The total stopping distance is the sum of the perception-reaction distance and the braking distance.
Braking distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. The braking distance is one of two principal components of the total stopping distance.
Reaction distance: Due to the delay in human reaction, however, there is always a time lag between seeing a hazard and braking the car. This time interval is known as reaction time, which is vary widely for different persons. During the reaction time the car travels a distance known as reaction distance.