Design speeds: The minimum design speed specified for various highway classes in plain terrain is typically taken equal to the ruling design speed of which other terrain category?

Introduction / Context:
Design speed is a primary control for horizontal and vertical geometry. Standards relate values across terrain categories to maintain consistency and safety when conditions worsen from plain to rolling to mountainous and steep terrains.

Given Data / Assumptions:

• We compare design speeds between plain and rolling terrains.
• Ruling and minimum values are as per conventional highway design practice.

Concept / Approach:
Plain terrain allows the highest speeds. Rolling terrain generally has lower ruling design speeds. The minimum design speed for plain terrain aligns with the ruling design speed for rolling terrain to ensure smooth transition in network performance.

Step-by-Step Solution:

Verification / Alternative check:
Tabulated values in highway design manuals typically show this alignment (e.g., a step-down of one category from plain-minimum to rolling-ruling).

Why Other Options Are Wrong:
Mountainous and steep terrains have even lower speeds; matching plain-minimum to those would be overly conservative and inefficient.

Common Pitfalls:
Confusing ruling with minimum values; not adjusting for terrain transitions leading to abrupt speed environment changes.

Final Answer:
Rolling terrain