Driving on the wrong side of the road sounds more like a recipe for traffic chaos than a traffic solution, but it’s an idea that’s starting to take hold in Australia.
Diverging diamond interchanges (DDI) are a twist on traditional interchanges that can move more traffic through an intersection and increase safety at the same time.
The standard diamond interchange is a familiar feature of Australian roadways. It is a road junction where a major road crosses a minor road via a bridge crossing. The two crossing roads are connected by the use of ramps that typically terminate either side of the bridge. Traffic lights are then used to regulate the traffic flow and conflicting movements.
The twist the DDI brings to the standard diamond interchange is along the minor road. Where the ramps terminate to create an intersection, traffic swaps from the left side of the road to the right in both directions. This cross over is controlled safely through the use of traffic lights. This creates a length of the roadway where the traffic is travelling on the opposite side of the road compared to normal.
This allows traffic turning right onto the ramp leading to the major road, to do so without needing to wait for traffic lights to stop oncoming traffic. When the traffic reaches the second intersection along the minor road, it crosses back over to the left hand side of the road to continue under normal conditions.
Gilbert Chlewicki, a director of US-based Advanced Transportation Solutions and essentially the creator of diverging diamond interchanges, outlined three advantages of the DDI to Arup and transport clients during a recent knowledge sharing visit to Australia.
The first is that they can move traffic faster and more efficiently.
This is because only two traffic light phases are typically required at intersections. At a traditional diamond interchange, there are three phases to accommodate the conflicting right hand turns. The reduction of one traffic signal phase can add significant efficiency and capacity to the intersections.
Secondly, they’re safer.
Data from the US confirms that crashes are reduced both in number and severity. DDIs reduce the conflict points by about half and reduce severe conflict points from 10 to two compared with a traditional diamond interchange.
Finally, they’re cheaper to build than traditional diamond interchanges.
DDIs let traffic peel off to the right instead of queuing in a turn lane while waiting for a green arrow. That means that those lanes are not needed on the bridge. Further, due to the efficiency of these intersections, fewer lanes are typically required to store queuing traffic.
These solutions have proven to be great retrofit solutions to existing diamond interchanges; existing bridges are reused whilst still increasing the traffic life of the interchange.
Another cost benefit is that DDIs have a small footprint. Compared with cloverleaf or interchange ramps or flyovers, they need much less land.
Gilbert Chlewicki says that in the US, reduced land purchase and building costs mean that DDIs cost on average $10 million less than the next best alternative.
DDIs have been popular in the US for several years where more than 60 are already in operation and many more are in planning and design stages. While there aren’t any yet in Australia, several road authorities are looking at the idea and can see the value.
A DDI is being considered for one location in Brisbane. It emerged as a strong option after we conducted a rigorous options analysis on four alternatives ranging from conventional upgrades to the unconventional. The DDI, when compared against the standard diamond interchange, came out cheaper, needed less land, was safer and provided 40 per cent more traffic capacity.
A DDI is currently being considered for the Bruce Highway Upgrade Caloundra Road to Sunshine Motorway project
We have also completed the concept design for a DDI in Melbourne, which likewise through analysis has proven to provide significant traffic improvements at minimal costs when compared to the conventional upgrades. This project has been endorsed to continue towards detailed design ready for construction.
In the US, public reaction to DDIs has generally been favourable, with surveys showing that 85 to 95 per cent of motorists think they’re a better and safer design. Road authorities also like them – usually after they’ve installed the first one they look for other opportunities to deploy the solution. ”Alex Borg Australasia Highways Skills Leader
However, DDIs are not a fix for all situations, They’re less suited to handling the high traffic volume created when two motorways meet or when most of the motorway traffic goes straight through the interchange and there’s little need to accommodate turns. The suitability of the DDI as a solution needs to be considered carefully for each specific site.
Many of Australia’s urban road networks are experiencing congestion and consequently delays and safety problems. Public funding for upgrades is also constrained, resulting in the need for innovative thinking about how we can deliver more for less and maximise the use of our existing assets. This is what currently drives interest in DDIs and other innovative treatments in Australia. Road authorities are starting to realise that they can get the traffic benefits at a much lower cost.