Module 3: Geometric design of highways

Lecture 15 Horizontal alignment II

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Radius of Horizontal Curve

The radius of the horizontal curve is an important design aspect of the geometric design. The maximum comfortable speed on a horizontal curve depends on the radius of the curve. Although it is possible to design the curve with maximum superelevation and coefficient of friction, it is not desirable because re-alignment would be required if the design speed is increased in future. Therefore, a ruling minimum radius $R_{ruling}$ can be derived by assuming maximum superelevation and coefficient of friction.

$$R_{\mathrm ruling}=\frac{v^2}{g(e+f)}$$ (1)

Ideally, the radius of the curve should be higher than $R_{ruling}$. However, very large curves are also not desirable. Setting out large curves in the field becomes difficult. In addition, it also enhances driving strain.