6.2 DWI Investigation & Implied Consent

Missouri DWI: Core Legal Concept

Quick Answer: Missouri criminalizes driving while intoxicated (see RSMo Chapter 577 themes, including RSMo 577.010). A blood alcohol content of .08% or more by weight of alcohol in the blood is prima facie evidence of an intoxicated condition—an official MPOLE sample-style fact. Investigate with observation, personal contact, SFSTs, arrest on probable cause, implied-consent advisement, then chemical testing.

Impaired driving enforcement protects Missouri roadways and is a high-stakes liability area for officers. MPOLE and academy testing emphasize legal thresholds, investigation sequence, and implied consent—not only “how to hold a flashlight.”

Prima facie evidence at .08%

Memorize the handbook-style formulation:

A BAC of .08% or more by weight of alcohol in the blood is prima facie evidence of an intoxicated condition.

“Prima facie” means the BAC reading is sufficient on its face to establish the intoxicated condition element unless overcome by other evidence in the case. Officers still need lawful investigation, a proper sample, and admissible proof. Do not confuse:

Practice-bank framing also treats 0.08% as the standard adult per se limit for drivers 21+. Under-21 and commercial standards differ (see CDL below).

Aggravated / enhanced themes (awareness)

Materials distinguish ordinary DWI from aggravated or enhanced fact patterns involving serious injury, very high BAC, prior convictions, or child passengers. Those factors affect charging level and penalties; they are not required to form basic PC for a standard DWI arrest when ordinary intoxication facts exist.

SFST Overview (Awareness Level)

The three NHTSA-validated Standardized Field Sobriety Tests taught nationwide and in Missouri academies are:

1. Horizontal Gaze Nystagmus (HGN) — observation of involuntary eye jerking as the eyes gaze to the side under standardized stimulus movement; scored with validated clues when properly administered.
2. Walk-and-Turn (WAT) — divided-attention heel-to-toe walk with instructional and walking-stage clues.
3. One-Leg Stand (OLS) — divided-attention balance test with standardized timing and clue scoring.

MPOLE awareness points

• Know the three validated tests by name; do not substitute “alphabet only” or “finger-to-nose only” as the validated triad.
• SFSTs are standardized—departures reduce evidentiary value.
• SFSTs support probable cause analysis; they are not a courtroom conviction by themselves.
• Always document instructions given, conditions (lighting, surface, weather), and subject statements about injuries or medications.

Preliminary breath tests (PBT) and agency-approved portable devices, where authorized, are screening tools with their own evidentiary limits—know your agency rules and that evidential chemical tests after arrest are the implied-consent focus.

Observation of Driving Patterns and Personal Contact

Build the case in layers:

Phase One — Vehicle in motion

Common alcohol-related driving cues (NHTSA-style training lists) include:

• Weaving, drifting, wide turns, almost striking objects
• Straddling center or lane lines; drifting during curves
• Stopping too far/short/jerky; accelerating or decelerating rapidly without reason
• Slow speed, braking erratically, delayed response to signals
• Driving without headlights at night; inconsistent signaling

Any one cue can justify a stop when it is also a traffic violation or contributes to RS of impairment; clusters of cues strengthen the impairment hypothesis.

Phase Two — Personal contact

After the stop, note:

• Odor of alcoholic beverage; bloodshot/watery eyes; slurred speech
• Fumbling with documents; delayed answers; admitting drinking
• Open containers; soiled clothing; urine/vomit odor
• Difficulty exiting; using the vehicle for balance

Phase Three — Pre-arrest screening

Administer SFSTs when safe and appropriate; consider alternatives or DRE involvement when drug impairment is suspected.

Probable Cause for Arrest and Processing Sequence

Probable cause for DWI arrest exists when facts would lead a reasonable officer to believe the person drove a vehicle while in an intoxicated or drugged condition under Missouri law. Totality matters: driving cues + personal-contact clues + SFST performance + admissions.

Standard processing sequence (exam-aligned)

1. Observation of driving/operation
2. Stop on lawful basis
3. Personal contact and impairment assessment
4. SFSTs (when conditions allow)
5. Probable cause determination
6. Arrest
7. Implied consent advisement for chemical testing
8. Chemical test (breath, blood, and/or urine as authorized)
9. Booking / reports / evidence

Exam trap: Jumping to an evidential breath test before arrest and implied-consent process, or skipping SFST observation when it was safe and available without documenting why, creates defense arguments. The commonly tested order is observation → PC assessment/SFSTs → arrest → implied consent warning → chemical test → booking.

Implied Consent Concepts, Chemical Testing, and Refusal

Implied consent idea

By driving on Missouri public roadways, a driver is deemed to have consented to chemical testing of breath, blood, or urine to determine alcohol or drug content when lawfully arrested for an applicable alcohol- or drug-related traffic offense—as framed in Missouri’s implied consent statutes and taught in POST academies. Exact statutory wording evolves; MPOLE expects the concept: lawful arrest + advisement + request for test.

Chemical testing

• Use approved instruments and qualified operators for evidential breath tests.
• Blood draws follow legal authority, medical protocols, and chain of custody.
• Observe deprivation/observation periods required by testing protocols when applicable.
• Document time of stop, arrest, advisement, test, and results.

Refusal consequences (awareness)

Refusing a chemical test after proper implied-consent procedures typically triggers administrative license consequences separate from the criminal case. Missouri training and practice items commonly treat a standard adult refusal theme as a one-year license revocation consequence pathway (exact eligibility, prior refusals, and commercial rules can modify outcomes—follow current DOR/RSMo process). Criminal prosecution for DWI can continue based on driving observations and SFSTs even without a chemical result.

CDL / Commercial Driver Heightened Standards

Commercial drivers are held to stricter alcohol standards and face severe CDL disqualifications for alcohol/drug violations. Training-level points:

• CDL is generally required for vehicles with GVWR 26,001+ lbs, vehicles designed for 16+ passengers, or hazmat requiring placards (among other categories).
• Federal/state commercial rules use a lower BAC threshold (commonly taught as 0.04% while operating a commercial motor vehicle) than the general .08% standard.
• Out-of-service and disqualification rules can apply even when a non-CDL driver might face different administrative results.
• Always identify whether the person was operating a commercial motor vehicle and hold a CDL; document vehicle type, load, and placards.

Drug-Impaired Driving Awareness

Alcohol is not the only impairing substance. Missouri DWI concepts include driving in a drugged condition or combined impairment.

Officer awareness cues beyond alcohol odor:

• Horizontal or vertical nystagmus patterns inconsistent with claimed intake; pupil size extremes; muscle tone changes
• Confused timeline, hallucinations, or stimulant-like agitation
• Drug paraphernalia; admissions to medication or illicit use
• Poor SFST performance with low or zero alcohol result

When drug impairment is suspected:

1. Still complete safe stop and SFST process when appropriate.
2. Summon a Drug Recognition Expert (DRE) if available and policy supports it.
3. Seek appropriate chemical testing (blood/urine) under implied consent or other lawful authority.
4. Document specific observations—not merely “acted high.”

Missouri DWI checklist for MPOLE

1. Can I articulate the stop basis?
2. What driving, contact, and SFST facts show intoxication or drug impairment?
3. Do I have probable cause to arrest under Missouri DWI law?
4. Did I give proper implied consent advisement before requesting the chemical test?
5. If refused, did I document refusal and start administrative process correctly?
6. If CDL/CMV, did I apply the heightened commercial standards?

Impaired-driving cases are won or lost on articulation and sequence. Know the .08% prima facie rule, the three SFSTs, implied consent, and commercial distinctions cold for MPOLE.