What is stopping sight distance and how is it calculated?

Stopping sight distance (SSD) is the minimum distance required for a driver to see an obstacle, react, and bring the vehicle to a complete stop. SSD = reaction distance + braking distance = V*t + V^2/(2*g*f), where V = speed, t = perception-reaction time (2.5 seconds), g = gravity, f = friction coefficient. SSD increases with speed and decreases with better braking friction. It governs vertical curve design, horizontal clearance, and obstacle-free zones.