1. Towards the Ideal Roundabout Design
SATC 2017
Towards the Ideal Roundabout Design
FH van Renssen Mowana Engineers
M Watters WCG DoT
CR Tichauer Mowana Engineers
DR C Krogscheepers ITS International
G Matthee ITS International

2. AGENDA Introduction Principles and Requirements (P&R’s)
Some critical P&R’s discussed
From here

3. Introduction More and more requests for roundabout designs
What do you get…
ALLSORTS…

4. Introduction WCG:DoT’s response: Planning and design principles
Volume 1 – International and national literature review, evaluation of existing roundabouts
Volume 2 – Planning document (P&R’s included)
Volume 3 – Roundabout Design Guidelines (later)
P&R’s developed
Design elements broken up

5. Format and layouts of P&R’sNo
Description 1A
Inscribed Circle Diameter (ICD) 1B
Design Vehicles 2A
Design Speeds and Speed Profiles 2C
Approach Alignment 2E
Sight Distances 2G
Entry Width 2H
Entry Angle 3A
Internal and External Apron Cross-section Detail 3C
Circulatory Roadway widths / Truck Accommodation 4A
Gradient / Vertical Alignment 4B
Cross-fall 4C
Approach Detail (Vane) 4D
Central / Splitter Island Treatment 4E
Surfacing and Pavement 5A
Pedestrian Movements and Accommodation 5B
Bicycle Movements 6A
Road Signs and Road Markings 7A
Lighting 8A
Access In and Close To Roundabout 9A
Left-turn Slip Lane
10A
Staged Implementation
11A
Directional Roundabout
12A
Public Transport Stops
12B
Preferential Lanes
13A
Abnormal Loads
14A
Signal Control

7. Focus of the design elements
To create a design which:
Safe environment
Optimal operation
ALL USERS

8. To be accommodated within circulatory lane
P&R: Design Vehicle
For consistency – SANRAL vehicles used
Vehicle Type
Requirement
Primary
design
vehicle:
BUS
To be accommodated within circulatory lane

9. Can encroach on truck apron or adjacent circulatory lane
P&R: Design Vehicle
Vehicle Type
Requirement
Secondary
Design
Vehicle:
WB-15
Can encroach on truck apron or adjacent circulatory lane

10. Ensure appropriate sight distances are achieved/not exceeded
P&R: Design Vehicle
For consistency – SANRAL vehicles used
Vehicle Type
Requirement
Sight
distance
design
vehicle: PV
Ensure appropriate sight distances are achieved/not exceeded

11. P&R: Design Speed and Speed Profiles
Many elements affect design speed through Roundabout
One of the most critical items which needs to be adhered to
Speed profile values on each approach should not exceed this speed
Design Speeds (km/h)
Single Lane
Double Lane
Urban
30 km/h
35 km/h
Rural
40 km/h

12. P&R: Design Speed and Speed Profiles (cont)
Aim: R1 < R2 < R3
R1 and R2 at no time more than 10 km/h difference
Any 2 conflicting speeds within 10 km/h, not more than 20 km/h
Method of calculating prescribed

13. P&R: Design Speed and Speed Profiles (cont)
Example:
Design Speeds (km/h)
Single Lane
Double Lane
Urban
30 km/h
35 km/h
Rural
40 km/h
R1 = 38 km/h
R2 = 34 km/h
R3 = 50 km/h
R4 = 25 km/h
R5 = 42 km/h
Min of SP

14. P&R: Approach alignment
In most instances entrance to roundabout dictates the speed through a roundabout
Extension of approach should be on right of CL

15. P&R: Approach alignment (cont)
Chicanes: Rural to Urban transition
Above 80 km/h, reduce in 20 km/h intervals
Not normal in Urban areas, but possible on arterials
Offset of chicane should be 7m (for double lane) – to prevent straight movement

16. P&R: Sight distances Approach Sight Distance (ASD):
Distance required to see yield line
This is a MINIMUM distance

17. P&R: Sight distances (cont)
Intersection Sight Distance (ISD):
Distance required to determine safe gap in conflicting traffic
This is a MINIMUM and MAXIMUM distance

18. P&R: Sight distances (cont)
Stopping Sight Distance (SSD):
Distance required to stop at any object/ intersection/ etc.
This is a MINIMUM distance

19. P&R: Sight distances (cont)
Left Turn Sight Distance to Pedestrians (LTSD):
3 m behind yield line to next full peds crossing
This is a MINIMUM distance

20. P&R: Sight distances (cont)
Combined sight distances

21. P&R: Internal and External Aprons (cont)
External truck apron
Internal truck apron

22. P&R: Internal and External Aprons (cont)
Internal aprons:
Determined by WB-15 wheel tracks (2 m minimum)
Cross-fall 2% outfall
Mountable kerb, 50 mm high at apron

23. P&R: Internal and External Aprons (cont)
Internal aprons:

24. P&R: Internal and External Aprons (cont)
Only if no other option available for controlling speed at entry and exit
Use WB-15 for tracking path
2% cross-fall (outfall) with V-channel
Pedestrian crossing to be clearly defined

25. P&R: Internal and External Aprons (cont)

26. P&R: Surfacing and pavement
Surface treatment can play important role in safety
Visual distinction between road, truck apron and peds/cycle crossings

27. P&R: Road Signs and Markings
SARTSM not consistent
Minimum required signs only in P&R
Rule remains – as little as possible
Not to obstruct peds and cyclists, but within legal requirements

28. P&R: Road Signs and Markings
Road markings
Fishhook arrows – get permission from minister
In double roundabouts use WM7-series for proper lane usage
Lane lines should guide lane selection
All kerbs to have GM8 markings
All markings shall be Thermo-plasic or cold plastic
On surfaces other than asphalt, proper priming of surfaces required before painting

29. P&R: Road Signs and Markings (cont)

30. From here…
Inconsistency in design standards used – need for South African standards long overdue
WCG: DoT in process of providing a tool
Evaluation of roundabout as intersection control measure
Consistent safe designs
Will be ready shortly for use in the industry
Recommended that other National, Provincial and Local governments also consider this for use in their areas

31. Towards the Ideal Roundabout Design
SATC 2017
Towards the Ideal Roundabout Design
Thank you
FH van Renssen Mowana Engineers
M Watters WCG DoT
CR Tichauer Mowana Engineers
Dr C Krogscheepers ITS International
G Matthee ITS International