Most traffic enforcement officers are trained in accord with National Highway Transportation Safety Administration (NHTSA) standards, which has compiled a list of certain “driving patterns” to alert police of potential drunk drivers. Indeed, the NHTSA has ranked the “symptoms” of drunk driving, in terms of the driving patterns a DUI driver is most likely to show, as follows:
65%: making a turn with a wide radius; straddling a center or lane marker.
60%: nearly striking an object or another vehicle; weaving
55%: driving outside the designated roadway; swerving
50%: drifting; driving more than 10% below the posted speed limit; stopping in traffic for no reason
45%: tailgating; driving with tires on center lane marker; erratic braking; driving into oncoming traffic
40%: incorrect signaling
35%: slow response to traffic signals; inappropriate stops (not in traffic); abrupt or illegal turns
30%: sudden acceleration or deceleration; driving without headlights after dark.
Also listed in the 60% category is “appearing to be drunk,” though NHTSA does not explain what that means.
Some of these behaviors are obviously illegal and would result in traffic stops in any event. In many cases, the DUI investigation proceeds from that point based on the appearance, statements and behavior of the driver. But even legal behavior, such as driving 10% under the speed limit or making an unusually wide turn, can trigger police attention, especially in conjunction with other “symptoms,” and they will often find a stop-worthy traffic infraction to pursue the DUI investigation.