Law enforcement officers employ a multi-faceted approach, combining observable driving behaviors, sensory cues, and direct interactions, to identify individuals driving under the influence.
Initial Observations: On the Road
Before initiating a traffic stop, police officers are trained to look for specific erratic or unusual driving patterns that often indicate impairment. These initial observations are crucial in establishing reasonable suspicion to pull a driver over.
Common pre-stop indicators of impaired driving include:
- Swerving or weaving within a lane or across multiple lanes.
- Driving unusually slowly or excessively fast for the prevailing conditions.
- Drifting over lane lines, center lines, or onto the shoulder.
- Making unusually wide turns or turns that are too sharp.
- Erratic braking, such as sudden stops or braking for no apparent reason.
- Failing to use headlights at night or driving with a broken headlight/taillight.
- Responding slowly to traffic signals, road signs, or other vehicles.
- Near-misses with other vehicles, parked cars, or roadside objects.
During the Traffic Stop
Once a vehicle has been pulled over, the officer's attention shifts to direct interaction with the driver. At this point, the police are looking for any and every sign of unlawful activity, including impaired driving.
Visual Cues
Officers carefully observe the driver's physical appearance and demeanor for immediate signs of intoxication:
- Bloodshot or watery eyes, often accompanied by dilated pupils.
- A flushed face or unusual paleness.
- Disheveled appearance or clothing.
- Fumbling with their driver's license, registration, or insurance documents.
- Difficulty exiting the vehicle (if asked to step out) or maintaining balance.
- Visible presence of open alcohol containers or drug paraphernalia within the vehicle.
Olfactory Cues (Smell)
The presence of certain odors can be a strong indicator of impairment:
- The smell of alcohol emanating from the driver's breath, person, or from within the vehicle's interior.
- The odor of marijuana or other illicit drugs.
Auditory Cues & Driver Interaction
Officers engage the driver in conversation to assess their cognitive and motor functions. They listen for specific vocal and behavioral indicators and look for contradictions or changes in the driver's story:
- Slurred or mumbled speech that is difficult to understand.
- Confused or incoherent responses to simple questions.
- Inconsistent or contradictory stories about their activities, destination, or where they are coming from.
- Repeating phrases or questions, indicating a lack of comprehension or short-term memory issues.
- Unusual mood swings, such as excessive joviality followed by anger or sadness.
Post-Stop Assessment: Field Sobriety Tests (FSTs)
If an officer suspects impairment based on the initial observations and interaction, they may ask the driver to perform a series of Standardized Field Sobriety Tests (SFSTs). These tests are designed to assess a driver's balance, coordination, and ability to follow instructions, all of which are critical for safe driving and are negatively affected by alcohol or drug impairment.
The three primary SFSTs are:
| Test Name | What it Assesses | Common Signs of Impairment |
|---|---|---|
| Horizontal Gaze Nystagmus (HGN) | Involuntary jerking of the eyes as they gaze to the side. | Lack of smooth pursuit, distinct nystagmus at maximum deviation, onset of nystagmus prior to 45 degrees. |
| Walk-and-Turn | Ability to follow instructions, balance, and coordination. | Missing heel-to-toe steps, stepping off the line, raising arms for balance, improper turns. |
| One-Leg Stand | Static balance and the ability to divide attention between physical and mental tasks. | Swaying, putting the foot down prematurely, hopping, using arms for balance. |
Officers also look for other non-standardized observations during these tests, such as general difficulty maintaining balance while standing still or a clear inability to follow simple instructions.
Chemical Tests
If, after the SFSTs, the officer still believes the driver is impaired, they may request a preliminary breath test (PBT) at the scene. While PBT results are typically not admissible as primary evidence in court, they provide an indication of the driver's Blood Alcohol Content (BAC).
For definitive evidence, officers will then request a more accurate chemical test, such as a breath, blood, or urine test, usually conducted at the police station or a medical facility. These tests provide a precise measurement of the driver's BAC or the presence of drugs in their system, which is crucial for legal proceedings.
