# Physiological and Behavioral Changes of Passive Fatigue on Drivers during On-Road Driving
###### Abstract  
Driver fatigue can be further categorized into passive fatigue and active fatigue based on the task-induced fatigue perspective, with its categorization necessary from a theoretical basis and practical needs. Passive fatigue is caused by mental underload and inactive task engagement, which is considered more hazardous. To facilitate the construction of the driver monitoring system (DMS), the current study aims to investigate the physiological and behavioral changes of passive fatigue. A total of thirty-six participants completed a 90 min driving task on a monotonous highway, during which subjective fatigue level, eye tracking indicators, and driving dynamics were recorded using the Stanford Sleepiness Scale, Smart Eye Pro, and CAN Bus system. Results showed that drivers reported higher levels of fatigue as driving duration increased. An increase in pupil diameters and gaze dispersions were observed during the task. Drivers gradually reduced the control of the vehicle, in which faster speed and lower speed compliance were witnessed. In addition, a compensatory process was found as passive fatigue increased. Drivers tended to lower their standards to maintain the lateral position but recovered their lateral control when they lost control of the car speed. The current study emphasizes the importance of investigating active and passive fatigue of drivers independently, and the unique physiological and behavioral changes accompanied by passive fatigue should be considered in designing driver monitoring systems.  
on-road study; driver fatigue; passive fatigue; eye tracking; gaze dispersion; driving  
on the characteristics of the tasks that operators engage in. Active fatigue is often caused by continuous tasks which impose a high cognitive workload and require frequent responses to respective circumstances, whereas passive fatigue is induced by prolonged tasks in which operators act as a supervisor of the system with little effort exerted [4]. The former was accompanied by mental overload and distress due to the engagement in intensive tasks, while the latter corresponded to mental underload and inactive task engagement [5].  
In the driving scenarios, factors such as dense traffic, poor visibility, and multi-tasking might lead to active fatigue, while passive fatigue might be related to monotonous highways, automated systems, and extended driving duration [6]. Yet, it is more valuable to explore passive fatigue of drivers in modern driving-related research. Firstly, compared with active fatigue, passive fatigue usually causes sleepiness due to its monotonous nature, which is believed to be more hazardous than the depletion of attentional resource without sleepy symptoms [3]. Secondly, with the continuous development of vehicle automation, passive fatigue may occur more frequently in future driving experiences. For instance, the impending conditional automation driving (SAE Level 3) vehicles will be equipped with systems that undertake the primary role during the driving task, where drivers will only be required to provide assistance when take-over requests are raised [7]. In general, with the development of autonomous vehicles, the advanced systems will greatly reduce drivers' workload and potentially relieve the experience of active fatigue; however, it will also create a more monotonous situation, leading to an increase in passive fatigue [8]. As the above, the present study focused on assessing passive fatigue of drivers.