Roadside Clearance Limit on Horizontal Curves with Transition Arcs

AASHTO guidelines for the geometric design of highways require that the insides of horizontal curves be clear of sight obstructions to provide required sight distance. However, the guideline's method for determining minimum roadside clearance is suitable only for middle sections of horizontal curves at sites where the circular arcs are equal to or longer than sight distance. For other sites and sections of horizontal curves, the guideline recommends the use of graphical or computational methods. The graphical method is tedious and time-consuming, and the recommended computational method was prepared for sites with circular arcs equal to or longer than sight distance. An analytical model was developed for clearance offsets for sites with circular arcs shorter than sight distance. Offsets were defined as normal ordinates from the driver path to a curve that demarcates the roadside area that accommodates sightlines. Variables in the model are sight distance, length of transition, length of circular curve, radius of circular curve, and driver location. Design charts for maximum and intermediate offsets were developed for use by practitioners with the model. Offsets from the charts for curves that have transition arcs were significantly shorter than offsets for equivalent simple curves. With these charts, the extra excavation costs associated with the resultant bigger offsets of other models can be avoided. Study results will be of value to engineers involved in design policy, highway maintenance, and design improvement for sites with insufficient sight distance.