Super-elevation computation from crossfall: a 12.5 m wide carriageway has its outer edge 50 cm higher than the inner edge. What is the required super-elevation expressed as a slope ratio?

Introduction / Context:
Super-elevation is the upward transverse inclination provided to counteract centrifugal effects on curves. It is commonly specified as a slope ratio (1 vertical in N horizontal) or as a percentage.

Given Data / Assumptions:

• Carriageway width (run) = 12.5 m.
• Height difference (rise) between outer and inner edges = 50 cm = 0.50 m.
• Uniform crossfall assumed across the width.

Concept / Approach:
Super-elevation e (as a ratio) equals rise divided by run. Converting the numeric result into the common “1 in N” form helps compare with standards and check reasonableness.

Step-by-Step Solution:

Verification / Alternative check:
Convert to percent: 0.04 * 100 = 4%. Compare to usual caps (e.g., 7–10% depending on context) to verify the value is realistic.

Why Other Options Are Wrong:
(a) “50 cm” is not a slope; (c) 1 in 400 equals 0.25%, far too small; (d) 1 in 40 equals 2.5%; (e) 1 in 24 ≈ 4.17% and does not match the computed value exactly.

Common Pitfalls:
Mistaking carriageway width with lane width; mixing units (centimetres and metres) without conversion.

Final Answer:
1 in 25