Determinants of horizontal curve design: Selection and sizing of horizontal curves on highways are governed primarily by which set of parameters?

Introduction / Context:
Horizontal curve design must ensure adequate safety and comfort while accommodating expected operating speeds. The main controls balance vehicle dynamics with pavement friction and cross-slope (superelevation).

Given Data / Assumptions:

• Standard design speed chosen for the facility type.
• Limits on lateral friction factor f.
• Maximum superelevation e set by climate, speed, and standards.

Concept / Approach:
The governing relationship is V^2 / (225 * R) = e + f in km/h-based units, linking speed, radius, superelevation, and friction. Hence, given speed, permissible e and f limit how small R can be. All these parameters act together to determine the minimum radius and transition length.

Step-by-Step Solution:
Fix design speed from functional classification.Adopt limits for e (e.g., 7%–10%) and f (e.g., 0.10–0.15 for comfort/safety).Compute minimum R satisfying e + f ≥ V^2 / (225 * R).

Verification / Alternative check:
Any change in speed, e, or f modifies the minimum radius and transition run-off, confirming that all listed factors govern the design.

Why Other Options Are Wrong:

• Options a–d individually matter, but none alone completely determines the curve without the others.

Common Pitfalls:

• Over-reliance on friction at high speeds; superelevation should carry a larger share of centripetal demand.
• Ignoring climate limits on maximum e in snow/ice regions.

Final Answer:
All the above