Vertical Curve - Stopping Sight Distance


Sight distance is dependent on the type of curve used and the design speed. For crest curves, sight distance is limited by the curve itself, as the curve is the obstruction. For sag curves, sight distance is generally only limited by headlight range. AASHTO has several tables for sag and crest curves that recommend rates of curvature, K, given a design speed or stopping sight distance. These rates of curvature can then be multiplied by the absolute slope change percentage, A to find the recommended curve length, Lm.

Without the aid of tables, curve length can still be calculated. Formulas have been derived to determine the minimum curve length for required sight distance for an equal tangent curve, depending on whether the curve is a sag or a crest.

Related formulas


Lmrecommended curve length (m)
Krates of curvature (dimensionless)
Aabsolute slope change percentage (m)