What Evidence Supports Marijuana DWI Charges?
Under Texas law, a person commits a DWI offense if they operate a motor vehicle while intoxicated. “Intoxicated” means not having the normal use of mental or physical faculties due to the introduction of alcohol, a controlled substance, a drug, or any combination of those.
Marijuana is a controlled substance under both federal and Texas law. Even though some states have legalized it for recreational or medicinal use, Texas hasn't.
At The Law Office of Justin K. Hall P.C. in Rockwall, Texas, I'm dedicated to fighting for your rights. If you've been charged with a DWI, I can prepare you for all of your options and provide guidance throughout the process.
Types of Evidence Used in Marijuana DWI Cases
Unlike alcohol-related DWIs, which rely heavily on breathalyzer results, marijuana DWIs depend on a broader and often more subjective range of evidence. Prosecutors must show that the driver was impaired at the time they were behind the wheel.
Common categories of evidence include:
Observations by law enforcement officers
Performance on field sobriety tests
Blood or urine test results
Statements made by the driver
The presence of marijuana or paraphernalia in the vehicle
Testimony from Drug Recognition Experts (DREs)
Each of these has limitations and can be challenged as part of a DWI defense strategy.
Officer Observations at the Scene
When officers suspect a driver is impaired by marijuana, they rely heavily on what they see, hear, and smell during the traffic stop. Their observations often form the basis of the arrest report and may include:
Bloodshot or glassy eyes
Slurred speech
The odor of marijuana in the vehicle or on the driver
Slow or lethargic movements
Poor motor coordination
These observations are subjective and can be influenced by factors such as fatigue, allergies, or even legal substances. Officers may also describe the driver’s behavior during questioning as confused, nervous, or delayed in response.
While these details can support a charge, they rarely stand alone in court without additional evidence.
Field Sobriety Tests
Field sobriety tests (FSTs) are often administered when marijuana impairment is suspected. These tests are designed to measure physical coordination and cognitive function.
The most common tests include:
Horizontal Gaze Nystagmus (HGN)
Walk and Turn
One-Leg Stand
It’s worth ing that the HGN test is more effective for detecting alcohol impairment than marijuana. This means results from the HGN may not carry as much weight in a marijuana DWI case.
FST performance can also be affected by medical conditions, injuries, anxiety, or even uneven terrain. Because of this, test results can be disputed as part of a DWI defense.
Chemical Testing
To support a marijuana DWI charge, law enforcement may seek chemical testing. In Texas, officers can request a blood or urine sample to test for the presence of THC, the psychoactive compound in marijuana.
Unlike alcohol, THC doesn’t have a well-established threshold that defines impairment. The presence of THC in the bloodstream doesn't always mean a person is under the influence at the time of driving. THC can remain in the system for hours, days, or even weeks depending on the frequency of use.
This presents a major challenge for the prosecution. Detecting THC doesn’t automatically prove the driver was impaired. Because of this, the timing of the test becomes a critical issue.
For instance:
Active THC (delta-9 THC) may indicate recent use
Inactive metabolites show past use but not necessarily impairment
The lack of correlation between THC levels and current impairment is a central issue in many DWI defense cases.
Drug Recognition Experts (DREs)
In some cases, a Drug Recognition Expert (DRE) is called in to evaluate the driver. These are law enforcement officers trained to detect drug impairment through a 12-step protocol that includes physiological measurements, eye examinations, and interviews.
DRE evaluations may include:
Checking pupil size and reaction to light
Measuring blood pressure and pulse
Observing muscle tone
Conducting coordination tests
Reviewing statements made by the driver
While DREs receive specific training, their evaluations are still subject to human judgment. Courts often examine their qualifications and the consistency of their findings. A DWI defense attorney may challenge the accuracy and reliability of DRE testimony, especially if the protocol wasn’t followed correctly.
Statements by the Driver
During a traffic stop or arrest, anything a driver says can be used as evidence. Admissions such as “I smoked earlier today” or “I didn’t think I was too high to drive” can become part of the case.
However, statements are only admissible if obtained legally. If law enforcement fails to provide Miranda warnings or engages in coercive questioning, those statements can be excluded.
DWI defense often involves reviewing bodycam footage and arrest reports to identify procedural errors related to driver statements.
Physical Evidence in the Vehicle
Police may also rely on physical evidence found in the car. This might include:
Marijuana or marijuana residue
Pipes, rolling papers, or other paraphernalia
Edibles or vape cartridges labeled with THC content
The presence of such items doesn’t necessarily prove impairment, but it can support the claim that marijuana was used recently. Still, this kind of evidence is circumstantial and must be weighed alongside other factors.
Challenges in Proving Marijuana Impairment
Unlike alcohol, marijuana affects people differently, and the signs of impairment are less consistent. This makes marijuana DWI cases more difficult to prove beyond a reasonable doubt.
Some of the primary challenges include:
No reliable THC threshold to define impairment
THC’s long detection window, even after effects wear off
Subjectivity of officer observations and field tests
Lack of standardized testing for marijuana impairment
These factors leave room for defense strategies that highlight the weaknesses in the prosecution’s evidence.
DWI Defense Strategies in Marijuana Cases
A strong DWI defense in a marijuana case often involves questioning the reliability and sufficiency of the evidence presented. Defense strategies may include:
Challenging the accuracy of field sobriety test results
Arguing that THC levels don’t prove current impairment
Highlighting alternative explanations for observed behavior
Disputing the qualifications or conclusions of a DRE
Suppressing unlawfully obtained evidence
In some cases, independent toxicology reports or expert witnesses may be used to refute the prosecution’s claims.
The goal is to show that the evidence falls short of proving the driver lacked the normal use of mental or physical faculties due to marijuana.
Probable Cause
Before officers can request a chemical test, they must have probable cause to believe the driver is impaired. That means their observations and any field tests must raise a reasonable belief of intoxication.
If a court finds that the officer lacked probable cause, any evidence collected afterward—including blood tests or DRE evaluations—may be inadmissible. This can significantly weaken the prosecution’s case and may result in a dismissal or reduction of charges.
Importance of Legal Representation
Because marijuana DWI cases rely heavily on interpretation and judgment, having a defense attorney with experience in DWI defense is crucial. Legal counsel can evaluate every piece of evidence, challenge improper procedures, and work to protect the driver’s rights throughout the process.
In Texas, where marijuana use is still illegal, prosecutors tend to pursue these cases aggressively. A well-prepared DWI defense can make a significant difference in outcome.
Reach Out Today
Marijuana DWI charges in Texas require a different approach than alcohol-related cases. I serve clients throughout Rockwall, Dallas, Collin, Hunt, and Kaufman, Texas. Reach out to me at The Law Office of Justin K. Hall P.C. today to get started on your DWI case.