Fatigue can lead to both hallucinations and optical illusions. A hallucination is a perception of something that is not really there, as distinct from an optical illusion, which is a misinterpretation of something real. Hallucinations are entirely created by the brain.
An alert driver who is travelling in heavy rain with dipped beam headlights, on a curvy and hilly road in the dark or in heavy snow with full beam headlights risks experiencing an optical illusion. A fatigued driver who is driving under the same (or completely different) circumstances risks both experiencing an optical illusion and having a hallucination.
A really fatigued driver can start to hallucinate when driving on a straight road in daylight.
Alcohol and drugs can also lead to hallucinations.