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:

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