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), ex hibited 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 Rat 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 curv e of Type IV experienced at no time a Rat grade, Types IT and IV verti cal 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 th e worst grades of braking initiation, the associated braking and stopp ing sight distances, and the resulting minimum rates of vertical curva ture. The new methodology can result in longer and Ratter 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 agai nst the blind acceptance of longer vertical curves. The inconsistencie s by AASHTO may have misrepresented various design parameters.