Understanding Speed Concepts

The relationships between design speed, operating speed, and posted speed limits are important considerations in the geometric design of highways and streets. Highway designers establish geometric design criteria by using a designated design speed. The ultimate objective is compatibility between design speed, operating speed, and posted speed limit–-or speed harmony. The design speed concept is intended to result in operating speeds that are consistent with the intended function of the highway or street and are therefore favorable with respect to safety and mobility. Because AASHTO's A Policy on Geometric Design of Highway Streets recommends using design values greater than the minimum for a designated design speed and because drivers choose to operate their vehicles according to perceived physical and operational limitations present along a roadway, higher than minimum design values may result in operational inconsistencies–-or speed discord. Speed discord is a condition in which the design speed is lower than the posted speed limit, lower than various operating speed measures, or both. Highway designers may not necessarily perceive such a relationship as problematic, but the public and enforcement personnel may express compliance or safety concerns for highway segments that experience speed discord. This paper provides key definitions of speed concepts used in the geometric design process and presents several case studies that demonstrate the concepts of speed harmony and speed discord.