Transition length on highways – controlling criteria The required length of a transition curve on a highway is governed primarily by which criterion(s)?

Introduction / Context:
Transition curves connect tangents to circular curves so that curvature does not change abruptly. Two serviceability considerations control how long a transition must be: vehicle dynamics (comfort and safety) and construction/operational needs for gradually introducing super-elevation.

Given Data / Assumptions:

• Design speed and curve radius are known for the alignment.
• Super-elevation is introduced progressively over the transition length.
• Comfort criterion limits the rate of change of radial acceleration felt by occupants.

Concept / Approach:
Criterion 1: Limit the rate of change of radial acceleration so that lateral jerk remains within acceptable comfort limits. Criterion 2: Provide adequate length to rotate the crossfall from normal camber to full super-elevation without exceeding construction and drainage constraints. The selected transition length should satisfy both criteria, and the larger of the two governs.

Step-by-Step Solution:
Compute L based on allowable rate of change of radial acceleration.Compute L based on required super-elevation runoff and rotation rate.Adopt the larger L to satisfy both comfort and construction needs.Hence, both (a) and (b) govern the design.

Verification / Alternative check:
Design manuals list both checks and require using the governing (greater) length. Field performance confirms that ignoring either leads to discomfort or impractical crossfall development.

Why Other Options Are Wrong:
Relying on just one criterion may yield too short a transition, causing either discomfort (jerk) or abrupt crossfall changes that are difficult to build and maintain.

Common Pitfalls:
Forgetting to coordinate transition with the super-elevation runoff; omitting lane-by-lane rotation implications on multi-lane highways.

Final Answer:
both (a) and (b)