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If you're sober, then you've got nothing to worry about, right? Not necessarily. The problem with being singled out of traffic as a possible DUI is that the officer has already formed certain expectations: He is psychologically predisposed to "see" what he expects to see -- and he expects to see an inebriated person behind the wheel. So reddish eyes from fatigue, alcohol on the breath from a single drink, and nervous fumbling with your wallet trying to get your driver's license will be seen as corraborating these suspicions. And then your future is going to depend upon performing well on field sobriety tests -- tests which are notoriously difficult for anyone to pass.

So how do you avoid being pulled over in the first place? Well, it helps to know exactly what the police are looking for. Most of them have been taught what driving symptoms indicate the relative probability that the driver is intoxicated. This training is usually based upon research conducted by the federal National Highway Traffic Safety Administration and reproduced in a booklet intended primarily for law enforcement, "Guide for Detecting Drunk Drivers at Night". The booklet contains a "DUI Detection Guide", which identifies the 19 most common and reliable initial indicators of drunk driving - along with the percentage probability that the driver exhibiting the symptom is, in fact, under the influence. The following is a list of the symptoms and the probability of intoxication. Thus, for example, the research indicates that "the chances are 65 out of 100" that a driver who is making a wide turn or straddling a lane line has a blood-alcohol concentration above the legal limit.

- Turning with Wide Radius 65
- Straddling Center or Lane Marker 65
- Appearing to be Drunk 60
- Almost Striking Object or Vehicle 60
- Weaving 60
- Driving on Other Than Designated Roadway 55
- Swerving 55
- Slow Speed (more than 10 miles per hour below limit) 50
- Stopping (without cause) in Traffic Lane 50
- Drifting 50
- Following too closely 45
- Tires on Center or Lane Marker 45
- Braking Erratically 45
- Driving Into Opposing or Crossing Traffic 45
- Signaling Inconsistent with Driving Actions 40
- Stopping Inappropriately (other than in lane) 35
- Turning Abruptly or Illegally 35
- Accelerating or Decelerating Rapidly 30
- Headlights Off 30

The NHTSA research indicates that symptoms are rarely seen in isolation; officers usually see a number of driving symptoms before pulling the suspect over. And the chances of a driver being intoxicated when multiple symptoms are observed can also be calculated: "When two or more cues are seen, add 10 to the highest value among the cues observed." For example, if the subject is observed to be weaving (60) and following too closely (45), there are 70 chances out of 100 that his blood-alcohol is above the legal limit.

Speeding, incidentally, is not a symptom of DUI. Because of the need for quicker judgment and reflexes, it may actually indicate sobriety. You may get stopped for speeding, but at least you won't have the officer approaching your car expecting to make a DUI arrest.

So if you're driving home late some night, keep in mind what the police are looking for -- and drive accordingly.