 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. Brake lag distance also needs to be taken into account. When operating a vehicle with air brakes, it takes about half a second for the mechanical operation to take place.