# Driving Behavior during Takeover Request of Autonomous Vehicle: Effect of Driver Postures
## 1 Introduction  
There has been much recent research and development in the field of automated driving technology. Its practical application is expected to reduce the number of traffic accidents caused by human carelessness. For example, there were 4863 traffic accidents on highways in Japan in 2021, with 136 fatalities; of these accidents, 43% occurred owing to inattention to the road ahead, 21% owing to inattention to motion, and 14% owing to failure to see the other vehicle due to insufficient safety checks [1]. In this context, it is expected that automated driving will not only reduce the number of accidents caused through driver carelessness but also reduce the number of road deaths associated with these accidents.  
The Society of Automotive Engineers (SAE) International defines six levels of autonomous driving systems, of which level 3 involves a change of control from the system to the driver under certain conditions [2]. In level 3 automated driving, the system makes a takeover request (TOR) which requires the driver to transition to manual operation when automatic driving becomes difficult because of functional limitations of the system or when the system is operating outside its design domain. The driver must be in a driving position to perform appropriate driving operations promptly when the TOR is made. The National Highway Traffic Safety Administration (NHTSA)'s guidelines for automated vehicles also state that for level 3 automated driving, the driver must be able to take control of the vehicle when notified by the vehicle [3]. Therefore, research is being conducted on the method and timing of TORs [4; 5; 6]. Additionally, there are different studies into ways of monitoring the driver's condition using a driver monitoring system [7; 8; 9].  
According to Japan's Road Traffic Law Amendment Act, drivers may use cell phones during level 3 automated driving. However, they are required to be able to immediately perform appropriate operations when there is a transition from automated to manual driving [10]. Tasks unrelated to driving operations can affect driving behavior [11; 12]. However, there are no regulations covering specific driver postures or the systems used to detect them, either in Japan's Road Traffic Law Amendment Act or in Automated Lane Keeping Systems Regulation No. 157 of the United Nations Economic Commission for Europe (UNECE) World Forum for Harmonization of Vehicle Regulations (WP. 29) [13].  
There have been tests of driver reaction times in different road conditions and in different situations, and studies on the effect of driver eye movements on reaction time [14; 15]. There are also studies on driver posture [16; 17]. However, few studies have investigated the relationship between driver posture and driver reaction time.  
[PERSON] et al. [18] studied the effects of four factors on transition time: the backrest angle, seat position, foot position, and arm position. The results showed that the driver's foot position had the biggest effect on the braking reaction time. It was also found that the driver's chin being placed on their hand had the biggest effect on the steering reaction time. Therefore, the relationship between driver posture and response to a TOR needs to be clarified.  
In this study, we focused on driving posture and conducted experiments using a driving simulator for a level 3 autonomous vehicle. The purpose of this study was to investigate the effect of driver posture before a TOR on driver reaction after the TOR. The driver's reaction included braking to avoid a collision with obstacles after the driving transition, and steering wheel operation to maintain the vehicle in its lane. A total of six different postures were set based on combinations of two types of upper-body posture and three types of foot posture.