Traffic Enforcement and Crash Investigation
Traffic Enforcement and Crash Investigation
This domain tests lawful traffic stops, impaired-driving investigation, crash-scene priorities, and documentation. Missouri enforcement uses probable cause for stops and arrests, standardized field sobriety tests, implied consent, and crash reports that support later prosecution and civil analysis.
Core rules
- A traffic stop needs reasonable suspicion of a violation or probable cause; the stop's scope must match its purpose.
- DWI requires operation and intoxication, proven by totality: driving, personal contact, statements, SFSTs, and chemical testing.
- Missouri's per se limit for adults is 0.08 BAC; commercial drivers and under-21 drivers have lower thresholds.
- Implied consent applies to chemical testing; refusal has administrative consequences and can be used as evidence in some proceedings.
- Crash priorities: render aid and secure the scene, preserve evidence, photograph and measure, identify witnesses, and document injuries and road conditions.
Worked example. A driver is in a ditch with slurred speech and an odor of alcohol. The officer observes six HGN clues, four walk-and-turn clues, and three one-leg-stand clues. The driver admits drinking. Reasoning: the SFSTs are validated only if properly administered and the officer documents conditions and clues. The totality supports arrest; implied consent then governs testing.
Traps
- Skipping the pre-test observation period or failing to document it.
- Treating a refusal as proof by itself without building the totality.
- Not ruling out medical or environmental causes.
- Failing to photograph road defects, sight lines, and debris.
- Saying "no injury" and skipping required reporting.
Exam focus
Articulate reasonable suspicion for the stop, the clues supporting impairment, and the crash facts that prove causation.