Establishing drivers' trust in the automated driving system is critical to the success of automated vehicles. The focus of this paper is learning what drivers of automated vehicles need to feel confident during braking events. In this study, 10 participants drove a test vehicle and each experienced 24 different deceleration settings. Prior to each drive, it was indicated to each participant what the expected brake starting and stopping positions would be. During each drive, participants maintained a set speed, and then stopped the vehicle when they saw a signal to apply the brakes. After each drive, the participants were asked what their perceived safety level was during the deceleration setting they just experienced. The results revealed that ‘jerk’ movements have significant influence on drivers' perceived safety. For this study, we have named this jerk movement impression jerk (IJ). Using IJ, clearly divides the secure and anxious feelings of the drivers along with individual differences.