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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

REVISED DESIGN PARAMETERS FOR STOPPING SIGHT DISTANCE

The geometric design process of a highway must provide an ample amount of stopping sight distance (SSD) at all points along the roadway for safe and comfortable traffic operations. The current SSD model used in the German policy on geometric design (RAS-L 1995) is based on a critical situation in which the driver detects a stationary object in the roadway, recognizes it as a hazard, and stops before striking it; evasive actions which occur often in real-world situations are not applied. Some of the variables and assumptions used in the SSD model are older than 20 years and are questioned because they no longer appear to represent realistic conditions. One of these questionable variables is the coefficient of friction between vehicle tire and wet pavement in relation to the operating speed. The SSD model is based on friction surveys in the 70s with the PIARC-Tire (blocked wheel measurement). The combination of modern vehicle tires, anti-lock-braking systems and modern road surfaces assumes to result in higher coefficients of friction and shorter braking distances than those used in the analytical model. The objective of the research sponsored by the Federal Highway Research Institute (BASt) was to investigate and quantify the possible reductions of SSD values without reducing traffic safety. First, the theoretical foundations of international policies are discussed. In braking studies on a crest vertical curve the average deceleration was determined. In friction surveys the PIARC-Tire was compared with a modern vehicle tire. A detailed macroscopic examination of object-related accidents was performed to provide security for the results of the braking studies and friction surveys. The results of the investigation show that the PIARC-Tire can no longer represent the characteristics of a modern vehicle tire. Furthermore, the average decelerations in stopping sight situations on a crest vertical curve are much higher than those estimated by the model. These findings suggest that the design values for SSD can be reduced without a notable handicap for traffic safety. The authors recommend the use of a constant average deceleration of 4.5 m/sq s and a perceive and reaction time of 2.0 s for the analytical calculation of the minimum SSD; the 1.3 times value of the minimum SSD should be ensured on 70% of the roadway to prevent a continuous high strain of the driver. The minimum SSD values must not be used at critical points like intersections. The reduced SSD values imply also the use of smaller minimum parameters for the crest vertical curves. The revised design parameters are recommended for use in the German design policy on geometric design. Accordingly the reduction of SSD could affect economical and ecological benefits: smaller minimum parameters for crest vertical curves decrease the road construction costs and the prejudice to the environment can be minimized (lower cut slopes).

  • Supplemental Notes:
    • The publisher's German name is Forschungsgesellschaft fur Strassen- und Verkehrswesen (FGSV).
  • Corporate Authors:

    Road and Transportation Research Association

    Postbox 50 13 62
    D-50973 Cologne,   Germany 
  • Authors:
    • Durth, W
    • Bernhard, M
  • Conference:
  • Publication Date: 2000-6

Language

  • English

Media Info

  • Features: Figures; Photos; References; Tables;
  • Pagination: p. 410-421

Subject/Index Terms

Filing Info

  • Accession Number: 00794796
  • Record Type: Publication
  • Report/Paper Numbers: FGSV 002/67
  • Files: TRIS
  • Created Date: Jun 27 2000 12:00AM