- Individual characteristics including medical conditions
- Internal body clock
- Lack of sleep or poor sleep
- What causes driver fatigue?
A task is monotonous when its stimulants don't change or the changes are predictable or there is a high level of repetition. Suburban highways where road environment changes are limited and there is a small volume of traffic match this definition. O'Hanlon and Kelly  pointed out that driving on a monotonous road is equal to a vigilance task, thus driver vigilance decrement is an expression of fatigue. Thiffault & Bergeron  found that in a monotonous driving situation, driver steering wheel movement is greater and occurs more often, showing that the fatigue effect and effect on driver vigilance caused by a monotonous road situation is relatively large. Also, driving on a relatively long and monotonous driving environment also has a clear negative effect on driver valid peripheral visual field .