Evaluation of Stopping Sight Distance Design Criteria

Glennon, John C. · 1969 · ROSA P / Texas Transportation Institute. Texas A&M University

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Summary

This 1969 report by John C. Glennon evaluates the validity of stopping sight distance (SSD) design criteria established in the 1965 American Association of State Highway Officials (AASHO) Policy. Motivated by concerns that standards based on data from 1934–1953 were outdated for modern high-speed highways, the study reviews the fundamental assumptions regarding driver perception-reaction time, friction factors, assumed speeds, and geometric measurements. The methodology involved a comprehensive review of existing literature and an analysis of empirical data, including skid trailer measurements from 500 pavements and 3,900 stopping distance tests. The evaluation specifically assessed whether AASHO’s design values—such as the 2.5-second perception-reaction time and specific friction coefficients—represented critical safety levels or merely average conditions. The findings indicate significant deficiencies in the existing standards. First, the 2.5-second perception-reaction time, derived from subjective extrapolations of laboratory studies, is likely insufficient for high-speed roadways where visual acuity degrades. Second, analysis of skid trailer data revealed that AASHO’s design friction factors correspond to only the 35th percentile of pavement conditions, meaning 35% of tested pavements could not provide adequate stopping distance. Third, the AASHO “assumed speed for conditions” lacks an objective basis; the report argues that wet pavement conditions should govern design, as friction is lower, rather than using arbitrary reductions in speed. Fourth, the standard 6-inch object height used for measuring sight distance on vertical curves is criticized as arbitrary and unrelated to operational safety needs, with the report suggesting a range of 0.1 to 0.3 feet or zero height for maximum safety. Finally, the study highlights a critical gap in designing for nighttime SSD on two-lane highways, noting the need to account for opposing vehicle headlights. The report concludes with recommendations to adopt a head-on collision criterion for two-lane highways, using friction factors representing the 15th percentile of pavement conditions and adjusted perception-reaction times for higher speeds. For multilane highways, it recommends retaining the stationary object criterion but using more conservative friction values. The study calls for further research into actual highway perception-reaction times and the relationship between design speed and critical driving speeds in wet conditions.

Key finding

AASHO design friction factors represented the 35th percentile of pavement conditions, meaning 35 percent of pavements could not provide adequate stopping distance to meet minimum stopping sight distance standards.

Methodology

review

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• visibility analysis litigation
• road geometry
• perception reaction time
• speed distance perception