The National Highway Traffic Safety Administration (NHTSA) has produced a pocket-size booklet intended primarily for law enforcement entitled Guide for Detecting Drunk Drivers at Night, DOT HS-805-711 (available free of charge from NHTSA, Administrative Operations Division, Room 4423, 400 Seventh Street, N.W., Washington, DC 20590). The booklet, based on NHTSA sponsored research, contains a DUI Detection Guide, which identifies the nineteen most common and reliable initial indicators of drunk driving — along with the probability that the driver exhibiting the symptom is, in fact, under the influence.
The following is a list of the symptoms and their related indicia of intoxication. Thus, for example, the research indicates that "the chances are 65 out of 100" that a driver who is straddling a center or lane marker has a blood-alcohol concentration of .10 percent or higher.
| 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 |
It may come as a surprise to jurors that, for example, 40 out of 100 drivers "appearing to be drunk" to police officers are, in fact, not under the influence. Similarly, 40 out of 100 drivers who are weaving or who almost strike another vehicle are also legally sober.
The report notes that symptoms are rarely seen in isolation; officers usually see a number of driving symptoms before pulling the suspect over. The NHTSA research indicates that 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 he has a BAC of .10 percent or more.
California DUI defense lawyers can come to some rather striking results by using this approach in cases where the officer has testified to particularly bad driving symptoms. Consider, for example, where the officer testifies to having observed all of the following in the defendant before pulling him over: The suspect traveled at 25 miles per hour when the speed limit was 55 miles per hour, left his headlights off, weaved in traffic, crossed the center divider four times, straddled the lane marker for a quarter of a mile, appeared to be drunk, and upon being pulled over, almost sideswiped a parked car.
On cross-examination, the NHTSA research figures can be used to show that 30 out of 100 individuals exhibiting these symptoms are not under the influence (i.e., add 10 percent to the highest figure of 60 percent, giving a figure of 70 percent).