Highway Geometrics – Purpose of Transition Curves on Horizontal Alignments Transition (spiral) curves are introduced on the approaches to circular horizontal curves primarily to achieve which of the following?

Introduction / Context:
On horizontal curves, vehicles undergo centripetal acceleration. A sudden change from zero lateral acceleration (on tangent) to a finite value (on curve) causes discomfort and safety issues. Transition curves (e.g., clothoids/spirals) gradually introduce curvature to provide a smooth change in lateral acceleration and its time rate of change, called jerk.

Given Data / Assumptions:

• Highway with tangent sections connecting to a circular curve.
• Design speed and comfort criteria require controlling jerk.
• Standard practice uses a clothoid where curvature varies linearly with length.

Concept / Approach:

Jerk is the derivative of acceleration with respect to time. With a transition curve, curvature increases progressively from 0 to 1/R, hence lateral acceleration increases smoothly, keeping jerk within allowable limits for comfort and safety. Secondary benefits include better aesthetics and easier superelevation run-off, but the primary motive is comfort (jerk control).

Step-by-Step Solution:

Verification / Alternative check:

Design manuals specify maximum rate of change of centrifugal acceleration; spiral transitions satisfy this criterion by construction.

Why Other Options Are Wrong:

• Increase jerk: opposite of the purpose.
• Minimize length: not the primary reason; sometimes transitions lengthen alignment.
• Simplify construction: not the main driver; layout is slightly more complex.

Common Pitfalls:

Confusing transition purpose with superelevation only; both curvature and cross-slope must be transitioned.

Final Answer:

Reduce jerk to allowable levels