A system for determining the shortest time needed for a driver to switch the foot from the accelerator pedal to the brake pedal in case of emergency and to push on the brake pedal. When the danger-imitating signal lamp is ignited, preferably, at random, the first photo-receiver receives the light signal and sends it to the signal amplifier, wherefrom the amplified signal is sent to the computer via the time counter. The computer begins to register the length of the light signal. As soon as the driver reacts to the light signal of the signal lamp, he or she moves his or her foot from the accelerator pedal to the brake pedal and pushes on the brake pedal, thus igniting the brake lights. Thus, the time interval between the moment of initiation of the first photo-receiver and the initiation of the second photo-receiver corresponds to the time of the driver's response to the light signal that imitates a danger plus time of transfer from the accelerator pedal to the brake pedal. The above-described test is repeated several times at different positions of the under-thigh support to find the position most optimal for the shortest braking time.