By Brent Balinski 

In less than a decade the US has gone from zero to 60-plus diverging diamond interchanges, a design that briefly channels motorists onto the wrong side of the road. 

Major construction is scheduled to begin this year on the billion-dollar Bruce Highway upgrade in Queensland, which is among the state’s “highest priority road projects". 

The project’s main site compound will be south-east of the busy Caloundra interchange, which sees an estimated 60,000 vehicles a day.

A new piece of roadway innovation will address traffic flow and improve safety at the busy site. First implemented in France then popularised in the US, the diverging diamond interchange (DDI), will be the first of its kind in the Australia, and see motorists defy years of practice by having them temporarily drive on the wrong side of the road.

Motorists will seamlessly cross over from the left side of the road carriageway to the right as they go over the interchange, then switch back to the left, explains a spokesperson from Queensland’s Department of Transport and Main Roads.

“One of the major advantages is that right turning traffic does not have to cross opposing traffic when making a right hand turn,” she tells inspire.

“The Caloundra Road interchange has a large amount of traffic turning right onto the Bruce Highway. Because the right-turning traffic does not have to give way to through traffic this significantly reduces delays, eases congestion and improves safety.”

Intersections can present risks, with the dangers (minimised through sensible driving) coming from conflicts such as right-angle crashes. Intersection crashes also usually result in more severe types of injuries, explains Professor Lynn Meuleners, Director of the Curtin Monash Accident Research Centre (C-MARC), which recently completed a study investigating the potential safety issues associated with the DDI.

“Injuries that occur as the result of crashes at an intersection usually result in more severe types of injuries, including head and spinal injuries, and they can be the result of possibly high speed or the impact to the side of the vehicle,” she says.

Advocates say DDIs lessen the number of conflict points from 26 (for a regular diamond interchange) to 14, and exposure to right-angle crashes.

Smaller and safer?

The DDI was popularised by Gilbert Chlewicki, Division Director at ATS/American, who first wrote a term paper in 2000 detailing the concept while a graduate student at the University of Maryland. Six months after turning in his work, and while on holiday in France, a bus tour to Versailles reminded him there’s nothing new under the sun when he drove through a DDI. Still, he was pleased to see that the DDI worked in the real world.

The first American installation was in Springfield, Missouri in 2009 and popularity has rapidly increased since. There are more than 60 in the United States and according to TMR, in excess of 100 world-wide.

Last year, an analysis of seven of the earliest DDIs in the US was further validation for Chlewicki and supporters of his work. Published in Transportation Research Record and comparing a total of 28 site years before and 19 after conversion, the researchers found a 33% reduction in all crashes, and a 41% reduction for injury crashes.

According to Chlewicki, there are three main advantages over other interchanges. These are safety, improving operations – under the right conditions – through requiring fewer movements prompted by traffic signals (two instead of three or more), and reduced cost.

“It cost a lot less to build the DDI, because you don’t need as much land for it; it’s a pretty compact design,” he tells inspire.

“You don’t need these big loop ramps anymore. And also the structure – the bridge – is a huge plus also. It can be a much smaller bridge, and often times, for retrofits, it uses the same bridge.”

Australian research

The safety issues around encouraging WA drivers to eschew years of tradition were examined in the recent C-MARC DIAMOND Study in collaboration with ARRB.

Funded via a Neurotrauma Research Program grant, the recently-published work involved 201 drivers aged between 18 and 80. Each participant underwent a driving simulator assessment which included driving through a DDI and two regular intersections. As well, participants completed a questionnaire and the NASA Taskload Index.

The research concluded that “minimal difficulties” were identified, consistent with international studies. There were no wrong-way or navigational errors, and only 16% self-reported any difficulty with the DDI. All drivers “performed well” on the NASA Taskload Index but more difficulty in terms of mental workload was experienced among older drivers than other age groups.

Among recommendations were that the introduction of DDIs in WA and the rest of Australia include community education, adequate signage, and targeted education for older drivers.

Education is part of the introduction at Caloundra.

“TMR will offer opportunities for Sunshine Coast motorists to familiarise themselves with how to drive through the interchange by the end of construction; however, a well-designed DDI will present the driver with an intuitive and familiar driving experience,” the Department’s spokeswoman said.

Limitations

The DDI has evidence showing its benefits, but it’s not an answer for every situation.

Alex Borg, a road designer at Arup, wrote last year that they are unsuited to the traffic volumes produced when two motorways meet, for example.

According to Chlewicki, the design shines when there’s a “high degree of right turns” (or left turns in his country) or if there’s a lot of traffic going in one direction and not so much in the other.

“If the peak line’s all going towards one side in the morning and they’re all coming back the other way in the afternoon [for example]. If it’s unbalanced, that helps out too,” he says, adding that it might not be the best (though could still be a good) solution if either of the above two conditions are lacking.

Whatever the interchange’s strengths and weaknesses, TMR expects their Australian first won’t be the last.

“Whilst a DDI solution doesn’t suit all circumstances, it is likely we will see more DDIs in Australia given the body of evidence supporting their successful use,” predicts their spokeswoman.

How it works

With the DDI, traffic moves along to a freeway overpass and in the ramp leading to the crossover, is directed into the right lane. Cars continue in the right-hand lane until they navigate the overpass and are then directed back into the regular lane.

Traffic in both directions moves on the right lane. It is directed at each crossover point via traffic lights, with few of these needed in total compared to a regular diamond interchange.

The angle of the crossover is not prescribed, and according to the comprehensive divergingdiamond.com website (maintained by Chlewicki’s ATS/American), “the only concern should be to have enough of an angle to avoid wrong way movements that could lead to a head-on collision.”

What is eliminated is what matters: the need to turn across oncoming traffic, and the need to wait for a signal to turn right.

According to the same website mentioned above, there is no real difference if the highway continues over or under the crossroad, with regular sight distance consideration applying if there’s a structure blocking the view of drivers.

Overall, less space is needed compared to a conventional interchange. In the case of the Caloundra site, TMR says, “The DDI has significantly reduced the impact on Beerwah State Forest from 24 hectares to less than eight hectares. This reduction has been very well received by environmental stakeholders.”

Image: Diverging Diamond Interchange completed in Fern Valley 2017. 

This article originally appeared in the April edition of inspire magazine. View more articles here.