1. Design Speed, Operating Speed, and Posted Speed Limit Practices
Research conducted by:
Kay Fitzpatrick,
Marcus Brewer,
and
Others

2. “Famous” Quote
“Finally I passed a road sign that read, WELCOME TO TEXAS – DRIVE FRIENDLY.”
“Was it my imagination, or did everyone on the road suddenly start driving faster?”
- Joan Bauer, Rules of the Road

3. Today’s Presentation Material primarily from NCHRP projects
Design speed – definition and selection
Influences on operating speed
Setting of posted speed limits
Development of criteria for higher design speeds

4. What is the Relationship?
Operating Speed
Design Speed
Posted Speed

5. Relationships
Operating Speed
Posted Speed
Design Speed

6. Relationships
Operating Speed
Design Speed
Posted Speed

7. Design Speed

8. Design Speed Examples of Where Used
Stopping sight distance
Horizontal curves and superelevation
Vertical grades and curves
Ramp acceleration and deceleration lane lengths
Roadside treatments

9. Design Speed Definition
Original definition
“…the maximum approximately uniform speed which probably will be adopted by the faster group of drivers but not, necessarily, by the small percentage of reckless ones.”
-1938 AASHO

10. Design Speed Definition
Previous definition
“…the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.”
-1994 Green Book, p. 62

11. Design Speed Definition
Current definition (for now…)
“…a selected speed used to determine the various geometric design features of the roadway.”
-2004 Green Book, p. 67
-2011 Green Book, p. 2-54

12. What’s the Design Speed?

13. Design Speed Assumed Relationship to Running Speed Note: Has Been Eliminated for Selected Criteria
Values from 2004 GB

15. Design Speed Relationships
Research has found 85th percentile speeds > design speed
Large majority of drivers = reasonable and prudent
Therefore - associated risk not excessive (even when higher than design speed) for typical situations

16. Design Speed AASHTO Policy on Selection
“Except for local streets where speed controls are frequently included intentionally, every effort should be made to use as high a design speed as practical to attain a desired degree of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts.”
-2004 Green Book, p. 67

17. Design Speed AASHTO Policy on Selection
“The selected design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use, and the functional class of the highway. In selection of design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts.”
-2011 Green Book, p. 2-54

18. Design Speed AASHTO Policy on Selection
Functional classification
Rural versus urban
Terrain type
“Some design features, such as curvature, superelevation, and sight distance, are directly related to, and vary appreciably with, design speed. Other features, such as widths of lanes and shoulders and clearances to walls and rails, are not directly related to design speed, but they do affect vehicle speeds.”
-2011 Green Book, p. 2-55

19. Design Speed Selection United States Practices – NCHRP 504
Mailout Survey
“What factors are considered when selecting a design speed for a new road?”
40 states responded
Answers could be distributed over a range of approaches

20. Design Speed Selection States Using An Approach
(max 40) %
Functional classification 36 90
Legal speed limit (+ value) 23 58
Traffic volume 15 38
Anticipated operating speed
Terrain 7 18

21. Influences on Operating Speed

22. Influences on Operating Speed Field Studies
79 tangent sites, most suburban/urban
Sites not near signals or horizontal curves (elements known to influence operating speed)
Free-flow speed measured with lidar
Roadway and roadside characteristics

23. Influences on Operating Speed Findings15 30 45 60
Posted Speed Limit (mph)
85th %-lie Speed (mph)
Local
Collectors
Arterial, C&G
Arterial, Shoulder
SL = 85th

24. Influences on Operating Speed Findings20 40 60 80
100 10 30 50 70
Cumulative Dist (%)
Arterials, 69 sites
Collectors, 20 sites
Locals, 13 sites
85th %-ile Speed (mph)

25. Influences on Operating Speed Findings75
Local
Collectors 60
Arterial, C&G
Arterial, Shoulders 45
85th %-ile Speed (mph) 30 15 30 60 90
120
150
180
Access Density (pts/mi)

26. Influences on Operating Speed Findings
Est. 85th = Intercept + Posted Speed Limit
Intercept 
Ro2
All Roadways
7.675
0.980
0.90
Suburban/Urban Arterial
8.666
0.963
0.86
Suburban/Urban Collector
21.131
0.639
0.41
Suburban/Urban Local
10.315
0.776
0.14
Rural Arterial
36.453
0.517
0.81

27. Influences on Operating Speed Recommendations
Several variables show influences:
Access density
Pedestrian activity
Absence of centerline or edge line markings
On-street parking
Median presence
Additional data needed

28. Setting of Posted Speed Limits

29. Posted Speed Limits MUTCD Guidelines
Within 5 mph of 85th percentile speed of free-flowing traffic
Road characteristics
Speed pace
Roadside development
Parking practices
Reported crash experiences

30. Posted Speed Limits Results from ITE Survey
85th percentile speed  predominant factor
Roadway geometry
Roadside development
Crash experiences
Political pressure

31. Posted Speed Limit Rural Roads
Source
Functional
Class
Vehicle Percentile (%)
When Speed =
Sites
Posted
+ 5 mph
+ 10 mph
NCHRP
Rural, Arterial
37 / 48* 70 91 9
FHWA
Rural, Minor Arterial 59 87 99
Rural, Principal Arterial 72 90 98 36
Rural 64 86 97
126

32. Posted Speed Limit Rural Roads
(mph)
Vehicle Percentile (%)
When Speed =
Sites
Posted
+ 5 mph
+ 10 mph 50 81 99
100 12 55 61 85 96
151 60 91 95 98 8 65 59 89 2 70 64

33. Posted Speed Limit Suburban/Urban Roads
Source
Functional
Class
Vehicle Percentile (%)
When Speed =
Sites
Posted
+ 5 mph
+ 10 mph
TxDOT
Suburban/Urban Arterial 32 69 91 35
NCHRP 92
Suburban/Urban Collector 23 57 86 22
Suburban/Urban Local 52 83 96 13

34. Posted Speed Limit Suburban/Urban Roads
Vehicle Percentile (%)
When Speed =
Sites
Posted
+ 5 mph
+ 10 mph 25 42 77 94 7 30 28 64 86 35 22 62 90 19 40 32 68 92 45 37 70 15 50 43 76 95 9

35. Operating and Posted Speed Summary of Field Study Findings
Previous findings:
85th %-ile speed exceeds posted
50th %-ile speed near posted
NCHRP/other studies:
Rural: 37 to 72% at posted speed
Suburban/urban: 32 to 52% at posted

36. Posted Speed Limit Zoning Reports
ITE TENC Committee 97-12
Request “speed zoning investigations your agency has recently conducted”
256 reports received
128 contained both 85th percentile speed value and speed limit recommendation

37. Posted Speed Limit Findings from ITE Survey

38. Posted Speed Limit Difference in Measured and Recommended

39. Posted Speed Limit Speed Zone Studies
128 speed zone studies  10% rounded
up and 31% rounded to nearest 5 mph 25 50 75
100 -5 5 10 15 20
Cumulative Frequency
Diff (85th – Recom SL)

40. Posted Speed Limit Comments
85th percentile speed is a “starting point”
Encourage changes in how speed limits are set?

41. Posted Speed Limit Enforcement is Key!

42. Questions?