Design Issues in Widening of Unspiraled Horizontal Curves

In highway alignment, when the traveled way is widened for unspiraled horizontal curves, a linear increase, widening from zero to its maximum value along the application length, leads to an undesired straight edge or an apparent kink, since there is no transition curve between the tangent section of the road and the main circular curve (unlike spiraled horizontal alignment where gradual and proportional offsets of widening can easily be accomplished with respect to the transition curve). Although compound curves or modified spirals can be employed to solve this problem, using complex curves for simple (unspiraled) alignment has not been a favored approach. This paper addresses this issue and proposes using a single circular arc on the inside edge of widening where possible. On trying many circular arcs iteratively until certain conditions are met, within a given range of superelevation runoff length, several arcs have been produced for each cell of the widening table provided by AASHTO. All the arcs satisfied the all-governing condition of sharing the same tangent with the main circular curve at the end of the application distance. Provided that the superelevation runoff length falls within the limits determined in the paper, the work presents a simple and time saving approach for designers to adapt. Although the computations are based on 7.2 m traveled way width, similar arcs can easily be produced by others for various widths. The paper, therefore, shows how to map the application domain of such arcs for given curve parameters.