Total stopping distance is the sum of your perception distance (how far the vehicle goes from when you see the hazard until your brain processes it), reaction distance (how far the vehicle goes from when your brain orders your foot to take action until your foot actually starts to brake), and braking distance (how far the vehicle continues to travel once you apply the brake). If your vehicle is equipped with air brakes, there is an additional lag of 0.4 seconds from the time you apply the brake pedal and the time that the brakes engage. The lag distance is the distance your vehicle travels during this lag, and it must be added to the total stopping distance.