How do cops know if you're driving high?

[VonBlakely]

Police determine if you're driving under the influence of marijuana through a multi-step process beginning with observed traffic violations and culminating in biological evidence. The initial reasonable suspicion for a stop often stems from erratic driving. Upon interaction, officers assess physical signs, conduct standardized field sobriety tests (SFSTs), and may utilize a Drug Recognition Expert (DRE) for a detailed evaluation, with the final proof typically being a blood test showing THC levels above a state's legal limit.

The investigation is systematic. After a lawful traffic stop, the officer engages the driver. Key indicators include the odor of raw or burnt cannabis, visible smoke or paraphernalia, and the driver's physical state. Common physical signs include red, glassy, or bloodshot eyes; dry mouth; slowed reaction time; impaired coordination; and inappropriate laughter or talkativeness. The driver's own admission of recent use is also considered significant evidence.

To objectify impairment, officers administer Standardized Field Sobriety Tests (SFSTs). While designed for alcohol, certain clues are sensitive to drug impairment. The Horizontal Gaze Nystagmus (HGN) test, which checks for involuntary eye jerking, is less reliable for cannabis alone but can indicate polydrug use. The Walk-and-Turn and One-Leg Stand tests assess balance, coordination, and the ability to follow instructions—all of which can be compromised by THC.

If SFSTs suggest impairment but a breath alcohol test is low or zero, a Drug Recognition Expert (DRE) may be called. This officer follows a strict 12-step protocol, including a toxicological interview, vital signs measurement, and detailed physical examinations (e.g., checking pupil size under different lights, muscle tone). The DRE forms an opinion on the category of drug causing impairment.

The most definitive evidence comes from biological samples. Unlike alcohol breathalyzers, there is no widely accepted roadside breath test for THC. Officers may use oral fluid (saliva) screening devices in some jurisdictions for preliminary detection. However, post-arrest blood testing is the primary method for obtaining quantifiable, admissible evidence of recent cannabis use. Legal limits for delta-9-THC in blood vary by state, typically ranging from 1 to 5 nanograms per milliliter.

The following table outlines the types of evidence and their roles in building a case:

Evidence Category	Specific Examples	Purpose & Role in Investigation
Observational Evidence	Odor of cannabis, red eyes, slow speech, admission of use, found paraphernalia.	Establishes initial probable cause for further testing and arrest.
Behavioral Evidence	Poor performance on SFSTs (balance, coordination issues); DRE evaluation conclusions.	Objectively demonstrates driver impairment and helps identify drug category.
Chemical Evidence	Positive oral fluid screen; Blood test confirming THC concentration over legal limit.	Provides scientific, quantitative proof of recent drug use for prosecution.

It's crucial to understand that probable cause for arrest is based on the totality of this evidence, not a single factor. A blood test alone, without evidence of actual impairment, may not be sufficient for a conviction in all jurisdictions. The process is designed to document both the presence of drugs and the resulting functional impairment of driving skills.

[Dylan]

As a defense attorney who handles these cases, I see the process from the other side. Cops are trained to build a chain of evidence. The car might swerve, giving them reason to pull you over. Once at the window, it's all about observation. That "green" smell? They'll note it. Bloodshot eyes? They'll note it. If you say you smoked a few hours ago, they absolutely note it. That's their initial ammo.

Then come the roadside tests. They're looking for more than just stumbling. Can you follow multi-step instructions? Do your eyes track smoothly? This isn't just a casual chat; it's a structured assessment. If they suspect drugs, they'll call a specialist officer or go straight for a blood warrant. In court, the prosecutor strings all these notes, videos, and the blood result together to tell a story of impairment from the moment you were stopped.

[Salvador]

Let me walk you through it like I was trained as a DRE, a Drug Recognition Expert. It's not guesswork; it's a clinical protocol. After the road-side tests, I take over in a controlled environment. Step one: a face-to-face interview. I'm listening to your speech, watching your eyes up close. I check your pupil size in different lighting—cannabis can cause them to dilate slower in light.

I take your blood pressure and pulse. THC can sometimes elevate the heart rate. I check your muscle tone; cannabis can cause relaxation. I perform detailed checks for horizontal gaze nystagmus and look for vertical nystagmus, which is a strong indicator of certain drug categories. Every step is documented. Based on all these physiological and behavioral signs, I form a professional opinion about which drug category is involved. That opinion, combined with a positive toxicology result, forms a robust case.

[PaisleyLynn]

From a medical and scientific standpoint, the challenge is that THC levels in blood don't correlate with impairment as neatly as alcohol. Cops know this, so they focus on functional impairment first. The field tests are essentially a neurological check—assessing your divided attention, balance, and fine motor skills, all functions THC affects.

The "how" involves bridging observable impairment to biological proof. The officer's observations create probable cause to request a blood draw. Modern lab testing can precisely measure active THC (delta-9) and its inactive metabolite. While a positive test shows use, the officer's documented observations of your driving and performance are what tie that use to impaired operation. Courts rely on this combined evidence.

[LeJake]

I learned this the hard way after a rough experience. I thought I was fine to drive hours after smoking. A cop pulled me over for a dim taillight, but it quickly became about more. He said he smelled "a faint odor of marijuana" and asked me to step out. My eyes were a bit red from being tired, and I got nervous. That nervousness made me fumble the heel-to-toe walk test.

I admitted I'd used cannabis earlier when he asked. That was my biggest mistake. They arrested me and took me for a blood test. The whole process felt invasive but very methodical. They weren't just accusing me; they were documenting everything—my speech, my movements, my own words. In the end, the blood test came back positive, and my admission and poor performance on the tests sealed it. They don't need you to be obviously "high"; they just need enough pieces of the puzzle to show you weren't driving normally and that drugs were in your system.