When should cruise control not be used?

Cruise control should not be used when you are tired because it can lead to decreased attention and slower reaction times. When you're fatigued, your ability to respond to sudden changes in traffic conditions diminishes. Using cruise control in such a state may result in a lack of engagement with the driving task, increasing the risk of an accident. Maintaining full control of the vehicle and staying alert is crucial for safe driving, and being tired undermines both of these factors. In contrast, using cruise control in other situations, like heavy traffic or under windy conditions, while also not advisable, does not directly relate to the driver's state of alertness and responsiveness compared to the fatigued state.