Standard equation of the cubic parabola (spiral) transition used on roads For a highway transition curve modeled as a cubic parabola joining a tangent to a circular curve of radius R with transition length L, which of the following is the standard Cartesian form relating offset y to tangent distance x?

Introduction / Context:
Highway transition curves are often represented by a cubic parabola (also called a cubic spiral) because it provides a linear rate of change of curvature with length, ensuring comfort and consistent development of centrifugal acceleration and super-elevation.

Given Data / Assumptions:

• Transition curve of length L connecting a tangent to a circular curve of radius R.
• Local coordinates: x measured along tangent, y as the offset to the transition.
• Small-angle assumptions applicable to derive a simple polynomial relation.

Concept / Approach:
For a cubic parabola transition, curvature k varies linearly with arc length s. Using small deflection approximations, the well-known Cartesian form emerges: y = x^3 / (6 R L). This relation is widely used in setting out offsets of a spiral from the initial tangent.

Step-by-Step Solution:
Take curvature k = s / (R L) so that k increases linearly from 0 to 1/R over length L.Relate small deflections: dy/dx ≈ θ, and dθ/ds = k.Integrating with boundary conditions at x = 0 gives y = x^3 / (6 R L).Therefore, the standard equation is y = x^3 / (6 R L).

Verification / Alternative check:
At x = L, the offset compares well with tabulated setting-out data for cubic transitions; alternatives with x^2 terms correspond to circular or parabolic approximations not giving linear curvature change.

Why Other Options Are Wrong:

• y = x^2 / (2R) and y = x^2 / (6 R L): quadratic forms relate to simple circular offsets or sag curves, not cubic transitions.
• y = x^3 / (3 R L): incorrect coefficient; does not match the derived boundary conditions for cubic parabola transitions.

Common Pitfalls:
Mixing clothoid (Euler spiral) notation with cubic parabola approximations; forgetting unit consistency; using large-angle trigonometry where small-angle assumption was applied.

Final Answer:
y = x^3 / (6 R L)