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

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

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

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

(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.

Inscribed Circle Diameter (ICD)

Inscribed Circle Diameter (ICD)

Circulatory Lane Widths

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.

Entry Lane Widths

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.

Truck Aprons

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.

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

External Truck Apron

Fastest Path The principles are generally ignored

Lordswalk Fastest Path

Brighton Fastest Path

Sunningdale Fastest Path

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.

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.

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.  

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.