Turner-Fairbank Highway Research Center

 
 
 
 
« back to the list

Investigation of Appropriate Objects Stopping Sight Distance

Product Type

Other Publication

Author

Karen B. Kahl

Date

Dec-92

Abstract

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.

Link To Other Publication

Link not available.

Keywords

Stopping Sight Distance
Object Height
Accident Data Base
HSIS


« Back to the List

HSIS Summaries

HSIS Summary Reports are two to eight pages in length and include a brief description of the issue addressed, data used, methodology applied, significant results, and practical implications.

Read More

Research Reports

A variety of research studies have been performed using data from HSIS. Many of the final reports prepared are now available electronically.

Read More

Technical Summaries

Research reports are often summarized in executive summaries, technical briefs, or other abbreviated formats. Included here are those road safety summaries that involved research using HSIS data.

Read More

Safety Analysis Tools

In addition to conducting research, HSIS resources are also used to develop products that can be used by practitioners in the analysis of safety problems.

Read More

Other Projects

HSIS data are sometimes used in research studies that result in other types of finished products, such as dissertations, theses, and conference proceedings.

Read More