Law enforcement has to have a reason for making a traffic stop. They cannot just pull someone over without cause. Often, the reason for a stop is simple and obvious: speeding, running a red light, or driving erratically. In order to make the stop, though, the officer must observe a driver breaking the law or exhibiting signs of impairment. They cannot simply assume because a vehicle is out late at night or because they saw a vehicle leave the parking lot of a bar or nightclub that the driver is impaired.

What are the signs of impairment that give an officer reasonable suspicion to make a DUI stop? The National Highway Transportation Safety Administration (NHTSA)--the same agency that developed the Standardized Field Sobriety Tests (SFST) used in a DUI stop--provides law enforcement agencies with a list of indicators in the booklet The Visual Detection of DWI Motorists.

While an outright traffic violation provides definite cause for a traffic stop, other signs are more subtle, and the NHTSA gives a statistical likelihood of a driver's impairment based upon the observation of one or more visual clues. Through its research into impaired driving behaviors, the agency identified 24 DWI cues divided into four categories:

  1. Problems maintaining proper lane position
  2. Speed and braking problems
  3. Vigilance problems
  4. Judgment problems

Problems with lane position are often one of the first indicators of impairment noted by law enforcement. Driving behaviors in this category include:

  • Weaving
  • Weaving across lines
  • Straddling a lane line
  • Swerving
  • Turning with a wide radius
  • Drifting
  • Almost striking an object or other vehicle

Struggling to maintain the lane can also be a sign of other types of distraction: texting, reading a map, picking up a dropped object, changing a radio station, talking to passengers in the back seat, etc. On its own, drifting across lanes does not mean that a driver is drunk. According to the NHTSA, lane problems indicates a .50 to .75 probability that the driver is impaired, depending on whether or not the officer also observes other cues. 

Everyone knows that if a driver is speeding, he or she has a high risk of being pulled over by police. However, speeds that are too slow may also be a signal for law enforcement. Driving cues in the category of "speed and braking problems" include the following:

  • Stopping too short, too far, or too jerky
  • Accelerating or decelerating for no apparent reason
  • Varying speed
  • Driving too far below the speed limit, identified as 10 mph or more below the posted limit

The NHTSA indicates that speed and braking problems demonstrate a statistical likelihood of DUI at .45 to .70.

"Vigilance problems" refer to a driver's failure or inability to notice traffic signs or signals or his or her surroundings. DUI cues in this category include:

  • Driving in opposing lanes or wrong way on one-way 
  • Slow response to traffic signals 
  • Slow or failure to respond to officer’s signals 
  • Stopping in lane for no apparent reason 
  • Driving without headlights at night 
  • Failure to signal or signal inconsistent with action

Vigilance problems demonstrate a .55 to .65 likelihood of impaired driving.

The last category, "judgement problems," are similar to those listed in other categories. These 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 officer 
  • Inappropriate or unusual behavior (throwing, arguing, etc.) Appearing to be impaired

The range of statistical likelihood of DUI is great for "judgement problems"--.35 to .90. This may depend upon the type of problem observed and its combination with any other DUI cue. According to the NHTSA, weaving plus any other cue makes the likelihood of impairment at least .65. The combination of any two cues gives a minimum statistical likelihood of .50.

The above indicators are those which law enforcement may use to make a traffic stop. Following the stop, there are other signs--including those observed during field sobriety tests--which may also indicate that a driver is intoxicated or impaired. Slurred speech, the odor of alcohol, balance problems, and more show a greater than .85 chance that the driver is impaired.

The best way to beat a DUI traffic stop is not to try to avoid the driving behaviors identified as DUI cues, but to avoid driving after drinking. If you do find yourself under arrest for DUI, call us for help.