In the AASHTO geometric design policy, the need for using unsymmetrical vertical curves because of clearance restrictions and other design controls is pointed out. Formulas for laying out these curves are presented in the highway engineering literature. However, no relationships are available concerning sight distance characteristics on these curves. A sight distance model for unsymmetrical crest curves has been developed to relate the available sight distance to the curve parameters, driver and object heights, and their locations along the curve. These relationships are used in a procedure for determining the available minimum sight distance. The model is used to explore the distinct features of sight distance profiles on unsymmetrical crest curves. To facilitate practical use, the model is used to establish design length requirements of unsymmetrical crest curves based on the stopping, decision, and passing sight distance needs presented by recent innovative approaches and by AASHTO. The model should prove useful in the design and safety evaluation of critical highway locations.
Abstract