Guide for Detecting Drunk Drivers at Night

          This booklet explains the visual cues contained in the DWI Detection Guide provide by the
U.S. Department of Transportation.

          The number given after each visual cue is the probability that a driver exhibiting that cue has a BAC equal or greater than 0.10. For example the 65 for the fisrt cue, Turning With Wide Radius, means that chances are 65 out of 100 that a driver who turns with a wide radius at night will have a BAC equal to or greater that 0.10.

Visual Cues

          Click on the cue to see a description of it!

1. Turning With Wide Radius 2. Straddling Center or Lane Marker
3. Appearing to be Drunk 4. Almost Striking Object or Vehicle
5. Weaving 6. Driving on Other Than Designated Roadway
7. Swerving 8. Speed Slower Than 10 M.P.H. Below Limit
9. Stopping Without Cause in Traffic Lane 10. Following Too Closely
11. Drifting 12. Tires on Center or Lane Marker
13. Braking Erradically 14. Driving Into Opposing Crossing Traffic
15. Signaling Inconsistantly With Driving Actions 16. Slow Response to Traffic Signals
17. Stopping Inappropiately (Other Than in Traffic Lane) 18. Turning Abruptly or Illegally
19. Accelerating or Decelerating Rapidly 20. Headlights Off

Turning With Wide Radius
Probabilty = 65%
During a turn, the radius defined by the distance between the turning vehicle and the center of the turn is greater than normal.
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Straddling Center or Lane Marker

Probabilty = 65%
The vehicle is moving straight ahead with the center or lane marker between the left-hand and right-hand wheels.
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Appearing to be Drunk
Probabilty = 60%
This cue is actually one or more of a series of indicators related to the personal behavior or appearance of the driver. Examples of specific indicators might include:
  • Eye fixation.
  • Tightly gripping the steering wheel.
  • Slouching in the seat.
  • Gesturing erratically or obscenely.
  • Face close to the windshield.
  • Drinking in the vehicle.
  • Driver's ahead protuding from vehicle.
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Almost Striking Object or Vehicle

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Probabilty = 60%
The observed vehicle almost strikes a stationary object or another moving vehicle. Examples include:
  • Passing abnormally close to a sign, wall, building, or other object.
  • Passing abnormally close to another moving and non-moving vehicle.
  • Causing another vehicle to maneuver to avoid collision.
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Weaving
Probabilty = 60%
Weaving occurs when the vehicle alternately moves toward one side of the roadway and then the other, creating a zig-zag course.The pattern of lateral movement is relatively regular as one steering correction is closely followed by another.
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Driving on Other than designated roadway
Probabilty = 55%
The vehicle is observed being driven on other than the roadway designated for traffic movement. Examples include:
  • Driving at the edge of the roadway,
  • Driving on the shoulder,
  • Driving off the roadway entirely,
  • Driving straight through turn-only lanes or areas.
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Swerving
Probabilty = 55%
A swerve is an abrupt turn away from a generally straight course. Swerving might occur direcly after a period of drifting when the driver discovers the approach of traffic in an oncoming lane or discovers that the vehicle is going off the road; swerving might also occur as an abrupt turn is executed to return the vehicle to the traffic lane. The illustration shows a swerve being executed to return to a lane after a period of drifting toward opposing traffic.
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Speed Slower Than 10 M.H.P.H. Below Limit
Probabilty = 50%
The observed vehicle is being driven at a speed that is more than 10 miles per hour below the specified speed limit.
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Stopping Without Cause in Traffic Lane
Probabilty = 50%
The critical element in this cue is that there is no observable justification for the vehicle to stop in the traffic lane; the stop is not caused by traffic conditions, traffic signals, an emergency situation, or related circumstances. Intoxicated drivers might stop in lane when their capability to interpret information and make decisions becomes severely impaired. As a consequence, stopping (without cause) in the traffic lane is likely to occur at intersections or other decision points.
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Follow Too Closely
Probabilty = 50%
The vehicle is observed following another vehicle while not maintaining the legal minimum separation.
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Drifting
Probabilty = 50%
Drifting is a straight-like movement of the vehicle at a slight angle to the roadway. As the driver approaches a marker or boundary (lane marker, center line, edge of the roadway), the direction of drift might change. As shown in the illustration, the vehicle driftss across the lane marker into another lane, then the driver makes a correction and the vehicle drifts back across the lane marker. Drifting might be observed within:
  • a single lane,
  • across lanes,
  • across the center lane,
  • onto the shoulder,
  • from lane to lane.
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Tires on Center or Lane Marker
Probabilty = 45%
The left-hand set of tires of the observed vehicle is consistently on the the center line, or either set of tires is sonsistently on the lane marker.
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Braking Erratically
Probabilty = 45%
The driver of the observed vehicle breaks unnecesarily, maintains pressure on the brake pedal ("riding the brakes"), or brakes in an uneven or jerky manner.
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Drifting Into Opposing or Crossing Traffic
Probabilty = 45%
The vehicle is observed heading into opposing or crossing traffic under one or more of the following circumstances:
  • driving in the opposing lane,
  • backing into traffic,
  • failing to yield the right-of-way,
  • driving the wrong way on a one-way street, as shown in the picture.
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Signaling Inconsistent With Driving Actions
Probabilty = 40%
A number of possibilities exist for the driver's signaling to be inconsistent with the associated driving action. This ccue occurs when inconsistencies such as the following are observed:
  • failing to signal a turn or lane change,
  • signaling opposite to the turn or lane change executed
  • signaling constantly with no accompanying driving action,
  • driving with four-way hazard flashes on.
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Slow Response to Traffic Signals
Probabilty = 40%
The observed vehicle exhibits a longer than normal response to a change in traffic signal. For example, the driver remains stopped at the intersection for an abnormally long period of time after the traffic signal has turned green.
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Stopping Inappropiately (Other than in Traffic Lane)
Probabilty = 35%
The observed vbehicle stops at an inappropiate location or under inappropiate conditions, other than in the traffic lane.
Examples include stopping:
  • in a prohibited zone;
  • at a crosswalk;
  • far short of an intersection;
  • on a walkway;
  • across lanes;
  • for a green traffic signal;
  • or for a flashing yellow traffic signal.
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Turning Abruptly or Illegally
Probabilty = 35%
The driver executes any turn that is abnormally abrupt or illegal. Specific examples include:
  • turning with excessive speed;
  • turning sharply from the wrong lane;
  • making a U turn illegally;
  • turning from outside a designated turn lane.
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Accelerating or Decelerating Rapidly
Probabilty = 35%
This cue encompasses any acceleration that is significantly more rapid than that required by the traffic conditions. Rapid acceleration might be accompanied by an abrupt stop. Also a vehicle might alternately accelerate and decelerate rapidly.
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Headlights Off
Probabilty = 30%
The observed vehicle is being driven with both headlights off during a period of the day when the use of headlights is reqired.
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