Summary of Design Speed, Operating Speed, and Design Consistency Issues

Abstract

Six papers in this Record were presented at a conference session during the January 1994 TRB annual meeting. They address various aspects of the relationships among design speed, operating speed, and design consistency. All six papers are examined to show the interrelationships of design speed, operating speed, and design consistency; what changes are needed in the current design process and current design policies; how the concept of design consistency can produce better designs; and what research is needed to develop these concepts further. The improvements involve three key principles. First, design speeds must not be arbitrary but must be selected realistically based on projected operating speeds. Second, if lower design speed is used, all aspects of the roadway and its environment must be consistent with the lower speed. Third, large differences in operating speed between successive roadway elements represent design inconsistencies that can lead to safety problems. These principles can be used to develop a design process that leads to safer, more efficient, and more consistent designs. The TRB Committees on Operational Effects of Geometrics (A3A08) and on Geometric Design (A2A02) jointly sponsored the January 1994 conference session to call attention to the many unresolved issues about the roles of design speed, operating speed, and design consistency in creating highway designs that operate efficiently and safely and meet the needs of drivers. Originally, two alternative topics were considered: relationship of design speed and operating speed and the role of formal design consistency criteria in creating good highway designs. These topics were so interrelated that it was difficult to address one without the other. A key insight from the papers was that in a consistent design the design speed selected for use in determining the geometrics of the facility is similar to the operating speed of the completed facility. To call attention to the central role of several speed-related parameters in creating consistent designs, the session was titled “Speed.” Key points of the papers are summarized, generalizing the principles and philosophy for creating consistent designs, and suggesting directions for research to develop the ideas into practical design policies that highway engineers can apply.