Vertical Cerves(Sag)

O Appearance and Comfort

• Appearance is an important consideration for both small and large changes in grade, similar to crest curves.
• Sag vertical curves are generally designed to meet comfort criteria at a minimum:
  • Unlit rural and urban roads: Sag curves should ideally be designed to satisfy headlight sight distance requirements.
  • Highways and freeways: Sag curves are typically designed to exceed headlight sight distance criteria for higher safety standards.

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O Drainage Design

• Proper drainage must be provided for both the road surface and the subgrade in all sag curves:
  • This is particularly critical for boxed pavements (pavement structures with confined layers).
  • When the lowest point of the sag is within a cut, ensuring adequate drainage can be challenging.
    • Generally, sag vertical curves should not be placed in cuts unless an appropriate underground drainage system is installed.
    • Land-locked sags require a secondary independent drainage system.
• Guidelines to prevent drainage issues:
  • For two-lane roads: Avoid sag curves longer than 750 m.
  • For kerbed roads: Sag curves with less than 0.3% grade should be limited to a maximum length of 30 m.

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O Ride Comfort and Acceleration

• Rapid changes in vertical acceleration can cause discomfort:
  • To minimize discomfort, the vertical acceleration on the sag curve is typically limited

            - low standard roads : a = 0.10g

            - Other urban and rural roads with street lighting : a = 0.05g