What Impaired Driving Looks Like (WI OWI Cues)

Why this page exists

We are a Wisconsin OWI-defense firm. We see clients after the arrest, not before. But the most common thing we hear from a first-time client is some version of “I didn’t think I was that impaired”. The research below is what the National Highway Traffic Safety Administration, the AAA Foundation, and the University of Iowa actually measure. Same science the Wisconsin State Patrol and the District Attorney’s office work from. If you are charged, knowing the science is the first part of building a defense; if you are not, knowing it might keep you out of one.

Alcohol impairment

The popular picture of drunk driving is perceptual - blurry vision, double-vision, slurred speech. The science is mostly cognitive: slower decision-making, worse judgment of speed and distance, overconfidence, and degraded lane-position control. That last one, the steering wobble, is what an officer is trained to look for and what dashcam footage in a 346.63 prosecution shows.

Drive Sober or Get Pulled Over (NHTSA press conference). Source: USDOTNHTSA on YouTube.

Wisconsin law - alcohol

§ 346.63(1)(a) - OWI: operating while under the influence of an intoxicant.
§ 346.63(1)(b) - PAC: prohibited alcohol concentration of 0.08 or higher (0.04 for commercial drivers; repeat/IID drivers have a reduced PAC threshold, while under-21 drivers face separate absolute-sobriety exposure under § 346.63(2m)).
§ 343.305 - the implied-consent statute. Refusing the chemical test starts a separate civil case with a 10-day clock to request a refusal hearing.

1st-offense OWI → 2nd offense → Refusal hearing →

Cannabis impairment

AAA Foundation research published in 2025 found that 84.8% of cannabis users will drive on the same day, and 53% will drive within an hour of using. Many do so believing cannabis affects driving less than alcohol. Federally sponsored research at the University of Iowa’s National Advanced Driving Simulator pushes back on that: a driver with a blood-THC concentration of 13.1 µg/L showed lane-weave equivalent to a driver at the 0.08 alcohol legal limit. (Lane-weave is the measurable signal; cognitive and reaction-time effects are harder to film, but worse.)

The World's Most Advanced Driving Simulator. Source: University of Iowa Media Production on YouTube.

Wisconsin law - cannabis

§ 346.63(1)(am) - Wisconsin is a zero-tolerance state for THC behind the wheel. Any detectable amount of a restricted controlled substance is sufficient to charge under the OWI statute. There is no per-se “legal limit” analogous to alcohol’s 0.08.
Recreational cannabis is illegal in Wisconsin, so a “but it’s legal where I bought it” argument does not move the needle in a Wisconsin OWI prosecution.
Combined alcohol-and-THC charges are common in Wisconsin and tend to draw enhanced sentencing exposure.

AAA Foundation cannabis fact sheet (PDF) → NADS Iowa: cannabis & alcohol study →

Combined alcohol and cannabis

The same NADS study tested six combinations of alcohol and vaporized cannabis at concentrations matched to what is found in arrested drivers. The headline finding: combined use is additive, not synergistic. Drivers using both substances together weave more than drivers using either substance alone, but the combination does not double the impairment of either component. In a Wisconsin prosecution, this matters for two reasons: (1) the State will present both substances together as evidence of impairment under § 346.63, and (2) toxicology timing is contested ground - cannabis metabolites linger long after impairment ends, and a defense investigation often turns on what the blood test actually proves.

National Advanced Driving Simulator highlights. Source: UI Driving Safety Research Institute on YouTube.

What the research can’t show

A driving-simulator clip can show steering wobble. It cannot show what the driver was thinking. The honest case against impaired driving is not the visual blur; it is the worse risk-assessment, the slower decision speed, and the overconfidence that comes with any psychoactive substance. AAA’s 2025 study found that 46.9% of cannabis users believe they drive the same on cannabis as sober. That belief is itself the most dangerous variable. Wisconsin juries hear it from defendants and from witnesses alike, and prosecutors know to cross-examine on it.

Frequently asked questions

What does an officer look for in a Wisconsin OWI investigation?

NHTSA training under DWI Detection and Standardized Field Sobriety Testing identifies three phases: (1) vehicle in motion (weaving, lane drift, slow response to traffic signals, speed below the limit, wide turns), (2) personal contact at the window (odor of alcohol, slurred speech, bloodshot eyes, fumbling with documents, admissions to drinking), and (3) pre-arrest screening (field sobriety tests, preliminary breath test under Wis. Stat. § 343.303). The officer is building probable cause for arrest under Wis. Stat. § 346.63(1)(a) (impairment) or § 346.63(1)(b) (PAC). The squad video captures phases 1 and 2; phase 3 typically happens off-camera on the shoulder.

What are the field sobriety tests in Wisconsin?

NHTSA recognizes three Standardized Field Sobriety Tests (SFSTs): Horizontal Gaze Nystagmus (HGN, the "follow my pen with your eyes" test), the Walk-and-Turn (9 steps heel-to-toe, turn, 9 steps back), and the One-Leg Stand (raise one foot 6 inches off the ground, count to 30). Officers also frequently use non-standardized tests (alphabet, finger-to-nose, finger count) which have no validated scoring under NHTSA standards. Field sobriety tests in Wisconsin are voluntary; refusing them is not an implied-consent violation under Wis. Stat. § 343.305 and carries no automatic license consequence.

