Geometric design standards: according to Indian Roads Congress recommendations, what is the typical ruling gradient adopted for highways in plain terrain?

Introduction / Context:
Ruling gradient is the desirable maximum longitudinal slope used for highway design under normal conditions, balancing construction economy with operating comfort and safety. It directly affects vehicle performance, sight distance, drainage, and fuel consumption, and thus is a cornerstone parameter in alignment design.

Given Data / Assumptions:

• Terrain: plain (low natural cross slope, relatively easy earthwork).
• Standard Indian practice (IRC) for two-lane and multi-lane highways.

Concept / Approach:
In plains, the ruling gradient is gentle because there is usually flexibility to optimize cut and fill. IRC guidance commonly adopts 1 in 30 (≈ 3.33%) as the ruling value, with steeper “limiting” gradients allowed locally if constraints warrant, and flatter gradients (e.g., approaches to intersections, bridges) used for special cases.

Step-by-Step Solution:
Identify the terrain class: plain. Recall standard ruling gradient for plains: 1 in 30. Choose the matching option.

Verification / Alternative check:
Design examples and tables use 1 in 30 for plains, 1 in 20 for rolling, with further adjustments for mountainous/steep terrains where gradients often increase due to constraints.

Why Other Options Are Wrong:

• 1 in 15 and 1 in 20: too steep for plains as a ruling (though may appear as limiting in hilly terrain).
• 1 in 45 and 1 in 100: very gentle; acceptable but not the standard ruling target for plains.

Common Pitfalls:

• Confusing “ruling” with “limiting” or “exceptional” gradient categories.

Final Answer:
1 in 30.