Stopping sight distance

Stopping sight distance

Stopping sight distance is a term used in highway design. It is defined as "the length of roadway ahead visible to the driver."American Association of State Highway and Transportation Officials (1994) "A Policy on Geometric Design of Highways and Streets" (pp. 117-118)]

ight distance

Sight distance, in the context of road design, allows a vehicle traveling at design speed to be able 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.

topping sight distance

Stopping sight distance is the sum of the brake reaction distance (the distance traveled between the time the driver sees an obstruction to when the brakes are applied) and the braking distance (the distance traveled while braking the vehicle 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.

Decision sight distance

The decision sight distance includes an additional distance -- the distance traveled while deciding that what is seen is a hazard and deciding what to do. 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.

Notes and references


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