legal advice

What do each of these driving patterns signify? Why are they evidence of impairment?
The driving patterns that police typically look for have been outlined by the National Highway Traffic and Safety Administration (NHTSA). NHTSA has delineated a number of “cues” that police officers can look for in detecting intoxicated drivers driving at night. The list presented below represents, in descending order, the visual cues giving rise to the probability that the person observed is driving while intoxicated. Often, police officers will observe several cues in conjunction, which increases the probability that the driver is intoxicated, that is, driving with a blood alcohol content (BAC), over the legal limit. Studies have indicated that cues by themselves or in conjunction with other cues, give rise to a statistical probability of intoxication.
However, it must be recognized that these cues are not definitive of intoxication. In fact, the following cues have between a 30-70% probability of not reflecting intoxication.
1. Turning with a Wide Radius: During the turn, the radius, defined as the distance between the turning vehicle and the center of the turn, is greater than normal. For example, in making the turn, the driver may go beyond the designated lane markers of the first lane they are supposed to be turning into. This cue indicates a 65% probability that the driver has a BAC at or above .10%.
2. Straddling Center or Lane Marker: Here, the vehicle is moving straight ahead with the center or lane marker between the right and left wheels. Essentially this means that the lane marker is between the right and left wheels. This cue indicates a 65% probability that the driver has a BAC at or above .10%.
3. Appearing to be Drunk: This factor is determined by the officer’s visual observations of the driver and most likely includes several different indicators, including: slouching in the seat, tightly gripping the steering wheel, driving with face close to windshield, and driver’s head sticking out of the driver’s window, in addition to other cues that may present themselves. This cue indicates a 60% probability that the driver has a BAC at or above .10%.
4. Inappropriate or Unusual Behavior: This cue indicates such inappropriate behavior as throwing objects, arguing, and other disorderly actions. This may also include drinking in the vehicle, or urinating on the side of the roadway. Varying Speed: The vehicle alternates between speeding up and slowing down. This cue indicates a 60% probability that the driver has a BAC at or above .10%.
5. Almost Striking Object or Vehicle: The vehicle is observed to almost strike a stationary object or another moving vehicle. For example, the driver may pass abnormally close to another object, which is visually observed by the officer, or the officer observes another vehicle having to maneuver to avoid a collision with the driver. This cue indicates a 60% probability that the driver has a BAC at or above .10%.
6. Weaving: Essentially weaving is demonstrated as driving in a zig-zag pattern, with the vehicle going from one side of the road to the other. This cue indicates a 60% probability that the driver has a BAC at or above .10%.
7. Weaving Across Lane Lines: Extreme cases of weaving when the vehicle wheels cross the lane lines before correction is made. This cue indicates a 60% probability that the driver has a BAC at or above .10%.
8. Driving on Other than Designated Roadway: Here, the vehicle is seen driving off the road entirely, for example in the shoulder or in the dirt, or through turn-only lanes. This cue indicates a 55% probability that the driver has a BAC at or above .10%.
9. Swerving: Swerving occurs in several different situations. For example, a swerve can occur when the car drifts off course and the driver swerves to put themselves back into the lane. This cue indicates a 55% probability that the driver has a BAC at or above .10%.
10. Slow speed (more than 10 mph below speed limit. This cue indicates a 50% probability that the driver has a BAC at or above .10%.
11. Stopping (without cause) in Traffic Lane: Here, the officer cannot observe any identifiable reason as to why the car is stopped in the traffic lane. This most often occurs at intersections because the intoxicated driver needs to make a decision as to which way to go and stops the car in order to do this. This cue indicates a 50% probability that the driver has a BAC at or above .10%.
12. Stopping Problems: Here, the vehicle stops either too far, too short, or stops in a jerky way. This cue indicates a 50% probability that the driver has a BAC at or above .10%.
13. Following Too Closely: Here, the vehicle is following another vehicle without allowing the legal minimum separation between the vehicles. This cue indicates a 50% probability that the driver has a BAC at or above .10%.
14. Drifting: A “drift” is defined as a straight line movement of the vehicle at a slight angle to the roadway. This cue indicates a 50% probability that the driver has a BAC at or above .10%.
15. Tires on Center or Lane Marker: This occurs where the vehicles tires are observed to be consistently on the center line or either tire is consistently on the lane marker. This cue indicates a 45% probability that the driver has a BAC at or above .10%.
16. Braking Erratically: Here, the driver will either “ride the breaks” or else brake unevenly or in a jerky manner. This cue indicates a 45% probability that the driver has a BAC at or above .10%.
17. Driving Into Opposing or Crossing Traffic: This cue occurs in several situations: driving in the opposite lane, backing into traffic, driving the wrong way on a one-way street, or failing to yield the right of way. This cue indicates a 45% probability that the driver has a BAC at or above .10%.
18. Improper or Unsafe Lane Change: Here the driver is observed to take risks or endanger others. The driver will frequently or abruptly change lanes without regard to other motorists. This cue indicates a 45% probability that the driver has a BAC at or above .10%.
19. Illegal or Improper Turn: This cue could mean several things: the turn is too fast, jerky, sharp, etc. It could also mean such maneuvers as turning sharply from the wrong lane, making a u-turn illegally, or turning from outside a designated turn lane. This cue indicates a 45% probability that the driver has a BAC at or above .10%.
20. Signaling Inconsistent with Driving Actions: This cue occurs in several situations: failing to signal a turn or lane change, signaling opposite to the turn or lane change executed, signaling constantly with no accompanying driving maneuver. This cue indicates a 40% probability that the driver has a BAC at or above .10%.
21. Varying Speed: The vehicle alternates between speeding up and slowing down. This cue indicates a 30% probability that the driver has a BAC at or above .10%.
22. Accelerating or Decelerating Rapidly: This cue is observed as any acceleration or deceleration that is significantly more rapid than what is required by traffic laws. This cue indicates a 30% probability that the driver has a BAC at or above .10%.
23. Driving Without Headlights at Night: The vehicle is observed to be driven with both headlights off, when the use of headlights would otherwise be required. This cue indicates a 30% probability that the driver has a BAC at or above .10%.
24. Stopping Inappropriately in Response to Officer: This cue is indicated by any number of factors including stopping at an inappropriate location or under inappropriate conditions, other than in a traffic lane. This cue indicates a 30% probability that the driver has a BAC at or above .10%.