Correlation Of Driving Behaviors With Alcohol Level

Accurately determining alcohol level based on driving behavior may be difficult. In one study, some behaviors typically associated with driving impairment were found in individuals well under the legal limit [Burns. M., Dioquino, T., A Florida Validation Study of the Standardized Field Sobriety Test (SFST) Battery, 1997, p. 27 (hereafter "SFST FL Validation 1997")]. Below is a chart detailing some of these behaviors:

Average BAC Associated With Certain Driving Behaviors

Driving BehaviorAverage BACPercentage
Equipment violation0.015%10.2%
Speed (high or low)0.036%28.6%
Running a stop sign/failure to yield, etc.0.038%32.7%
Failure to maintain a single lane0.040%8.2%

In addition to this study, the National Highway Traffic Safety Administration (NHTSA) provided a list of driving behaviors and their related probability of discriminating "nighttime drunk drivers" with sober drivers. Although the probability is numerically represented, the NHTSA list provides the number "primarily to emphasize the importance of a particular cue." The cues were determined to relate to drivers who may have a BAC of .10 percent or greater (U.S. Department of Transportation, National Highway Traffic Safety Administration, Guide to Detecting Drunk Drivers at Night, DOT HS 805-711, 1982, pp 1-8).

Below is a chart of such cues and the probability that they occur:

Driving Cues And Probability Of .10 Percent BAC

Turning with a wide radius: The distance between the turning vehicle and the center of the turn is greater than normal.65
Straddling center or lane marker: Vehicle is moving straight ahead with the center or lane marker between the left and right wheels65
Appearing to be drunk: Eye fixation, tightly gripping wheel, slouching in seat, erratic or obscene gestures, face close to windshield, drinking in the vehicle, driver's head protruding from the vehicle60
Almost striking object or vehicle: Passing abnormally close to an object or vehicle, causing another vehicle to maneuver to avoid collision60
Weaving: Moving from one side of the roadway to the other in a zigzag course60
Driving on other than designated roadway: Driving at the edge of roadway, on the shoulder, off the roadway, straight through turn-only lanes55
Swerving: An abrupt turn from a straight course55
Slow speed: Moving slower than 10 mph below limit50
Stopping abruptly: Stopping without cause in traffic lane; stopping without observable justification50
Following too closely: Following another car without maintaining minimum legal separation50
Drifting: Driving in a straight line but at an angle to the roadway.50
Tire placement: Tires on center or lane marker; the left hand set of tires is consistently on the center line, or either set is consistently on the lane marker.45
Braking: Braking erratically; braking in an uneven/jerky manner, unnecessarily maintaining pressure on the brake pedal; riding the brakes.45
Driving into opposing or crossing traffic: Driving into opposing lane, backing into traffic, failing to yield the right of way, driving the wrong way on a one-way street.45
Signaling inconsistent with actions: Failing to signal a lane change, signaling opposite to the action, driving with hazard flashers, etc.40
Slow response to traffic signals: A longer than normal driver response to a change in the traffic signal.40
Stopping inappropriately: Vehicle stops at an inappropriate location or under inappropriate conditions other than in the traffic lane (flashing yellow light, green light, in a crosswalk etc.).35
Turning abruptly or illegally: Turning with excessive speed, turning sharply from the wrong lane, illegal U-turn, turning outside a designated turn lane.35
Accelerating or decelerating rapidly: Abrupt stop, breaking traction, acceleration or deceleration more rapid than required by traffic conditions.30
Headlights off: Driving with the headlights off at a time of day when headlights are required.30

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