👉Understanding Fundamentals of Vertical Alignment: Symmetric Parabolic Curves in Road Design

🚧When designing roads, vertical alignment ensures safe and smooth elevation changes.

💫One of the most widely used types is the symmetric parabolic vertical curve.

🏷What Is a Symmetric Parabolic Vertical Curve?

⏺This curve connects two different roadway grades (slopes) with a smooth, gradual transition, shaped like a parabola.

⏺It's commonly used for both crests (upward humps) and sags (dips) in the road.

📜Key Characteristics of vertical curves:

✅Symmetry Around the PVI

-The curve is evenly split around the Point of Vertical Intersection (PVI) — meaning half the curve lies before the PVI, and half after.

✅Units of Measurement

-Distances are given in stations (standard surveying units, e.g., Km 1+120).

-Grades are entered as percentages (e.g., +3%, -2%).

-They are denoted as G1 or G2

✅Horizontal Distance Measurement

-All curve lengths are measured horizontally, not along the slope of the road.

-This ensures accurate and consistent design regardless of the grade.

-The length of the vertical curve (LVC) is a key factor in the design of the vertical alignment of roads.

-The minimum length LVC is twice the design speed (LVC =2Vd)


 🚗 Understanding these principles is essential for designers to achieve the following key outputs:

-Ensuring driver comfort and safety

-Minimizing vehicle strain

-Maintaining visibility (sight distances) and drainage performance

Via Yonatan Tadesse, PMP®

@etconp