Can field sobriety tests be wrong?

Frequently. NHTSA's own validation studies put the combined SFST accuracy at roughly 91% only when administered exactly per protocol on a flat, dry, well-lit surface to a healthy adult under 65. Real-world conditions degrade the accuracy substantially: roadside slope, ice/snow, gravel shoulder, headlight glare, the driver's footwear, age over 65, weight over 50 lbs over ideal, inner-ear conditions, and back/leg/knee injuries all produce false-positive "clues." Medical conditions like vertigo, BPPV, and certain prescription medications affect HGN independent of any alcohol. Officer scoring is also subjective and frequently inconsistent with the squad video.

What does the breath test actually measure?

The Intoximeter EC/IR II used by Wisconsin agencies measures alcohol concentration in deep-lung (alveolar) breath using infrared spectroscopy, then converts to a blood-equivalent reading using a 2,100:1 partition ratio (assumed average ratio of alcohol concentration in blood to alcohol concentration in breath). The reading is reported in g/210L of breath, which equates to g/100mL of blood under Wis. Stat. § 340.01(46m). The 2,100:1 ratio is an average; individual partition ratios range from roughly 1,300:1 to 3,100:1, which means the same actual blood-alcohol level can produce breath readings off by 30% in either direction.

Can I refuse field sobriety tests in Wisconsin?

Yes, and there is no automatic license consequence for doing so. Field sobriety tests are not implied-consent tests under Wis. Stat. § 343.305; that statute applies only to the post-arrest evidentiary breath or blood test. Refusing field sobriety tests will almost certainly mean the officer arrests on what probable cause exists from observation and the preliminary breath test, but it eliminates the most subjective, error-prone source of probable cause from the case file. The preliminary breath test under Wis. Stat. § 343.303 is also refusable without automatic license consequence; only the post-arrest § 343.305 chemical test triggers the 12-month refusal revocation.

What is the legal BAC limit in Wisconsin?

Under Wis. Stat. § 346.63(1)(b), the standard prohibited alcohol concentration (PAC) for adult drivers in personal vehicles is .08. For CDL holders operating a commercial motor vehicle, the limit is .04 under § 346.63(5). For drivers under 21, the limit is anything more than .00 under § 346.63(2m) (Absolute Sobriety). At BAC ≥ .15 on a 1st offense, IID is mandatory under § 343.301; at BAC ≥ .17 on a 3rd or higher offense, the BAC enhancement under § 346.65(2)(g) doubles, triples, or quadruples the mandatory minimum jail and fine.

How accurate is the preliminary breath test (PBT) in Wisconsin?

PBT accuracy is materially worse than the post-arrest evidentiary Intoximeter EC/IR II. The PBT is a portable handheld device used roadside as a screening tool, not as a final evidentiary instrument. PBT readings can be off by .02 to .03 BAC compared to the post-arrest test, which is why under Wis. Stat. § 343.303 the PBT result is admissible only to show probable cause, not to prove impairment or guilt. The same statutory rule now applies to roadside oral-fluid (saliva) screening under 2025 Wisconsin Act 99. The PBT result is the basis for the arrest decision; the post-arrest Intoximeter or blood test is what the OWI conviction actually rests on.

What is the difference between the PBT and the evidentiary breath test?

The PBT under Wis. Stat. § 343.303 is a handheld roadside screening device used before arrest; result admissible only for probable cause; refusable without automatic license consequence; not evidentiary for trial. The evidentiary breath test under Wis. Stat. § 343.305 is the Intoximeter EC/IR II (or blood draw) administered post-arrest at the station or hospital; result admissible to prove the OWI at trial; refusal triggers automatic 12-month license revocation; subject to a 20-minute observation period and operator certification. The two tests are at different stages of the stop, governed by different statutes, and have very different consequences.

Can I challenge the breath-test result in a Wisconsin OWI?

Yes, on multiple grounds. Common challenges: (1) failure to observe the mandatory 20-minute deprivation period before the test (ensures no mouth alcohol from belching, vomiting, or recent drinking); (2) calibration gaps exceeding the 240-hour certification window required under Wis. Admin. Code Trans 311; (3) operator certification lapses; (4) mouth-alcohol contamination from GERD, dental work, or medications; (5) instrument malfunction documented in the maintenance log; (6) violation of the 2,100:1 partition ratio assumption where individual physiology produces a non-standard ratio. A successful challenge can result in suppression of the breath result entirely, often collapsing the PAC charge under Wis. Stat. § 346.63(1)(b).

Charged with OWI in Wisconsin?

We are Cafferty & Scheidegger, S.C., a Racine criminal-defense firm. Free, confidential consultations, answered 24/7. If you have a refusal-hearing deadline running, call now - the 10-day clock starts at arrest, not at hire.

Call (262) 632-5000 Request a consultation →

What impaired driving actually looks like