Why are horizontal and vertical curves used in roadway design and how are they defined?

Why are horizontal and vertical curves used in roadway design and how are they defined?

• A. To align with terrain and sight distances; horizontal curves change direction in plan, vertical curves connect grade lines.
• B. To decorate the roadway.
• C. To increase speed at curves.
• D. Horizontal curves change grade; vertical curves change direction.

Answer: (A)