What does fatigue primarily affect in terms of safe driving?

Fatigue affects longer reaction time primarily because it impairs cognitive function and slows down the brain's processing speed. When a driver is fatigued, their ability to respond swiftly to surrounding stimuli diminishes. This can lead to slower reflexes when needing to make quick decisions, such as braking or steering to avoid a hazard. Research indicates that drivers who are fatigued may take longer to perceive a situation and react appropriately, significantly increasing the likelihood of accidents.

In contrast, while vision clarity, judgment accuracy, and speed control can also be negatively affected by fatigue, the immediate consequence of slowed reaction time is more critical in the context of driving safety. A driver might still have decent visibility or judgment when fatigued, but their delayed response can have dire consequences if a sudden situation arises on the road.