Understanding Speed Concepts: Key Definitions and Case Study Examples.

The relationships between design speed, operating speed, and posted speedlimits are important considerations in the geometric design of highways and streets. Geometric designers establish geometric design criteria usinga 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 aretherefore favorable with respect to safety and mobility. Because the American Association of State Highway and Transportation OfficialsÆ Policy onGeometric Design of Highway Streets recommends the use of design values greater than the minimum for a designated design speed, and because driverschoose to operate their vehicles based on 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 where the design speed is lower than the posted speed limit, various operating speed measures, or both. Although such a relationship is notnecessarily perceived as a problem by designers, enforcement personnel may observe operating speeds that exceed the posted speed limit, particularly if the posted speed limit is established based on local regulations, or if transportation agencies are reluctant to post a regulatory speed that exceeds the design speed. The public may also express safety and compliance concerns when operating speeds exceed the posted speed limit. The key to creating speed harmony is to produce an alignment and cross-section where the design speed, operating speed, and posted speed limit are related insuch a manner that is consistent with the intuition of drivers, enforcement personnel, and designers alike. The purpose of this paper is to provide key definitions of speed concepts used in the geometric design process, and to present several case studies that demonstrate the concepts of speedharmony and speed discord.