Which of the following is NOT a reason to stop a driver for a suspected DUI?

The correct choice highlights that maintaining a consistent speed within the posted limit does not provide sufficient grounds for a DUI traffic stop. Law enforcement is trained to identify specific driving behaviors that may indicate impairment, and simply driving at or below the speed limit does not suggest any level of impairment or dangerous driving.

In contrast, the other options describe behaviors that are often associated with impaired driving. For instance, driving without headlights at night can indicate a lack of awareness or control, while slow response to a stop sign might suggest that the driver is not fully attentive. Sudden lane changes without signaling can reflect erratic driving patterns typically associated with someone who may be under the influence. Therefore, the presence of these traffic violations justifies an officer's decision to initiate a traffic stop for suspected DUI.