Human Factors Guidelines for Road Systems: Second Edition (2012) / Chapter Skim
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Pages 27-45
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Key Components of Sight Distance .
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KEY COMPONENTS OF SIGHT DISTANCE Introduction Sight distance (SD) is the distance that a vehicle travels before completing a maneuver in response to some roadway element, hazard, or condition that necessitates a change of speed and/or path.
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Discussion Before drivers can execute a maneuver, they must first recognize that some action is required and decide what that action should be. Therefore, this mental activity -- perception, cognition, and action planning -- precedes an overt vehicle control action and takes some amount of time.
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DETERMINING STOPPING SIGHT DISTANCE Introduction Stopping sight distance (SSD) is the distance from a stopping requirement (such as a hazard)
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Discussion The PRT stage is significantly influenced by visibility conditions. In particular, the distance at which drivers can see an unilluminated, unreflectorized hazard depends on their headlights, their sensitivity to contrast, and their expectation of seeing the hazard.
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DETERMINING INTERSECTION SIGHT DISTANCE Introduction Providing stopping sight distance at intersections is fundamental to intersection operation. In addition, drivers also require an unobstructed view of the entire intersection, including any traffic control devices, and sufficient lengths along the intersecting highway to permit the driver to anticipate and avoid potential collisions with other vehicles.
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Discussion The two types of sight triangles used in calculating ISD are described below. Approach Sight Triangles: According to AASHTO (1)
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DETERMINING WHEN TO USE DECISION SIGHT DISTANCE Introduction According to AASHTO (1, page 3-6) , decision sight distance (DSD)
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Discussion Because some driving situations are particularly challenging (e.g., merging in moderate traffic during a lane drop) , drivers require additional time to plan and execute the necessary maneuvers, or additional "safety margin" to compensate for errors they may make in the process.
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DETERMINING PASSING SIGHT DISTANCE Introduction According to AASHTO (1) , passing sight distance (PSD)
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Discussion The PSD encompasses both a PRT and an MT component. Mean PRTs to initiate a pass, measured from when PSD was available until when the right tire crossed the centerline, have been found to vary from 3.6 to 6.0 s, depending on the particular site on two-lane rural highways( 2)
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INFLUENCE OF SPEED ON SIGHT DISTANCE Introduction Although posted speed has been found to have the strongest association with operating speed, some visual aspects or driving-task demands associated with the roadway environment can "unconsciously" influence drivers' speed choice. Consequently, if operating speeds on a roadway significantly exceed design speed, sight distances on that roadway may be inadequate.
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Discussion The design of a road affects drivers' speeds through two major mechanisms. First, the design creates the driving task.
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KEY REFERENCES FOR SIGHT DISTANCE INFORMATION Introduction Sight distance requirements, issues, and subtopics have been covered extensively in a range of standard reference sources for roadway design and highway. It is important for roadway designers and traffic engineers to recognize that most of the information presented in this chapter has been adopted from these other sources and for users of this HFG to know where to go to find alternative sources of sight distance information.
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Discussion The HFG focuses on key aspects of sight distance from the roadway users' perspective and is not intended to provide a comprehensive or definitive presentation of sight distance. Additional data sources follow: A Policy on Geometric Design of Highways and Streets (1)
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WHERE TO FIND SIGHT DISTANCE INFORMATION FOR SPECIFIC ROADWAY FEATURES Introduction The following table lists the information required to diagnose sight distance for specific roadway features. Although the roadway designer and the traffic engineer work with distances, sight distance needs actually originate from driver MT needs and speed choice.
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Discussion The sight distance diagnostic procedure consists of a systematic on-site investigation technique to evaluate the highway environment to support sight distance needs. The highway location is surveyed, diagrammed, and divided into component sections based on specific driving demands (e.g., need to perform a specific maneuver)
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WHERE TO FIND SIGHT DISTANCE INFORMATION FOR INTERSECTIONS Introduction The following table lists the information required to diagnose sight distance at various intersection types. Although the roadway designer and the traffic engineer work with distances, sight distance needs actually originate from driver MT needs and speed choice.
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Discussion The sight distance diagnostic procedure consists of a systematic on-site investigation technique to evaluate the highway environment to support sight distance needs. The highway location is surveyed, diagrammed, and divided into component sections based on specific driving demands (e.g., need to perform a specific maneuver)
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