Traffic Enforcement and Motor Vehicle Law
This domain tests DC Code Title 50 and DCMR Title 18 (Traffic and Parking), the impaired-driving statutes, and whether your traffic stop produces a case that survives a motion to suppress.
Stops and enforcement
Whren v. United States: a stop is valid if the officer has reasonable suspicion or probable cause of a traffic violation, regardless of the officer's subjective motive. The initial violation must be documented in your report, because it is the legal justification for everything that follows. DC uses automated speed and red-light enforcement, which produces civil citations by mail rather than police stops; know the difference. Default urban speed limits and posted school-zone limits drive enforcement decisions.
Impaired driving
- OUI (DC Code § 50-2201.05b): operating or being in physical control of a vehicle while impaired by liquor, drugs, or a combination, or with a blood alcohol concentration of 0.08 or more.
- Aggravated OUI: a higher concentration (0.20 or above), or an elevated risk version such as driving with a child or at excessive speed.
- Lower thresholds: commercial drivers and drivers under 21 face stricter standards, including zero-tolerance handling for underage drivers.
- Implied consent: a refusal triggers administrative revocation of the permit and, under South Dakota v. Neville, may be used against the driver at trial.
- Field sobriety tests are voluntary; document the observations (odor, eyes, speech, coordination, admissions) that support probable cause independently.
Other core duties
DC's hands-free law restricts handheld phone use while driving. Occupant restraint and child-restraint requirements apply. Leaving after colliding requires stopping, providing identifying information, and rendering aid - this is a separate offense from the collision itself.
Worked example
You stop a driver for running a red light. On approach you smell alcohol, see bloodshot watery eyes and slurred speech, and hear an admission of two drinks. The driver shows six clues on the horizontal gaze nystagmus test and declines the breath test after the implied-consent advisement. You arrest for OUI. Your report documents the red light, each impairment observation, the test administration and clues, the advisement, and the refusal - the refusal is both an administrative consequence and trial evidence.
Traps
Failing to document the initial violation; treating a preliminary breath test as the evidentiary test; skipping or mangling the implied-consent advisement; confusing a mailed automated citation with a custodial traffic stop.