Minimum radius
Minimum radius connects design speed, superelevation, and available side friction. Smaller radii require higher lateral acceleration, which may trigger comfort or stability warnings.
Image source: MUTCD W1-4L.svg, public domain, used here as a curve warning reference.
Rmin = V^2 / (127 x (e + f))A common highway design approximation. V is km/h, R is meters, e is superelevation rate, and f is side friction factor.- If radius is too small, first check whether the target speed is too high.
- Where terrain allows, increasing radius usually feels more natural than forcing higher superelevation.
- Successive small-radius curves should also be checked for transitions and sight distance.