On sag (valley) vertical curves for highways, the available sight distance is primarily determined on what basis?

Introduction / Context:
Vertical curve design ensures drivers can see far enough ahead to stop safely. For summit (crest) curves, the limiting condition is daytime line-of-sight over the crest. For sag (valley) curves, the constraint is the reach of vehicle headlights at night, because the road surface falls away and objects are illuminated only within the headlight beam geometry.

Given Data / Assumptions:

• Standard headlight mounting height and upward beam angle per highway design practice.
• Stopping sight distance (SSD) criteria apply for safety.
• Driver eye height and obstacle height are accounted for differently on crest vs. sag curves.

Concept / Approach:
On sag curves at night, available sight distance equals the illuminated distance along the roadway, which depends on headlight height and beam spread. The design uses headlight sight distance as the control value, ensuring the provided sight distance equals or exceeds SSD at the design speed. Therefore, night-time conditions govern sag curve length and K-value selection.

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