Stopping sight distance is a term used in highway design. It is defined as "the length of roadway ahead visible to the driver."

Sight distance

Sight distance, in the context of road design, is how far a road user (usually a vehicle driver) can see before the line of sight is blocked by a hill crest, or an obstacle on the inside of a horizontal curve or intersection. Insufficient sight distance can have implications for the safety or operations of a roadway or intersection.

During highway design and traffic safety investigations, highway engineers compare the available sight distance to how much sight distance is needed for the situation. Depending on the situation, one of three types of sight distances will be used.

Stopping sight distance

Stopping sight distance is the distance traveled while the vehicle driver perceives a situation requiring a stop, realizes that stopping is necessary, applies the brake, and comes to a stop

Braking distance

Braking distance refers to the distance a vehicle will travel from the point where its brakes are fully applied to when it comes to a complete stop...

. Actual stopping distances are also affected by road conditions, the mass

Mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 of the car, the incline

Incline

Incline, inclined, inclining, or inclination may refer to:* Incline, California* Inclined plane, a flat surface whose endpoints are at different heights* Inclined orbit, an orbital plane is tipped away from the equator...

 of the road, and numerous other factors. For design, a conservative distance is needed to allow a vehicle traveling at design speed to stop before reaching a stationary object in its path. Typically the design sight distance allows a below-average driver to stop in time to avoid a collision. In the United States, the driver's eye is assumed to be 42 inches above the pavement, and the object height is 24 inches (about the height of vehicle taillights). For bicycle facilities, the cyclist's eye height is assumed to be at 1.4 m (4.5 ft), and the object height is 0 inches, since a pavement defect can cause a cyclist to lose control.

Decision sight distance

Decision sight distance is used when drivers must make decisions more complex than stop or don't stop. It is longer than stopping sight distance to allow for the distance traveled while making a more complex decision. The decision sight distance is "distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its threat potential, select an appropriate speed and path, and initiate and complete the required maneuver safely and efficiently. Ideally roads are designed for the decision sight distance -- typical times detecting and understanding a situation is about 6 to 10 seconds and 4 to 5 seconds to perform the right maneuver.

Intersection sight distance

Intersection sight distance is the decision needed to safely proceed through an intersection. The distance needed depends on the type of traffic control at the intersection, and the maneuver (left turn, right turn, or proceeding straight). All-way stop

All-way stop

An all-way stop is an intersection system used predominantly in the United States of America, Canada and South Africa where traffic approaching it from all directions is required to stop before proceeding through the intersection...

intersections need the least, and uncontrolled intersections require the most.