Revised Design Parameters for Vertical Curves

A Policy on Geometric Design of Highways and Streets, by the American Association of State Highway and Transportation Officials (AASHTO), exhibited severe disparities between its recommended procedures for the computation of braking distances on grades and the design controls of vertical curves. The braking distances it advocated for the design of vertical curves are fully consistent with flat terrains. The grades of the tangents only influenced the curvatures needed to achieve braking distances and thus stopping sight distances on flat grades. A vehicle traveling on a crest vertical curve of Type II, or a sag vertical curve of Type IV, experienced at no time a flat grade. Types II and IV vertical curves join tangents with identical grade orientations. This study pinpoints the inconsistencies of the previous procedures and proposes conciliatory procedures. A new methodology computes simultaneously the worst grades of braking initiation, the associated braking and stopping sight distances, and the resulting minimum rates of vertical curvature. The new methodology can result in longer and flatter curves than currently utilized given the design parameters recommended by AASHTO. The writers recommend the calibration of design parameters of vertical curves for the new methodology prior to implementation and guard against the blind acceptance of longer vertical curves. The inconsistencies by AASHTO may have misrepresented various design parameters.