The following excerpt was taken from the 1994 edition of AASHTO's __A
Policy on Geometric Design of Highways and Streets__ (p. 153).

The minimum radius is a limiting value of curvature for a given design speed and is
determined from the maximum rate of superelevation and the maximum side friction factor
selected for design (limiting value of f).

Use of sharper curvature for that design speed would call for superelevation beyond the
limit considered practical or for operation with tire friction beyond what is considered
comfortable by many drivers, or both. Thus, the minimum radius of curvature is a
significant value in alignment design. The minimum radius of curvature is also a necessary
and important control value for determination of superelevation rates for flatter curves.

The minimum radius Rmin can be calculated directly from the simplified curve formula
introduced earlier under the section, "Side Friction Factor."