Which of the following is a common sign of driver fatigue?

Frequent yawning is a well-recognized sign of driver fatigue because yawning is a physiological response to decreased levels of alertness and drowsiness. When a person is feeling tired, the body attempts to increase oxygen intake and promote alertness by eliciting yawning. This behavior often indicates that the driver needs rest, as it reflects a struggle to maintain wakefulness and focus on the task at hand.

In contrast, increased focus and alertness would suggest that the driver is not fatigued. Rapid heart rate is not directly correlated with fatigue; instead, it could result from stress, anxiety, or physical exertion. Increased speed might indicate reckless driving behavior that could result from fatigue but is not a direct symptom of it. Hence, frequent yawning stands out as a clear, observable sign that a driver is at risk of being fatigued and could benefit from a break.