Design Speed and Operating Speed

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Design Speed and Operating Speed

• AASHTO Subcommittee on Design
• 2004 Annual Meeting
• Snowbird, Utah
• June 10, 2004
• presented by Don T. Arkle

2
Design Speed
a selected speed used to determine various
selected design features of the roadway. (Fambro
et al, 2001 Green Book)
3
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.
4
Posted Speed
the legal speed limit for which a section of road
is signed.
5
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
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NCHRP Report 504
Relationship between operating speed and posted
speed can be defined
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NCHRP Report 504
Relationship between design speed and operating
speed or posted speed can not be defined with the
same level of confidence
8
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
9
NCHRP Report 504
In selecting the design speed of a new road, the
functional class and legal speed limit were the
most commonly used.
10
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.
11
NCHRP Report 504
As posted speed increases, the 85th percentile
speed increases
12
NCHRP Report 504
Studies have shown that 85th percentile speeds
typically exceed posted speeds.
13
NCHRP Report 504
When operating speeds are used to determine
posted speeds, there are occasions where the
posted speed exceeds the design speed.
14
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

15
NCHRP Report 504
A majority of states surveyed use functional
classification in selecting design speed
16
NCHRP Report 504
One-third of states responding use operating
speed to select design speed
17
NCHRP Report 504
One-half of state responding use anticipated
posted speed to select design speed