Highway Geometrics – Superelevation for High Speed with Mixed Traffic Limits Determine the design rate of superelevation for a horizontal curve of radius R = 450 m under mixed traffic conditions at a speed of 125 km/h. Apply practical limits for mixed traffic.

Introduction / Context:
Superelevation (e) counters the lateral acceleration experienced by vehicles on curves. The theoretical rate needed for a given speed and radius follows from equilibrium of forces, but practical design for mixed traffic imposes an upper cap to accommodate slow-moving vehicles, heavy vehicles, and safety margins in rain.

Given Data / Assumptions:

• Design speed v = 125 km/h.
• Curve radius R = 450 m.
• Mixed traffic conditions with a capped maximum e.

Concept / Approach:

The theoretical no-friction superelevation is e_theory = v^2 / (225 R) (v in km/h, R in m). Compute e_theory and then apply the practical limit for mixed traffic. If e_theory exceeds the cap, adopt the capped value.

Step-by-Step Solution:

Verification / Alternative check:

Design charts similarly show that for very high speeds on modest radii, the required theoretical e exceeds the cap; designers therefore use the maximum permissible e and manage the remainder with speed control, signage, or increased radius.

Why Other Options Are Wrong:

• 1.0: Impossible; would imply a 100% cross slope.
• 0.05: Below the needed maximum; less conservative than the cap.
• 0.154: Theoretical value without practical cap; not used for mixed traffic.

Common Pitfalls:

Using e_theory directly without checking the allowable maximum; ignoring speed management measures when e is capped.

Final Answer:

0.07