Design guidelines for horizontal sightline offsets

When the sightlines along horizontal roadway curves are restricted, there are challenges to find acceptable solutions. Designers compensate for the limitations on driver sight distance in various ways, including: accepting shorter sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs; as a result, many highway agencies have used the design exception process to address the trade-offs for sight distance in such situations. This project conducted research to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered. In NCHRP Project 15-59, MRIGlobal, assisted by Larson Pennsylvania Transportation Institute, was asked to: (1) evaluate the safety and operational performance as well as the trade-offs and risks associated with state-of-the-practice mitigation treatments when horizontal sightline offset criteria are not met; and (2) recommend updates to AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) for horizontal sightline offset design criteria, and guidance on curved roadway alignment adjacent to barriers or other types of impediments that may impact the driver’s line of sight. The TRB report is online at http://www.trb.org/Main/Blurbs/179404.aspx