Turner-Fairbank Highway Research Center

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Investigation of Appropriate Objects Stopping Sight Distance

Product Type

Other Publication


Karen B. Kahl




Stopping sight distance (SSD) is an integral part of highway design process because it is required at all points along the roadway. The current SSD model requires the driver to see an object in the roadway and stop before striking it. A four-inch object was used prior to the current six-inch object and the height was originally a compromise between excavation costs and the ability of the driver to see the roadway ahead. Currently, the justification for the six-inch object states that it is the lowest object that could create a hazardous; however, this definition represents a hypothetical hazardous situation. Accident history can best determine a realistic hazardous situation by revealing the type of objects that are encountered on the roadway, and the circumstances surrounding the accident. Three types of accidents were used from databases in to states: other object, animal, and evasive action accidents. These accident types represented situations where the driver was likely to encounter the SSD design condition: an unexpected hazard in the roadway. Small objects were encountered 0.07 percent of the time in reportable accidents. When the accident characteristics from these accidents were compared to all accidents it became apparent that light conditions were a major contributory factor. Many of these accidents occurred at night when the sight distance was limited to the headlight illumination distance. Increasing the vertical curve length would not provide additional safety in these cases. The roadway alignment in the accidents studied was primarily straight and level, therefore, AASHTO ’s assumption that the roadway geometry restricted sight distance to small objects was not substantiated. This research suggests that the object height and the accident conditions represented by the AASHTO design situation do not depict a realistic accident encounter. Perhaps, alternative factors such as the driver, the vehicle, visual angle, comfort and appearance need to be investigated to determine their appropriateness to the SSD model. These alternatives could provide a realistic situation for highway designers.

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Stopping Sight Distance
Object Height
Accident Data Base

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