## 1 Introduction  
Autonomous cars are expected to become a new solution for safety, environmental, and traffic-related problems. Further, it is an important technology related to all core industries in the 4 th industrial revolution, including the sharing economy, artificial intelligence, and the Internet of Things [1]. The popularization of autonomous vehicles combined with electric or fuel cell technology is expected to play a key role in efficient energy consumption and the construction of sustainable eco-friendly smart cities [2]. According to the SAE J3016 standard, autonomous driving can be classified into six levels: Level 0 (completely manual) to Level 5 (completely autonomous); the classification criteria include the driving responsibilities of the driver and driving support function level of the system [3]. Up to Level 2, the driver must always supervise vehicle control. At Level 3, the autonomous driving system can temporarily take over the control of the vehicle under certain conditions. Nevertheless, at Level 3, the driver must be able to take control at any time while keeping an eye on the driving situation, and the autonomous driving system must determine whether the driver can start driving [4]. Recently, UNECE/WP.29, the international vehicle safety standard-setting organization (International Conference on Harmonization of Vehicle Standards), announced the world's first binding international regulation on Level 3 autonomous driving, which is summarized below [5]:  
* Automated lane-keeping system (ALKS) definition and activation criteria
* System stability, including failsafe reaction
* Driver availability recognition system and human-machine interface (HMI)
* Autonomous driving system event recording and cyber security  
California, U.S.A., is one region where research on autonomous driving is actively conducted. There have been licensed autonomous driving tester programs on public roads since 2014, and an unmanned autonomous driving test program has been operated since 2018 [6]. As of 25 February 2021, there were a total of 56 autonomous-vehicle testing permit holders and seven autonomous-vehicle driverless testing permit holders registered with the California Department of Motor Vehicles (DMV). Every year, both types of permit holders must track and report the number of times their vehicles needed to disengage from the autonomous mode during the tests. The top-level group drove about 1200 miles per disengagement in 2015 [7]; in 2020, they drove more than 23,000 miles per disengagement on average [8]. However, even in 2020, the autonomous driving system still frequently failed to respond except for the top group (on average, they drove 80 miles per disengagement) (Figure 1). The top causes for this were system failure (52%), driver factors (30%), and external environmental factors (11%) [9]. Takeover control between the system and the driver is one of the most critical functions in terms of the safety of a highly automated vehicle. The first step in this process is determining how ready the driver is to start driving [10]. The driver monitoring system (DMS) has been included as a primary safety item in Euro NCAP's 2025 Roadmap [11]. Research using heart rate [12], respiration rate [13], and gaze response [14,15] is in progress to measure the cognitive load, distraction, and drowsiness of a person in the driver's seat. The approach in which sensors are attached to the driver's body decreases accuracy due to movement and causes inconvenience when driving. Although these shortcomings are being addressed [16], a monitoring system using a camera has been commercialized. The takeover process proceeds with non-driving-related tasks (NDRTs), a disengagement scenario, and warnings raised through HMI as shown in Figure 2[17,18]. Quantitatively evaluating the situation wherein the driver concentrates on other tasks in the autonomous driving mode is essential.