We analyzed drivers’ braking behavior to minimize over-dependence on the system in the design of the Forward Collision Avoidance Assistance System. There are various factors that influence over-dependence on the system. We focused on the braking algorithm to minimize interference between driver operation and system actuation in this study. It was proven that all subjects initiated the braking operation in advance of system actuation when the system was actuated at a Time to Collision of 1.7s or less and when the car approached a stationary obstacle at 60km/h. We propose that Time to Collision is an effective state variable for analyzing the dependence level on the system, and that a Time to Collision setting of less than 1.7s is optimal for the onset timing to minimize over-dependence on the system.