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Design Speed, Operating Speed, and Posted Speed Relationships

Transportation Research Board (TRB), National Cooperative Highway Research Program

Speed is used both as a design criterion to promote consistency and as a performance measure to evaluate highway and street designs. Geometric design practitioners and researchers are, however, increasingly recognizing that the current design process does not ensure consistent roadway alignment or driver behavior along these alignments. The goals of the NCHRP 15-18 research project were to reevaluate current procedures, especially how speed is used as a control in existing policy and guidelines, and then to develop recommended changes to the design process.

Objectives completed included the following:

• Review current practices to determine how speed is used as a control and how speed-related terms are defined. Also identify known relationships between design speed, operating speed, and posted speed limit.
• Identify alternatives to the design process and recommend the most promising alternatives for additional study.
• Collect data needed to develop the recommended procedure(s).
• Develop a set of recommended design guidelines and/or modifications for the AASHTO A Policy on Geometric Design of Highways and Streets (commonly known as the Green Book).

Strong relationships between design speed, operating speed, and posted speed limit would be desirable, and these relationships could be used to design and build roads that would produce the speed desired for a facility. While the relationship between operating speed and posted speed limit can be defined, the relationship of design speed with either operating speed or posted speed cannot be defined with the same level of confidence. The strongest statistical relationship found in NCHRP Project 15-18 was between operating speed and posted speed limit for roadway tangents. Several variables other than the posted speed limit do show some sign of influence on the 85^th percentile free-flow operating speed on tangents. These variables include access density, median type, parking along the street, and pedestrian activity level.

For more information relating to design speed and posted speed, read FHWA’s “Relationship between Design Speed and Posted Speed.”