Highway design — definition of stopping sight distance (SSD) Stopping sight distance on a highway is defined as the minimum length of roadway visible ahead to:
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AEnable a driver to stop the vehicle safely without collision
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BBe visible to the driver during night driving only
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CEqual the height of the object above the road surface
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DEqual the height of the driver’s eye above the road surface
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ECover the length between successive signposts only
Answer
Correct Answer: Enable a driver to stop the vehicle safely without collision
Explanation
Introduction / Context:Stopping sight distance (SSD) is a core safety parameter in geometric highway design. It ensures that drivers can perceive a hazard and stop the vehicle before reaching it under design conditions, accounting for reaction time and braking performance on the selected grade and surface condition.
Given Data / Assumptions:
- SSD covers perception-reaction time plus braking distance.
- Design values assume a standard driver reaction time, design speed, and coefficient of friction (and grade).
- SSD must be available continuously along the alignment in both day and night conditions.
Concept / Approach:SSD = distance travelled during perception-reaction + braking distance. In symbols: SSD = vt_r + v^2/(2gf) (on level), adjusted by grade when applicable: SSD = vt_r + v^2 / (2g(f ± G)) with G as grade (rise +, fall − when using consistent sign convention). Designers ensure crest curves, sag curves, and lateral clearances provide at least the computed SSD for the design speed.
Step-by-Step Solution:State definition: minimum visible distance ahead to stop safely without collision.Relate to components: perception-reaction + braking.Note design implications: determines vertical curve lengths and obstacle clearance.
Verification / Alternative check:Check that provided sight distance ≥ computed SSD for the design speed across all alignment elements; otherwise, reduce speed or increase curve/clearance.
Why Other Options Are Wrong:Night-only visibility is not the definition; SSD applies at all times.Object height and driver eye height are inputs for visibility envelopes, not the definition of SSD itself.Signpost spacing is unrelated to SSD.
Common Pitfalls:
- Ignoring grade effect on braking distance.
- Assuming wet friction values for dry design speed (or vice versa), mis-sizing SSD.
Final Answer:Enable a driver to stop the vehicle safely without collision