Reaction time

Reaction time is the interval between a stimulus occurring and a person responding. In a ruler-drop test, the stimulus is the ruler beginning to fall. Receptors in the eyes detect its movement, information travels to the brain, and the brain sends impulses to the muscles that close the fingers.

There is a delay between noticing the hazard and responding by pressing the brake pedal. This delay is the driver's reaction time. Typical human reaction times are about 0.2–0.9 seconds, although they vary between people and with the conditions of the test.