Building upon a previous NHTSA study, researchers interviewed officers from across the United States and developed a list of more than 100 driving cues that have been found to predict blood alcohol concentrations, or BACs, of 0.08 percent or greater. The list was reduced to 24 cues during three field studies involving hundreds of officers and more than 12,000 enforcement stops. The driving behaviors identified by the officers are presented in the following four categories:
1) Problems in maintaining proper lane position,
2 ) Speed and braking problems,
3) Vigilance problems, and
4) Judgment problems.
The cues presented in these categories predict that a driver is DWI at least 35 percent of the time. For example, if you observe a driver to be weaving or weaving across lane lines, the probability of DWI is more than .50, or 50 percent. However, if you observe either of the weaving cues and any other cue listed in this booklet, the probability of DWI jumps to at least .65, or 65 percent. Observing any two cues other than weaving indicates a probability of DWI of at least 50 percent, although some cues, such as swerving, accelerating for no reason, and driving on other than the designated roadway, have single-cue probabilities greater than 70 percent. Generally, the probability of DWI increases substantially when a driver exhibits more than one of the cues.
Problems in Maintaining Proper Lane Position
Maintaining proper lane position can be a difficult task for an impaired driver. For example, we have all seen vehicles weaving before. Weaving is when the vehicle alternately moves toward one side of the lane and then the other. The pattern of lateral movement can be fairly regular, as one steering correction is closely followed by another. In extreme cases, the vehicle’s wheels even cross the lane lines before a correction is made. You might even observe a vehicle straddling a center or lane line. That is, the vehicle is moving straight ahead with either the right or left tires on the wrong side of the lane line or markers.
Drifting is when a vehicle is moving in a generally straight line, but at a slight angle to the lane. The driver might correct his or her course as the vehicle approaches a lane line or other boundary, or fail to correct until after a boundary has been crossed. In extreme cases, the driver fails to correct in time to avoid a collision.
Course corrections can be gradual or abrupt. For example, you might observe a vehicle to swerve, making an abrupt turn away from a generally straight course, when a driver realizes that he or she has drifted out of proper lane position, or to avoid a previously-unnoticed hazard.
A related DWI cue is almost striking a vehicle or other object. You might observe a vehicle, either at slow speeds or moving with traffic, to pass unusually close to a sign, barrier, building, or other object. This cue also includes almost striking another vehicle, either moving or parked, and causing another vehicle to maneuver to avoid a collision.
Turning with a wide radius, or drifting during a curve, is the final cue in this category of driver behaviors. A vehicle appears to drift to the outside of the lane, or into another lane, through the curve or while turning a corner. Watch for this cue and stop the driver when you see it. Many alcohol-involved crashes are caused by an expanding turn radius, or drifting out of lane position during a curve.
Speed and Braking Problems
The research showed that braking properly can be a difficult task for an impaired driver. For example, there is a good chance the driver is DWI if you observe any type of stopping problem. Stopping problems include,
- Stopping too far from a curb or at an inappropriate angle,
- Stopping too short or beyond a limit line, and
- Jerky or abrupt stops.
Impaired drivers also can experience difficulty maintaining an appropriate speed. There is a good chance the driver is DWI if you observe a vehicle to,
- Accelerate or decelerate rapidly for no apparent reason,
- Vary its speed, alternating between speeding up and slowing down, or be
- Driven at a speed that is ten miles per hour or more under the limit.
Vigilance concerns a person’s ability to pay attention to a task or notice changes in surroundings. A driver whose vigilance has been impaired by alcohol might forget to turn his or her headlights on when required. Similarly, impaired drivers often forget to signal a turn or lane change, or their signal is inconsistent with their maneuver, for example, signaling left, but turning right.
Alcohol-impaired vigilance also results in motorists driving into opposing or crossing traffic, and turning in front of oncoming vehicles with insufficient headway.
Driving is a complex task that requires accurate information about surrounding traffic conditions. Failing to yield the right of way and driving the wrong way on a one way street are dangerous examples of vigilance problems.
