Diverging Diamond Interchanges: Driving Innovation in Australia

When it comes to road design, a simple idea can sometimes unlock transformational change. The Diverging Diamond Interchange (DDI) is one such idea. By briefly shifting traffic onto the “wrong” side of the road, it reduces congestion, improves safety, and makes life easier for drivers, pedestrians, and cyclists alike. 

Once considered radical, the DDI is now spreading across the world, and in 2020, Australia opened its very first. 

What is a DDI? 

A diverging diamond interchange, also known as a double crossover diamond, is a type of road interchange where traffic crosses to the opposite side of the road on either side of a freeway. This counterintuitive move eliminates the need to turn across opposing traffic when entering or exiting. 

The benefits are significant: 

• Safety: Conflict points drop from 26 (in a conventional diamond interchange) to just 14. Studies in the US show a 33% reduction in all crashes and a 41% reduction in injury crashes after DDIs are introduced. 

• Efficiency: Right turns no longer need dedicated signals, which means fewer traffic lights, shorter clearance times, and smoother flows. 

• Cost and space: DDIs require less land, smaller bridges, and often can be retrofitted to existing sites. 

The design first appeared in France in the 1970s but only gained traction in the US from 2009, when Missouri opened the first American DDI. Today, there are over 100 in operation worldwide. 

Australia’s First Diverging Diamond Interchange 

Australia joined the global movement in December 2020, with the opening of a DDI on the Bruce Highway at Caloundra Road on the Sunshine Coast. 

The interchange forms part of the $932 million Bruce Highway Upgrade (Caloundra Road to Sunshine Motorway), a project jointly funded by the Federal and Queensland Governments. Designed by Arup and Jacobs, and built by Fulton Hogan Seymour Whyte Joint Venture, it represents a landmark in Australian road innovation. 

For Stephen O’Brien, Seymour Whyte’s Pre-Contracts Manager, the project became one of the most defining achievements of his career. 

“I was involved from the Expression of Interest through to the Tender phase, where I served as the Traffic and Staging Lead. I later returned as Engineering Manager and later as Construction Manager, remaining on the project through to Practical Completion. That full-circle involvement gave me a deep understanding of both the design intent and the delivery realities of constructing Australia’s first Diverging Diamond Interchange.” 

A Radical Design Becomes Reality 

When Stephen first heard about the DDI, he admits he was intrigued – and a little baffled. 

“I’d read an article from Arup about this new type of interchange and was instantly fascinated,” he said. “At first, it was a complete head-wreck — watching videos where people drove on the ‘wrong’ side of the road — but once it clicked, I was completely sold and couldn’t wait to get on and build it.” 

With more than 60,000 vehicles a day and a particularly high proportion of right-turning traffic onto the Bruce Highway, the site was plagued by congestion and safety risks. The DDI offered a smarter solution: vehicles could move more freely with fewer stops, while right turns no longer clashed with through-traffic. 

Engineering the First of its Kind 

Being the first DDI in Australia meant there was no room for error. The design, verification and approvals all required careful collaboration between delivery partners and the Department of Transport and Main Roads (TMR). 

“The design team faced huge challenges, and both they and TMR deserve a lot of credit for getting it designed, verified, and approved,” Stephen said. 

From a construction standpoint, the biggest challenge came in staging – sequencing the works while maintaining traffic flow. 

“When I came back to the project, I overhauled our staging strategy to make the transition to the DDI as seamless as possible. Saving so much of the surrounding national park meant our new footprint overlapped the existing one with very different levels. At one stage, we considered a temporary roundabout, but instead we created what we called the ‘Superloop’ — essentially reconfiguring an existing ramp to Caloundra Road without changing the operation of the interchange.” 

Stephen credits the success of the project to the strength of the team and the collaborative approach on site. 

"I was incredibly lucky to have a fantastic team around me — people who were as passionate as I was about making the project a success. Everyone, from site engineers to supervisors, was fully invested in getting it right. That energy really showed in the way we tackled challenges together.” 

Sustainability and Community at the Core 

The Caloundra DDI wasn’t just about cars. The project reused 130,000m³ of material and reduced its footprint on the Mooloolah River National Park by 29 hectares, preserving ecologically and culturally significant land. 

It also incorporated infrastructure for pedestrians, cyclists, carpoolers and buses, embedding sustainable transport options into the design. 

“Another standout achievement was how the project actually gained community support as it progressed — quite often, it’s the other way around,” Stephen noted. “Transparency and consistent communication built real trust with the public. The team did an exceptional job of keeping people informed and engaged.” 

Lessons for Future Projects 

The Caloundra DDI set a precedent for future projects – with DDIs now being delivered at Burleigh and Bracken Ridge. Stephen has since shared his experience with those teams, as well as international designers exploring the model. 

“The common message I’ve shared with all of them is simple: communication is everything,” he said. “When you’re introducing something genuinely new, be upfront about that. You’re asking the public to come on the journey with you — and trust is earned through honesty.” 

Delivering something new isn’t easy, but for Stephen, it’s where true progress happens. 

“Innovation means overcoming the bias toward the familiar,” he reflected. “When we collaborate to overcome fear and challenge convention, we achieve real progress — delivering more with less and helping the industry become more productive and sustainable. It’s a reminder that progress in our industry depends on the courage to create, not just repeat.” 

The Legacy 

The Caloundra DDI shows what’s possible when bold design, collaboration, and communication come together. It’s a tangible example of progress: delivering a safer, more efficient interchange that also respects its environment and community. 

And for Stephen O’Brien and the team at Seymour Whyte, it stands as proof that innovation doesn’t just change infrastructure – it changes how we think about what’s possible.