Unsymmetrical sag (vertical) curves may be required at complex interchanges and other highway locations because of clearance and other controls. No relationships are available for designing or evaluating these curves on the basis of sight distance needs, so sight distance models for unsymmetrical sag curves are developed for headlight and overhead obstacle controls. For headlight control, the model relates the minimum sight distance, vertical curve parameters, and vehicle and object characteristics. For overhead control, the model relates the available sight distance, sag curve parameters, vertical clearance and location of overhead obstacle, and locations and heights of driver eye and object. A procedure for calculating the minimum sight distance is presented. The distinct characteristics of sight distance on unsymmetrical sag curves are examined. To facilitate practical use, graphs and tables of the minimum sight distance for headlight and overhead controls are established. The length requirements and sight distance characteristics of symmetrical and unsymmetrical sag curves were found to be quite different. The developed models should be valuable in the evaluation of safety and operation of unsymmetrical sag curves.
Abstract