Road widening on curves in hilly terrain: where should the additional width be provided to best address off-tracking and safety?

Introduction / Context:
On horizontal curves, vehicles off-track toward the inside of the curve. In hilly terrain, space and stability concerns make the location of curve widening especially important for safety and constructability.

Given Data / Assumptions:

• Hilly region alignment with retaining wall/slope on the valley side.
• Objective is to accommodate off-tracking and improve maneuverability.
• Conventional two-lane hill road with standard design speeds.

Concept / Approach:
Widening is preferably provided on the inner side of the curve. This aligns with the direction of off-tracking and improves sight distance by cutting into the hill rather than expanding toward the valley side, which could demand heavy protection works.

Step-by-Step Solution:

Verification / Alternative check:
Design practice notes and safety audits in hilly regions consistently recommend inner-side widening for better control and reduced protective works.

Why Other Options Are Wrong:
Outer-side widening can be expensive and hazardous on valley side; equal widening splits benefits but is less efficient; “less on outer and more on inner” partly true but the question asks where it is done—the accepted concise answer is inner side.

Common Pitfalls:
Ignoring off-tracking magnitude and construction risk on valley side embankments.

Final Answer:
on the inner side