Median design to provide stopping sight distance along horizontal curves in urban freeways

A new approach to design the roadway and its roadside is necessary to provide both stopping sight distance and design flexibility along horizontal curves. This study focuses on median, dividing, and side strips to provide the stopping sight distance along not-straight paths. According to the Italian standard for road design, a procedure for estimating the needed widening for urban freeways is presented: it is based on the diagram speed along symmetric transition curves (clothoid-arc-clothoid). Different geometrical and functional factors have been considered to identify the most critical sections where the largest widening needs: they are generally along the clothoid stretches before circular arcs whose design speed is far below the maximum one. The implementation of the proposed procedure gives the required net widening at different abscissae along the transition curve. The geometric strip connection between current and widened sections could be modeled according to a 3 section-approach along the clothoid. The results can be a valid help both for technical practice (e.g., safe rearrangement of the roadway) and for the correct development of design and construction process (e.g., early prediction of the areas to be expropriated).