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Sight Distances

Sight Distances. Distance a driver can see ahead at any specific time Must allow sufficient distance for a driver to perceive/react and stop, swerve etc when necessary. Important Sight Distances. Stopping Decision Passing Intersection Crossing RR. GB Question.

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Sight Distances

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  1. Sight Distances Distance a driver can see ahead at any specific time Must allow sufficient distance for a driver to perceive/react and stop, swerve etc when necessary

  2. Important Sight Distances • Stopping • Decision • Passing • Intersection • Crossing RR

  3. GB Question • Sight distance assumes drivers are traveling at: • A. The posted speed limit • B. 10 mph above the speed limit • C. The 85% percentile spot speed of the facility • D. The design speed of the facility

  4. GB Question • Stopping sight distance is composed of two distances, what are they? • Distance traveled during perception/reaction time • Distance required to brake (stop) vehicle

  5. GB Question • AASHTO GB recommends 2.5 seconds, this is adequate for conditions that are more complex than the simple conditions used in laboratory and road tests, but is not adequate for what? • Is not adequate for the most complex conditions encountered in actual driving.

  6. Stopping Sight Distance (SSD) • Required for every point along alignment (horizontal and vertical) – Design for it, or sign for lower, safe speed • Available SSD = f(roadway alignment, objects off the alignment, object on road height • SSD = PRD + BD (with final velocity V2 = 0)

  7. Criteria for Sight Distance • Driver eye height: for passenger vehicle’s = 3.5 ft above surface • Height of object in roadway = 2 feet (SSD) – why? • Height of opposing vehicle = 3.5 feet (PSD)

  8. Stopping (emergency) – SSD (Table 3-1) Source: A Policy on Geometric Design of Highways and Streets (The Green Book). Washington, DC. American Association of State Highway and Transportation Officials, 2001 4th Ed.

  9. Decision Sight Distance • When situation is unexpected or driver makes unusual maneuvers or under difficult to perceive situations • Requires higher P/R time • Depends on type of maneuver made and roadway setting (urban vs. rural)

  10. Decision (controlled stop, speed/path/route change) – DSD (Table 3-3) Source: A Policy on Geometric Design of Highways and Streets (The Green Book). Washington, DC. American Association of State Highway and Transportation Officials, 2001 4th Ed.

  11. Important Sight Distances (cont.) Intersection (turning/crossing) - see Ch 9 of GB

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