What is a spiral curve and why is it used in highway design?

A spiral (transition or clothoid) curve has radius varying from infinity at the tangent to the circular curve radius (R) at the spiral-curve junction. Spiral equation: L*R = A^2 (constant). Benefits include: gradual introduction of centrifugal force, natural vehicle path following, smooth superelevation transition, and improved aesthetics. Spiral length depends on design speed and radius. Used for curves with high design speeds where sudden curvature change causes driver discomfort. Minimum length based on rate of change of lateral acceleration.