Horizontal clearance offsets to objects higher than sight lines

Sight distance is an important design element of curved highway sections. To ensure that design sight distance is provided on curved sections, AASHTO guidelines require that the insides of horizontal curves be cleared of sight obstructions. However, the guidelines'analytical method for determining the extent of clearance is suitable only for the middle sections of horizontal curves that are longer than sight distance. The guidelines suggest the use of computational or graphical methods for determining the clearance ofTsets for cases in which the AASHTO model is inapplicable. The computational method is useful only for curves that are longer than sight distance but have unknown analytical aspects. The graphical method works for all lengths of horizontal curves but is tedious, time-consuming, and outdated. An analytical model for the determination of horizontal clearance offsets for sections that are not covered by the AASHTO model was developed and verified. The offsets were derived as ordinates from driver path to a roadside curve that acts as a boundary for the area that accommodates sight lines. Main factors in the equations were driver location, sight distance, curve radius, and curve length. With the use of the analytical model, a new design chart was developed for use by practitioners on both short and long simple curves. These results have dual applications: for controlling the proximity of high objects to highways and for input into research on the acceptable heights of roadside objects (e.g., safety barriers) so that they do not block sight distance.