DesignSpeedand Operating Speed AASHTO Subcommittee on Design 2004 Annual Meeting Snowbird, Utah June 10, 2004 presented by Don T. Arkle
Design Speed a selected speed used to determine various selected design features of the roadway. (Fambro et al, 2001 Green Book)
Operating Speed the actual speed of a group of vehicles over a certain section of road. The 85th percentile distribution of observed speeds is the most frequent used measure of the operating speed associated with a particular location or geometric feature.
Posted Speed the legal speed limit for which a section of road is signed.
NCHRP Report 504 Design Speed, Operating Speed and Posted Speed Practices Kay Fitzpatrick Paul Carlson Marcus Brewer Mark D. Wooldridge Shaw-Pin Miaou Transportation Research Board, Washington D.C., 2003
NCHRP Report 504 Relationship between operating speed and posted speed can be defined
NCHRP Report 504 Relationship between design speed and operating speed or posted speed can not be defined with the same level of confidence
NCHRP Report 504 Design Speeds appear to have minimal impact on operating speeds unless a tight horizontal radius or a low K-value is present
NCHRP Report 504 In selecting the design speed of a new road, the functional class and legal speed limit were the most commonly used.
NCHRP Report 504 The safety review demonstrated that there are known relationships between safety and design features. Therefore, design features should be selected with some consideration of anticipated operating speed.
NCHRP Report 504 As posted speed increases, the 85th percentile speed increases
NCHRP Report 504 Studies have shown that 85th percentile speeds typically exceed posted speeds.
NCHRP Report 504 When operating speeds are used to determine posted speeds, there are occasions where the posted speed exceeds the design speed.
NCHRP Report 504 Factors used to select design speed • AASHTO – Functional Classification, Rural vs Urban, Terrain • States – Green Book, legal limit, legal limit plus a value (5 mph), anticipated volume, anticipated operating speed, development, cost, and consistency • International – anticipated operating speed, feedback loop
NCHRP Report 504 A majority of states surveyed use functional classification in selecting design speed
NCHRP Report 504 One-third of states responding use operating speed to select design speed
NCHRP Report 504 One-half of state responding use anticipated posted speed to select design speed