Module 3: Geometric design of highways

Lecture 16 Horizontal alignment III

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Type of transition curve

Different types of transition curves are spiral or clothoid, cubic parabola, and Lemniscate. IRC recommends spiral as the transition curve because:

1. it fulfills the requirement of an ideal transition curve, that is;
   1. rate of change or centrifugal acceleration is consistent (smooth) and
   2. radius of the transition curve is $\infty$ at the straight edge and changes to $R$ at the curve point ( $L_s \propto \frac{1}{R}$) and calculation and field implementation is very easy.

Length of transition curve

The length of the transition curve should be determined as the maximum of the following three criteria: rate of change of centrifugal acceleration, rate of change of superelevation, and an empirical formula given by IRC.