What is 'stopping sight distance' (SSD)?

Stopping Sight Distance (SSD) is defined as the total distance a vehicle needs to come to a complete stop before hitting an object on the roadway. This distance incorporates both the reaction time of the driver and the braking distance.

When a driver perceives a need to stop—due to an obstacle, another vehicle, or a traffic signal—the reaction time is the time it takes for the driver to recognize the situation and begin applying the brakes. This reaction time contributes to the distance traveled during this period, known as the perception-reaction distance.

Once the brakes are applied, the vehicle continues to travel until it comes to a full stop, which is determined by the vehicle's speed, the road surface conditions, and the braking efficiency. This distance is referred to as the braking distance.

Together, the perception-reaction distance and the braking distance sum up to form the total stopping sight distance, which is crucial in road design to ensure that drivers have adequate visibility and distance to safely stop their vehicles. This is essential in preventing accidents and enhancing road safety.

The other options describe different concepts related to vehicle dynamics and road design but do not accurately define stopping sight distance. For instance, the distance a vehicle travels during an emergency does not consider the full context