Stopping sight distance (SSD) is an integral part of the highway design process because it is the minimum sight distance required at all points along the roadway. The current SSD model uses a critical sight distance situation in which the driver detects a small object in the roadway, recognizes it as a hazard, and stops before striking it. Some researchers have questioned the assumptions and variables used in the SSD model because they do not appear to represent a realistic, real-world situation when combined in the existing model. The 150-mm (6-in.) object is one of these questionable variables because this height was not based on the probability of encountering such an object in the roadway environment. The objective of this research was to investigate the characteristics of objects encountered in the roadway that represent a realistic hazard for the driver. A detailed examination of accident data was performed to evaluate the characteristics. Three types of accidents in two states were studied: other-object, animal, and evasive action. The study results showed that only 0.07% of the reportable accidents involved small objects in the roadway. More than 90% of these objects accidents occurred at night on straight, flat roadways (conditions in which sight distance is not limited by the roadway's geometry), and they did not result in serious injuries. These findings suggest that accidents with small objects are neither frequent enough nor severe enough to justify their use as the critical situation in the SSD model. The authors recommend that the object height used in the design should represent the smallest object that poses a hazard to the driver: the minimum legal taillight height of a vehicle.


  • English

Media Info

  • Features: References; Tables;
  • Pagination: p. 25-30
  • Monograph Title: Geometric design, roadside safety features, roadside hardware monitoring, and scenic loop tours
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00714988
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Dec 26 1995 12:00AM