Highway engineering (Nagpur plan, hilly regions): According to the Indian Roads Congress recommendations under the Nagpur Plan, are the standard carriageway widths in hilly terrain the same across National Highways, State Highways, and Major District Roads, or do they differ by road class?

Introduction / Context:
Geometric design standards under the Nagpur Plan (as adopted by the Indian Roads Congress) differentiate between plain/rolling and hilly terrain. In hilly regions, design widths are often constrained by topography, but higher-order roads (for example, National Highways) still carry stricter or higher standards than lower-order roads, reflecting traffic volume, safety, and service requirements.

Given Data / Assumptions:

• Nagpur Plan–era IRC recommendations are being referenced.
• Terrain category: hilly region.
• Road classes compared: National Highways (NH), State Highways (SH), and Major District Roads (MDR).

Concept / Approach:

IRC guidance associates carriageway widths with design speed, traffic category, and road class. Even in hilly terrain (where widths often reduce), the standard widths for National Highways are not universally identical to those for State Highways and Major District Roads. In practice, NH typically retains a wider or more stringent width standard, with SH and MDR permitted somewhat narrower widths due to lower design speeds and traffic volumes.

Step-by-Step Solution:

Verification / Alternative check:

Comparative tables in IRC manuals show class-wise standard widths and shoulders differing by hierarchy, even when reduced for hilly conditions (single-lane, intermediate-lane, or two-lane prescriptions vary by road class).

Why Other Options Are Wrong:

• “Same for National Highways / SH / MDR / all classes”: Overgeneralizes; class-wise differences are retained even in hilly terrain.

Common Pitfalls:

• Assuming “hilly” means uniform reduction across all classes; hierarchy still matters.
• Confusing shoulder width changes with carriageway width prescriptions.

Final Answer:

Different for National Highways.