# Driving Behavior during Takeover Request of Autonomous Vehicle: Effect of Driver Postures
## 4 Discussion  
### Reaction Time  
Comparing the forward-BFF (0.80 s) and sideways-BFF (1.03 s) postures, the forward-CL (1.22 s) and sideways-CL (1.47 s) postures, and the forward-S (1.52 s) and sideways-S (1.51 s) postures, it was found that the braking response was slower in the sideways position. This could be because the drivers tended to be conscious of steering in the sideways position in a situation in which it was difficult to shift to the steering action itself. In fact, 10 of the 20 participants (50%) showed slower braking responses in the sideways posture compared with the forward posture for each of the foot postures. The results obtained in this study are consistent with those reported by [PERSON] et al., which showed that 29 out of the 36 participants avoided a collision due to the sudden appearance of an obstacle by steering only when the TOR warning was sounded [15]. This suggests that the driver may prioritize steering in TOR warning situations.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Foot Position**} & \multicolumn{2}{c}{**Forward**} & \multicolumn{2}{c}{**Sideways**} & \multirow{2}{*}{**\(p\)-Value**} \\  & **Distance from the Ego** & & & & **Distance from the Ego** & \\  & **Vehicle to the Obstacle [m]** & & & & **Vehicle to the Obstacle [m]** & \\ \hline BFF & 70.2 & 3.3 & 59.8 & 3.1 & 0.013 * \\ CL & 61.0 & 4.1 & 52.6 & 3.7 & 0.016 * \\ S & 57.4 & 4.7 & 48.1 & 3.1 & 0.020 * \\ \hline \hline \end{tabular}
\end{table}
Table 7: Distance from the ego vehicle to the obstacle.  
Figure 6: Leg posture comparisons of distance from the ego vehicle to the obstacle. **\(p\) < 0.01/***\(p\) < 0.001.  
In both the forward and sideways postures, the steering reaction time was slower in the S posture (forward: 1.19 s; sideways: 1.51 s) than in the CL posture (forward: 1.01 s; sideways: 1.29 s), and slower in the CL posture (forward: 1.01 s; sideways: 1.29 s) than in the BFF posture (forward: 0.94 s; sideways: 1.2 s). Comparing the forward-BFF (0.94 s), CL (1.01 s), and S (1.19 s) postures with the sideways-BFF (1.20 s), CL (1.29 s), and S (1.51 s) postures, respectively, shows that the S posture significantly delays the steering reaction time. This suggests that when the driver was in a situation in which it was difficult to shift to braking, the steering wheel grasp may also have been delayed. These results suggest that the condition of the driver's upper body may have affected the steering as well as the braking. Similarly, the driver's foot posture affected not only the braking, but also the steering.  
In fact, the steering reaction times for the sideways-BFF and the forward-S postures were similar. Therefore, even if the upper body is forward-facing, the amount of influence on the steering reaction time is equivalent to the sideways posture (BFF) if the subject is in an S posture.  
Braking reaction times were compared with the results from previously reported experiments [14]. In the compared experiments, the driver's collision avoidance was mostly limited to braking because of the difficulty of steering, due to vehicles in the adjacent lane. The median brake reaction time for collision avoidance ranged from 0.81 s to 1.15 s, depending on the distance from the vehicle ahead (10-50 m). In this experiment, the median braking reaction time ranged from 0.79 s to 1.67 s, indicating that the effect of driving posture on the reaction time was greater than in other experimental conditions.