Speed is a fundamental concept in transportation engineering. The Green Book, MUTCD, and other references use various aspects of speed (e.g., design speed, operating speed, running speed, 85th percentile speed) depending on the application, but the definitions of these aspects have not always been consistent between documents. These inconsistencies resulted in ambiguous and sometimes conflicting policies. Design speed is a critical input to the Green Book’s design process for many geometric elements. For some of these elements, however, the relationship between the http://gulliver.trb.org/publications/nchrp/nchrp_rpt_504.pdf This report may be accessed by Internet users at design speed and the actual operating speed of the roadway is weak or changes with the magnitude of the design speed. Setting a design speed can be challenging, particularly in a public forum, and alternative approaches to design may be beneficial and should be explored. Under NCHRP Project 15-18, the Texas Transportation Institute compiled and analyzed industry definitions for speed-related terms and recommended more consistent definitions for AASHTO’s Green Book and the MUTCD. The researchers surveyed state and local practices for establishing design speeds and speed limits and synthesized information on the relationships between speed, geometric design elements, and highway operations. Next, researchers critically reviewed geometric design elements to determine if they should be based on speed and identified alternative designelement-selection criteria. Geometric, traffic, and speed data were collected at numerous sites around the United States and analyzed to identify relationships between the various factors and speeds on urban and suburban sections away from signals, stop signs, and horizontal curves (all elements previously found to affect operating speeds). In addition to including the survey of practice and information on the relationships between speed and various geometric and traffic factors, this report suggests refinements to the Green Book in the following areas: design speed definitions; information on posted speed and its relationship with operating speed and design speed; how design speed values are selected in the United States (noting that anticipated posted speed and anticipated operating speed are also used in addition to the process currently in the Green Book, which is based on terrain, functional class, and rural versus urban); changes to functional class material; and additional discussion on speed prediction and feedback loops. The included CD-ROM contains the following appendixes: (A) Suggested Changes to the Green Book; (B) Mailout Survey; (C) Design Element Reviews; (D) Previous Relationships Between Design, Operating, and Posted Speed Limit; (E) Field Studies; (F) Driving Simulator Study; (G) Selection of Design Speed Values; (H) Operating Speed and Posted Speed Relationships; (I) Distributions of Roadway and Roadside Characteristics; and (J) Alternatives to Design Process. (Author/publisher)
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