1. 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. Design Speed, Operating Speed and Posted Speed Practices
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

6. NCHRP Report 504
Relationship between operating speed and posted speed can be defined

7. 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