Introduction
More than a million people have died in traffic crashes in the United States since 1966, the year of the National Traffic and Motor Vehicle Safety Act, which led to the creation of the National Highway Traffic Safety Administration (NHTSA).
During the late 1960’s and early 1970’s more than 50,000 people lost their lives each year on our nation’s streets, roads and highways. Traffic safety has improved considerably since that time: the annual death toll has declined substantially, even though the numbers of drivers, vehicles, and miles driven all have increased. When miles traveled are considered, the likelihood of being killed in traffic during the 1960’s was three to four times what it is today.
The proportion of all crashes in which alcohol is involved also has declined. The declines in crash risk and the numbers of alcohol-involved crashes are attributable to several factors, including the effectiveness of public information and education programs, traffic safety legislation, a general aging of the population, and law enforcement efforts.
NHTSA research contributed to the improved condition, in part, by providing law enforcement officers with useful and scientifically valid information concerning the behaviors that are most predictive of impairment. Continued enforcement of Driving While Intoxicated (DWI) laws will be a key to saving lives in the future. For this reason, NHTSA sponsored research leading to the development of a new DWI detection guide and training materials, including a new training video. Many things have changed since 1979, but like the original training materials, the new detection guide describes a set of behaviors that can be used by officers to detect motorists who are likely to be driving while impaired.
Building upon the 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 (BAC) of 0.08 percent or greater. The list was reduced to 24 cues during 3 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:
- Problems in maintaining proper lane position
- Speed and braking problems
- Vigilance problems
- 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. 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.
This booklet contains:
- The DWI Detection Guide
- A summary of the research that led to the guide
- Explanations of the 24 driving cues
- A description of post-stop cues that are predictive of DWI
The research suggests that these training materials will be helpful to officers in:
- Detecting impaired motorists
- Articulating observed behaviors on arrest reports
- Supporting officers’ expert testimony
| DWI DETECTION GUIDE
Weaving plus any other cue: p = at least .65 Any two cues: p = at least .50 |
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| Problems Maintaining Proper Lane Position | p=.50–.75 |
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| Speed and Braking Problems | p=.45–.70 |
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| Vigilance Problems | p=.55–.65 |
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| Judgment Problems | p=.35–.90 |
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| Post Stop Cues | p > .85 |
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| p > .50 when combined with any other cue: | |
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| The probability of detecting DWI by random traffic enforcement stops at night has been found to be about 3 percent (.03). | |
Problems in Maintaining Proper Lane Position
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| Weaving |
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| Drifting |
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| Swerving |
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| Turning With a Wide Radius |
Maintaining proper lane position can be a difficult task for an impaired driver. For example, we have all, at one time, seen vehicles weaving. 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
- Jerky or abrupt stops
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| Stopping Beyond a Limit Line |
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| Signaling Inconsistent With Driving Actions |
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| Driving Into Opposing or Crossing Traffic |
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
- Be driven at a speed that is 10 miles per hour (mph) or more under the limit
Vigilance Problems
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 on his or her headlights 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.
Judgment Problems
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| Turning Illegally |
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| Driving on Other Than the Designated Roadway |
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| Appearing To Be Impaired |
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 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.
Summary
To summarize, the DWI cues related to problems in maintaining proper lane position include:
- Weaving
- Weaving across lane lines
- Straddling a lane line
- Drifting
- Swerving
- Almost striking a vehicle or other object
- 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
- 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
- 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
- Appearing to be impaired
Post-Stop Cues
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 or officer must repeat questions
- Providing incorrect information or changes answers
- Odor of alcoholic beverage from the driver