Geometric standards for hill roads: What is the normal formation width adopted for a hill road carrying one-way traffic?

Introduction / Context:Formation width (roadway width) in hill roads must balance safety, traffic needs, and cost in constrained terrain. For one-way operation (single-lane in plan), standard formation widths are smaller than for two-lane roads, but still must accommodate vehicle envelope, drainage, and lateral clearance.

Given Data / Assumptions:

• Road is in hilly terrain.
• Traffic operation: one-way (single-lane).
• Standard IRC-type guidance for formation width.

Concept / Approach:

Single-lane carriageway width in hills is typically around 3.0–3.75 m, with shoulders and parapets as applicable. The formation width (carriageway plus side clearances and drains) for one-way roads is commonly about 3.6 m, accommodating typical vehicle width plus margins.

Step-by-Step Reasoning:

Verification / Alternative check:

IRC hill road standards provide typical values; local agencies may refine for heavy vehicle share or safety parapets, but 3.6 m remains a common norm for one-way formation width.

Why Other Options Are Wrong:

• 4.8 m and above: characteristic of wider or two-lane standards.
• 6.6–8.0 m: suitable for two-lane or special designs, not simple one-way hill roads.

Common Pitfalls:

• Confusing formation width (roadway) with carriageway only or with overall benching width on hill slopes.

Final Answer:

3.6 m.