 In order to manage speed, you need to understand the four factors involved in stopping a vehicle. Perception distance is the distance a vehicle travels from the time you see a hazard until your brain recognizes it. The perception time for an alert driver is approximately 3 fourths of a second. Reaction distance is the distance a vehicle travels from the time your brain tells your foot to move from the accelerator until your foot hits the brake pedal. The average driver has a reaction time of 3 fourths of a second.