producing false positives and negatives 45.5% of the time. That’s barely better than flipping a coin.
Another study showed that two sets of trained DREs disagreed with each other up to 31% of the time about whether a suspect was impaired. And remember: these are the very people prosecutors call “experts.”
Placebo Studies Show Shocking False Positive Rates
The problem doesn’t stop there. According to Marcotte & Fitzgerald (2025), in a randomized, double-blind study of marijuana impairment using trained police officers, nearly half (48.5%) of sober, placebo participants were wrongly identified as impaired by law enforcement.
Let me repeat that: Nearly one out of every two sober people were accused of being high.
That kind of error rate isn’t just unacceptable—it’s dangerous. It turns innocent people into criminals, and it lets flawed science masquerade as justice in our courts.
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Why This Matters for HB 393, Bentley’s Law, and Child Support Cases
Recent legislative changes in Texas—especially HB 393 (also known as Bentley’s Law)—add a whole new layer of severity to DWI prosecutions. Under this law, individuals convicted of Intoxication Manslaughter can be ordered to pay child support for the children of the person killed in the crash. On its face, the intent is noble. But what happens when someone is convicted based on junk science?
Imagine being falsely labeled “impaired,” wrongly convicted in a DWI case involving marijuana, and then forced to pay child support for a tragedy you didn’t cause—all because the state relied on a police officer’s gut feeling rather than real science.
Marijuana ≠ Alcohol: Why The Comparison Fails
The system for detecting alcohol impairment—using blood alcohol concentration (BAC)—has at least some scientific grounding. BAC levels correlate reasonably well with impairing effects. But that logic falls apart with marijuana. THC is fat-soluble, not water-soluble like alcohol. It lingers in the body for days or even weeks after use. Someone can test “positive” for THC and be stone-cold sober.
Even more importantly, there’s no established threshold that equates a certain blood level of THC with impairment. Some chronic users can function normally with high levels in their system, while a novice might be impaired at much lower levels. The law just hasn’t caught up.
And yet, DREs continue to use subjective evaluations—like blood pressure, pupil size, muscle tone, and “paranoia”—to support impairment claims, often without any corroborating toxicology data.
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The Solution? Real Science—Not Roadside Guesswork
Both McNichol and Marcotte call for a scientifically validated method to detect actual marijuana impairment. We’re not there yet. But the science community is working toward it, with research into new psychomotor testing tools, advanced cognitive screens, and validated biomarkers.
Until then, relying on outdated, inaccurate, and often biased tools like FSTs and DRE evaluations is not just lazy—it’s unjust.
If You’re Accused of DWI Involving Marijuana, You Need a Real Defense
Here in Galveston, DWI cases are prosecuted aggressively. Prosecutors often bring in DREs, toxicologists, and sometimes “experts” with questionable credentials. Don’t let this be your undoing.
At Zendeh Del & Associates, PLLC, we’ve built the largest criminal defense firm in Galveston on a reputation for exposing weak cases and flawed science. As a DWI Super Lawyer, I’ve fought these cases across Texas and won, often by holding the state’s so-called “experts” to real scientific standards.
We understand the law. We know how to cross-examine DREs. We’ve read the studies. We know the tactics prosecutors use. And we don’t back down.
Whether you’re facing a first-time marijuana DWI, an intoxication manslaughter allegation involving HB 393 and Bentley’s Law, or a case with potential child support implications—we can help.
Call us now at (409) 740-1111. We answer 24/7. Don’t let pseudoscience destroy your future.
Let us fight for you like your future depends on it—because it does.
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