Superelevation with Friction – Compute e for Given Speed and Radius A vehicle travels at 60 km/h on a horizontal curve of radius R = 128 m. The pavement's side-friction factor is f = 0.15. What superelevation e should be provided (use e + f = v^2 / (127 R), with v in km/h and R in m)?

Introduction / Context:
On horizontal curves, part of the lateral acceleration is balanced by roadway banking (superelevation e) and the remainder by side friction f between tires and pavement. Design aims to keep both within safe limits.

Given Data / Assumptions:

• Speed v = 60 km/h.
• Radius R = 128 m.
• Side-friction factor f = 0.15.
• Design relation: e + f = v^2 / (127 R).

Concept / Approach:

Compute the right-hand side from speed and radius, then subtract the allowable friction contribution to find the required superelevation e.

Step-by-Step Solution:

Verification / Alternative check:

Checking with design tables for v = 60 km/h and R = 125–130 m yields similar e values around 7%, confirming the calculation.

Why Other Options Are Wrong:

• 0.05: Too low, would overload friction.
• 0.09 or 0.11: Higher than required and may be uncomfortable or drain poorly.

Common Pitfalls:

Confusing the constants 127 vs 225; forgetting to subtract f from the total required e + f; rounding e to an impractically fine precision (use 7%).

Final Answer:

e = 0.07