1. TOWARDS THE IDEAL ROUNDABOUT DESIGN
C KROGSCHEEPERS, M WATTERS, CR TICHAUER,
FH VAN RENSSEN, G MATTHEE

2. Content
Background
Review of Existing Roundabouts – Sample of 37
Inscribed Circle Diameter
Circulatory Lane Width
Entry Lane Width
Truck Aprons
Fastest Paths
Road Signs
Road Markings
Conclusions

3. Why are we still looking at roundabouts?
Confusion?
Inconsistency?
Guidelines?
Data?

8. REVIEW OF ROUNDABOUTS: SAMPLE of 37
Summary of Roundabouts investigated   
Urban
Rural
Single
Double
City of Cape Town 11
George 4 1
Other small towns 3
Part of the process to develop a Roundabout Guideline for Western Cape

9. (approach, entry, circulatory and exit positions)
Process: What did we do?
Reviewed Geometric elements:
ICD, CID, Aprons, Lane widths, Gradients at various locations on the approaches, Sight distances, Entry angles, Splitter island lengths and widths, Road markings and Signs, etc.
Did Speed measurements
(approach, entry, circulatory and exit positions)
Visual assessment
Operations, accident evidence (broken glass, damaged kerbs, damaged signs, etc.)
Interviews with agency.

10. Inscribed Circle Diameter (ICD)

11. Inscribed Circle Diameter (ICD)

12. Circulatory Lane Widths

13. Circulatory Lane Widths
Circulatory lane widths. Widths vary considerably. At some single lane roundabouts, the width was greater than 8m resulting in too small a central island diameter. When coupled with a poorly designed truck apron (no vertical separation) the advantage of controlling speed through the roundabout is compromised.

14. Entry Lane Widths

15. Entry Widths
Entry lane width. Too wide entry width negates any benefit achieved through the correct design of other elements resulting in too high entry speed. In instances where the wide entry width is coupled with a wide circulatory lane, the measured speeds through the roundabout often exceeded 60 km/h.

16. Truck Aprons

17. Truck Aprons
Truck apron not serving the purpose of design
The purpose of the truck apron clearly misunderstood by some
Truck apron often considered cosmetic rather than necessary.
External truck - useful - have a positive effect on the entry speeds.

18. Truck Aprons
Truck apron with better design, but too wide circulatory lane
Truck apron with correct design for use by trucks only with a narrow circulatory roadway

19. External Truck Apron

20. Fastest Path
The principles are generally ignored

21. Lordswalk Fastest Path

22. Brighton Fastest Path

23. Sunningdale Fastest Path

24. Road Signs
The variation in the use of road signs and marking reflects either the lack of knowledge, or the lack of attention to detail or the lack of clear guidance (within the SADC-RTSM) for the implementation of road signs and road markings.

25. Road Markings
The variation in the use of road signs and marking reflects either the lack of knowledge, or the lack of attention to detail or the lack of clear guidance (within the SADC-RTSM) for the implementation of road signs and road markings.

26. Conclusions
Lack of a clear consistent design standards
ICD generally within guidelines, entry and circulatory lane widths are generally too wide.
Potential for too high speeds - compromise in safety.
Pedestrians: No attention - unsafe conditions.
Truck aprons: Not used for the purpose intended.
Road signs and markings: Confusion
Many variables influence speed and safety. Correct application of all elements are vital for a safe design.

27. Conclusions
Could designs contribute to the perception that roundabouts are not safe?
Assessment of 37 roundabouts suggest: Yes.
Both in terms of vehicle only and ped crashes.
Clearly begs for design standards to provide safe and efficient designs.
Update SADC-RTSM for roundabouts.