1. Perception distance, reaction distance, and braking distance
Total stopping distance is a combination of perception distance, reaction distance, and braking distance.
More info:
Total Stopping Distance is the sum of the perception distance, reaction distance and braking distance.
Once a driver find a need to slow down or stop, a small amount of time passes.
In short, it is the distance your vehicle travels from the time you see a hazard and press on the brake until the vehicle stops.