BRITANNIA’S DOUBLE ROUNDABOUT
by John Satterley
BACK in 2005 a planned remake of the notorious Britannia roundabout was scrapped because the fix would cause too much environmental damage. It was to be a high-tech traffic light solution costing $8.84 million, but in the process destroying 66 trees, including 16 deemed significant and one endangered.
The roundabout, just east of the CBD, is an intersection of five roads used by more than 50,000 vehicles a day. In November 2013 the Department of Planning, Transport and Infrastructure (DPTI) completed a remake that cost $3.2 million and involved acquiring 58 square metres of parkland, the removal of a small eucalypt from the original roundabout and four plane trees. No trees were removed from the parklands.
There are now two roundabouts about 40 metres apart – one at the bottom of Kensington Road, the other at Wakefield Street. They were installed by 32 DPTI field service workers and 73 contractors. Within six weeks of opening, DPTI confirmed that traffic on the Dequetteville Terrace-Wakefield Street approach had increased by 20% in the afternoon peak.
In June this year the Institute of Public Works Engineering Australasia recognised the upgrade with an Excellence Award in design and/or construction of a public works project. The citation noted there had been a 36 per cent reduction in crashes between November 2013 and March this year and an overall 10 per cent increase in traffic moving through the intersection.
Andrew Excell, Metropolitan Regional Manager in the Transport Services Division who was part of the project team told CCF SA Magazine: “What we found throughout the whole process was the sense of excitement and commitment of everyone, from the traffic control personnel all the way through to the (then) Chief Executive, Rod Hook.”
Claudio D’Agostini, the division’s network strategist at the time, started the traffic modelling of the intersection, work that took almost two years to complete. Then traffic systems engineer Dr Kun Zhang took a further two years to complete the modelling work.
The project team relied on AIMSUN, micro transport simulation software developed in Spain to model the current behaviours of traffic at the intersection. This is called calibrating the model and aimed to achieve the real behaviour being replicated in the traffic model. SIDRA was used to model queues and delays.
Dr Zhang observed aggressive behaviour by road users as they entered the intersection, i.e. they were not giving way; they were just selecting a gap. “He took out all the give-way rules from the model and then the model matched the real world in terms of the traffic flows,” said Mr Excell.
This enabled the model to be used in the development of the two roundabouts scheme to determine if they would work in maintaining the traffic capacity and flow. The models were tested with increasing increments of traffic volumes to determine what capacity the two roundabouts could handle.
This was determined by identifying when there was a drop in the level of service and/or when the travel times through the intersection increased due to delays. Additional modelling was undertaken to confirm the AIMSUN modelling using LinSig software and the ARCADY (Assessment of Roundabout Capacity and DelaY) software.
Mr Excell said the traffic models showed that with two roundabouts there would be an improvement to the capacity of the intersection and a reduction of the travel times on some legs of the roundabouts. DPTI worked on the road design of the scheme and an extended design domain process was adopted to enable the new scheme to fit within the existing kerb lines where possible. The extended design domain process enables design values to be lower than minimum values traditionally specified for new roads in Austroads road design guidelines.
Construction Project Manager Peter Tan was assigned to deliver the project. The Adelaide City Council, Norwood Payneham and St Peters Council and Burnside Council which border the intersection were consulted as were the RAA and the SA Motor Sports Board which runs the annual Clipsal 500 (the intersection forms Turn Nine of the course).
A major challenge was constructing the roundabouts while keeping 50,000 vehicles moving. “We managed to maintain two lanes into the original roundabout during peak hours and undertook the majority of construction at night,” Mr Excell said. There were 77 traffic management plans. Eight properties including the Britannia Hotel underwent a Property Condition Assessment before construction began. On site were two vibration monitors to alert construction personnel to stop work should heavy vibration occur.
More than 1.6 kilometres of conduits (installed with underground boring) will provide for CCTV coverage by the Traffic Management Centre at Norwood and for future needs. Six gantries 5.5m to 6.5m high with 4.5m footings were erected for overhead signage on all approaches to the roundabouts. They show a symbolic representation of the roundabouts with arrows to get people to the correct lanes.
A behavioural scientist from road-research entity ARRB Pty Ltd was commissioned to ensure the signage would be understood by the community. DPTI is trialling a new gantry protection system – a 0.5m high buffer around the base of the gantry on Fullarton Road (North) containing numerous metal cylinders that absorb energy in a collision.
Major works started in September 2013. Tan said traffic control was enhanced to ensure the community knew how to get through the construction site and that workers were well protected.
The existing pavement was overlaid with new asphalt and detailed survey level work ensured a smooth surface. Roughness counts taken following the work “are extremely pleasing” with an average count of 30, which exceeds DPTI’s master specifications of 50 for urban low speed environments.
The roundabout at Turn Nine was removed for the staging of the Clipsal 500 this year and replaced afterwards. Recycled rubber was considered (the ACC has one near the Adelaide Zoo) but after further investigation it was considered the rubber wouldn’t last and glass reinforced concrete was chosen.
The main players
- SA Department of Planning, Transport and Infrastructure (DPTI)
- Planners and designers: SA Department of Planning, Transport and Infrastructure (DPTI)
- Construction: SA Department of Planning, Transport and Infrastructure (DPTI)
- Asphalt: Fulton Hogan
- Gantry: Sev-Style Engineering
- Gantry protection buffer: Automotive Safety Engineering Pty Ltd
- Gantry signage: Road Management Solutions
- Glass reinforced concrete: GRCC Pty Ltd
- Landscaping: Landscape Construction Services Pty Ltd
- Property condition assessment: TMK Engineering
- Time lapse photography: Logic Films Traffic control: FILCON Safety Group SA
- Tree removal: Urban Virons Group
- Vibration monitoring: Resonate Acoustics
- Underground boring and conduits: Trenchless Pipelaying Contractor