Stopping sight distance is a term used in highway design. It is defined as "the length of roadway ahead visible to the driver."[1]
Contents |
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 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. Actual stopping distances are also affected by road conditions, the mass of the car, the incline 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).[1] 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.[2]
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. [1] 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 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 intersections need the least, and uncontrolled intersections require the most.