A driver whose vigilance has been impaired by alcohol also might respond more slowly than normal to a change in a traffic signal. For example, the vehicle might remain stopped for an unusually long period of time after the signal has turned green. Similarly, an impaired driver might be unusually slow to respond to an officer’s lights, siren, or hand signals.
The most extreme DWI cue in the category of vigilance problems is to find a vehicle stopped in a lane for no apparent reason. Sometimes when you observe this behavior the driver will be just lost or confused, but more than half of the time the driver will be DWI-maybe even asleep at the wheel.
Operating a motor vehicle requires continuous decision-making by the driver. Unfortunately, judgment abilities can be affected by even small amounts of alcohol. For example, alcohol-impaired judgment can cause a driver to follow another vehicle too closely, providing an unsafe stopping distance.
Alcohol-impaired judgment also can result in a driver taking risks or endangering others. If you observe a vehicle to make improper or unsafe lane changes, either frequently or abruptly, or with apparent disregard for other vehicles, there is a good chance the driver’s judgment has been impaired by alcohol.
Similarly, impaired judgment can cause a driver to turn improperly. For example, misjudgments about speed and the roadway can cause a driver to take a turn too fast, or to make sudden corrections during the maneuver. These corrections can appear to the observer as jerky or sharp vehicle movements during the turn.
Alcohol-impaired judgment can affect the full range of driver behaviors. For example, the research found that impaired drivers are less inhibited about making illegal turns than unimpaired drivers.
Driving on other than the designated roadway is another cue exhibited by alcohol-impaired drivers. Examples include driving at the edge of the roadway, on the shoulder, off the roadway entirely, and straight through turn-only lanes.
In some cases, impaired drivers stop inappropriately in response to an officer, either abruptly as if they had been startled, or in an illegal or dangerous manner.
In fact, the research has shown that there is a good chance a driver is DWI if you observe the person to exhibit any inappropriate or unusual behavior. Unusual behavior includes throwing something from the vehicle, drinking in the vehicle, urinating at the roadside, arguing with another motorist, or otherwise being disorderly. If you observe inappropriate or unusual behavior, there is a good probability that the driver is DWI.
The final cue is actually one or more of a set of indicators related to the personal behavior or appearance of a driver. These indicators include, gripping the steering wheel tightly, driving with one’s face close to the windshield, slouching in the seat, and staring straight ahead with eyes fixed. Some officers routinely scrutinize the faces of drivers in oncoming traffic, looking for the indicators of impairment. If you observe a driver who appears to be impaired, the research showed that there is an excellent probability that you are correct in your judgment.
To summarize, the DWI cues related to problems in maintaining proper lane position include:
- Weaving across lane lines,
- Straddling a lane line,
- Almost striking a vehicle or other object, and
- Turning with a wide radius, or drifting during a curve.
The DWI cues related to speed and braking problems include:
- Stopping problems (too far, too short, too jerky),
- Accelerating for no reason,
- Varying speed, and
- Slow speed.
The DWI cues related to vigilance problems include:
- Driving without headlights at night,
- Failure to signal a turn or lane change, or Signaling inconsistently with actions,
- Driving in opposing lanes or the wrong way on a one-way street, Slow response to traffic signals,
- Slow or failure to respond to officer’s signals, and
- Stopping in the lane for no apparent reason.
The DWI cues related to judgment problems include:
- Following too closely,
- Improper or unsafe lane change,
- Illegal or improper turn (too fast, jerky, sharp, etc.),
- Driving on other than the designated roadway,
- Stopping inappropriately in response to an officer,
- Inappropriate or unusual behavior, and
- Appearing to be impaired.
In addition to the driving cues, the following post-stop cues have been found to be excellent predictors of DWI:
- Difficulty with motor vehicle controls,
- Difficulty exiting the vehicle,
- Fumbling with driver’s license or registration,
- Repeating questions or comments,
- Swaying, unsteady, or balance problems,
- Leaning on the vehicle or other object,
- Slurred speech,
- Slow to respond to officer/officer must repeat questions,
- Provides incorrect information or changes answers, and
- Odor of alcoholic beverage from the driver.