What do police look for before they stop someone on suspicion of DUI?

Law enforcement officers are trained to identify specific visual cues associated with impaired driving before initiating a traffic stop. Understanding what officers look for helps explain how many DUI stops begin — and why certain driving behaviors substantially increase the risk of a DUI investigation.

The NHTSA Visual Detection Guide

The National Highway Traffic Safety Administration (NHTSA) has developed a standardized guide for law enforcement officers to detect impaired drivers based on observable driving behaviors. Pennsylvania law enforcement, including state police and local departments in the Erie region, are trained using this framework. The NHTSA guide identifies four categories of cues, each with a statistical correlation to a BAC at or above 0.08%.

Problems Maintaining Proper Lane Position

Lane maintenance difficulties are among the most reliable visual cues for impaired driving and include:

• Weaving — making a series of alternating turns across the roadway
• Weaving across lane lines
• Straddling a lane line
• Drifting — a gradual, unsteady movement across the lane
• Swerving — an abrupt turn away from a straight course
• Almost striking a vehicle or object
• Turning with a wide radius

Speed and Braking Problems

• Stopping problems — stopping too short, too far into the intersection, or in a jerking manner
• Accelerating or decelerating for no apparent reason
• Varying speed — alternating between speeding up and slowing down
• Slow speed — driving significantly below the posted limit
• Driving without headlights at night

Vigilance Problems

• Driving in opposing lanes or wrong way on one-way streets
• Slow response to traffic signals
• Stopping in a lane for no apparent reason
• Driving in prohibited areas
• Signaling inconsistently with driving actions
• Inappropriate or slow response to officer’s signals

Judgment Problems

• Following too closely
• Improper or unsafe lane changes
• Illegal or improper turns
• Driving on other than the designated roadway
• Stopping inappropriately in response to an officer

DUI Checkpoints in Pennsylvania

In addition to patrol-based stops, DUI arrests in Pennsylvania occur at sobriety checkpoints. Pennsylvania courts have held that properly conducted checkpoints are constitutional. However, checkpoints must meet specific legal requirements, including advance public notice, a neutral and predetermined vehicle selection method, and limitations on the duration and intrusiveness of the stop. Failure to comply with these requirements can render a checkpoint stop unconstitutional.

The Relevance of This Information to DUI Defense

In any DUI case initiated by a traffic stop, the officer’s basis for the stop is a threshold legal issue. If the only observation was a minor weave that equally suggests distraction or fatigue rather than impairment, or if the officer’s stated reason for the stop is legally insufficient, a motion to suppress all evidence from the stop may be viable. Reviewing dashcam footage — which many Pennsylvania law enforcement vehicles are equipped with — against the officer’s report of the driving behavior is an important early step in DUI defense.